[{"data":1,"prerenderedAt":926},["ShallowReactive",2],{"layer:eclipses:discover":3},{"layer":4,"contentHash":905,"dependencyHashes":906,"approval":920,"releaseId":925},{"schemaVersion":5,"conceptId":6,"locale":7,"depth":8,"revision":5,"title":9,"subtitle":10,"summary":11,"objectives":12,"estimatedMinutes":18,"plate":19,"blocks":40,"sourceIds":900,"reviewStatus":901,"authoring":902},1,"eclipses","en","discover","An eclipse is a shadow that finds you","Two shadows, two kinds of eclipse, and how to watch one without hurting your eyes","Meet eclipses as what they really are: shadows. Learn whose shadow falls on what in solar and lunar eclipses, why the eclipsed Moon turns red, why we don't get one every month, and the safe ways to watch the Sun.",[13,14,15,16,17],"Explain an eclipse as a shadow, and name the umbra and the penumbra.","Say whose shadow falls on what in a solar eclipse and in a lunar eclipse.","Tell a solar eclipse from a lunar one using at least three clues from a photograph or description.","Give the reason there isn't an eclipse every month, using the Moon's 5.1° tilt and the two nodes.","List the safe and unsafe ways to watch a solar eclipse, and set up a pinhole or colander projection.",30,{"title":20,"rows":21},"Lesson plate",[22,25,28,31,34,37],{"label":23,"value":24},"Depth","Discover",{"label":26,"value":27},"Reading time","≈ 30 minutes",{"label":29,"value":30},"Prior knowledge","Light travels in straight lines (Light)",{"label":32,"value":33},"Chapters","9",{"label":35,"value":36},"Labs","Shadow lab, eclipse lab ×3, sort, match",{"label":38,"value":39},"Safety","Solar viewing rules — read them",[41,45,51,57,63,68,73,76,101,133,138,143,146,182,187,193,198,213,218,221,246,250,255,259,270,275,278,314,332,392,397,402,406,409,420,433,438,443,479,484,527,532,535,562,566,595,600,603,608,637,641,646,649,668,683,687,737,868,872,888],{"id":42,"type":43,"markdown":44},"intro-hook","prose","It is the middle of the day. The sky is bright, the crows are noisy, the shadows under the neem tree are sharp and black.\n\nThen, slowly, something starts to go wrong.\n\nThe light turns thin and strange, like an old photograph. The air cools. The shadows under the tree stop being blurry blobs and become hundreds of tiny **crescents** scattered on the ground. The crows go quiet and fly to their branches. A cow lies down. Somebody's street light, confused, flickers on.\n\nAnd if you are standing in exactly the right place on Earth, the last sliver of the Sun vanishes, a ring of pearly light appears where the Sun used to be, and for two or three minutes it is **night in the middle of the morning**.\n\nThat is a total solar eclipse. And here is the surprising thing about it: nothing has exploded, nothing has been switched off, and nothing has gone wrong at all. Something very ordinary has happened. **A shadow has landed on you.**",{"id":46,"type":47,"variant":48,"title":49,"markdown":50},"intro-promise","callout","observation","What this lesson gives you","By the end you will be able to do four things that most adults cannot:\n\n1. Say exactly whose shadow falls on what, in both kinds of eclipse.\n2. Look at a photograph and say instantly whether it is solar or lunar.\n3. Explain why we do not get an eclipse every single month.\n4. Watch a solar eclipse **without damaging your eyes**, and help other people do the same.\n\nThe safety part is not a footnote. It is the most important thing on this page, and it comes up again and again.",{"id":52,"type":53,"conceptId":54,"relation":55,"explanation":56},"conn-light","connection","light","helps_understand","An eclipse is a shadow, and a shadow only exists because light travels in straight lines.",{"id":58,"type":59,"title":60,"eyebrow":61,"navLabel":62},"ch1","chapter","Everything here is made of shadows","Chapter 01","1 Shadows",{"id":64,"type":43,"markdown":65,"help":66},"shadow-basics","Stand outside on a sunny day and look at your own shadow. You have made it by doing something very simple: you got in the way.\n\nLight from the Sun travels in **straight lines**. It cannot bend around you to fill in the space behind you, so that space stays dark. That dark space is your shadow, and it is not a thing lying on the ground — it is a **long invisible tunnel of darkness** stretching away from you, which only becomes visible where it hits something.\n\nNow hold that thought, because it is the whole lesson:\n\n- Every object lit by the Sun trails a shadow behind it. Your body. A tree. A building. A mountain.\n- **Earth has one.** It is a cone of darkness about 1.38 million kilometres long, pointing away from the Sun.\n- **The Moon has one too.** It is a much thinner cone, only about 374,000 kilometres long.\n\nAn eclipse happens when one of those two cones lands on the other body — and somebody is standing where it lands.",{"simplerExplanation":67},"Light goes straight, so anything solid leaves a dark tunnel behind it. Earth and the Moon each trail one of these tunnels through space. When one tunnel hits the other world, that is an eclipse.",{"id":69,"type":47,"variant":70,"title":71,"markdown":72},"def-eclipse","definition","Eclipse","An **eclipse** happens when one body moves into the shadow of another, or blocks our view of it.\n\nThere are only two that we can see well from Earth:\n\n- **Solar eclipse** — the **Moon's** shadow falls on **Earth**. The Sun is hidden.\n- **Lunar eclipse** — the **Earth's** shadow falls on the **Moon**. The Moon goes dark.\n\nA useful memory hook: an eclipse is named after **the thing that goes dark for you**, not the thing casting the shadow.",{"id":74,"type":43,"markdown":75},"umbra-penumbra-intro","Hold your hand up in front of a small torch in a dark room and look at the shadow on the wall. Now do it again with a big frosted table lamp. The second shadow has a soft, fuzzy grey edge.\n\nThat fuzzy edge is telling you something important. A shadow has **two parts**:\n\n- The **umbra** is the properly dark middle. Stand in the umbra and the light source is **completely** hidden.\n- The **penumbra** is the grey fringe around it. Stand in the penumbra and you can still see **part** of the light source peeping past the edge of the blocker.\n\nThe words come from Latin: *umbra* means \"shadow\" and *paene* means \"almost\", so *penumbra* is the \"almost-shadow\".\n\nBoth kinds of eclipse come in two flavours for exactly this reason. Stand in the Moon's umbra and you get **totality**. Stand in its penumbra and you only get a **partial** eclipse: a bite taken out of the Sun.",{"id":77,"type":78,"tone":79,"items":80},"spec-two-shadows","spec","blue",[81,85,89,93,97],{"label":82,"big":83,"value":84},"Umbra","Full dark","The light source is completely hidden. This is the narrow core of the shadow cone.",{"label":86,"big":87,"value":88},"Penumbra","Part dark","Part of the light source is still visible past the blocker's edge. Much wider, much softer.",{"label":90,"big":91,"value":92},"Why two?","Big source","The Sun is a huge disc, not a pinprick. Different edges of it are blocked in different places.",{"label":94,"big":95,"value":96},"In a solar eclipse","≈ 160 km","The Moon's umbra lands as a spot only about 160 km wide. The penumbra spot is thousands of km wide.",{"label":98,"big":99,"value":100},"In a lunar eclipse","2.6 Moons","Earth's umbra at the Moon's distance is about 2.6 Moon-widths across — easily big enough to swallow it.",{"id":102,"type":103,"component":104,"componentVersion":5,"config":105,"objective":127,"textAlternative":128,"help":129},"lab-shadow-basics","interactive","shadow-lab",{"objects":106,"source":119,"maxDistanceCm":120,"challenges":121},[107,111,115],{"id":108,"label":109,"heightCm":110},"ball","Small ball (Moon)",4,{"id":112,"label":113,"heightCm":114},"globe","Large ball (Earth)",12,{"id":116,"label":117,"heightCm":118},"card","Card disc",8,"both",300,[122,125],{"prompt":123,"targetRatio":124},"Make the shadow exactly twice as wide as the ball.",2,{"prompt":126,"targetRatio":5},"Make the shadow the same width as the ball.","See how a shadow has a dark umbra and a soft penumbra, and how its size changes as you move the object.","This lab puts a lamp, an object and a screen in a row. You choose the object and slide it between the lamp and the screen, and the shadow on the screen is drawn with a black centre (the umbra) and a grey fringe (the penumbra).\n\nWith a **point** source (a tiny bright bulb), the shadow has crisp edges and almost no grey fringe.\n\nWith an **extended** source (a wide, frosted lamp, more like the real Sun), a grey penumbra appears around the black umbra. Move the object closer to the screen and the umbra grows sharper and smaller; move it closer to the lamp and the shadow spreads out and the grey fringe widens.\n\nReadings you can take from the lab: a 4 cm ball 20 cm from a lamp, with the screen 60 cm from the lamp, casts a shadow 4 × 60 ÷ 20 = **12 cm** wide. Slide the ball back to 30 cm and the shadow shrinks to 4 × 60 ÷ 30 = **8 cm**. At 40 cm it is **6 cm**. The pattern: the further the object is from the lamp, the smaller its shadow on a fixed screen.",{"hints":130},[131,132],"Try the point source first, then switch to the extended source and watch the grey fringe appear.","The umbra is where *none* of the lamp is visible. The penumbra is where *some* of it is.",{"id":134,"type":47,"variant":135,"title":136,"markdown":137},"careful-torch","try_it","Two-minute experiment: make a penumbra","You need a dark room, a phone torch, a table lamp with a shade off (a bare bulb), a small ball or an orange, and a white wall.\n\n1. Hold the ball about 30 cm from the wall. Shine the **phone torch** at it from a metre away. Look at the shadow: hard edges, almost no fringe.\n2. Now light it with the **bare bulb** instead. A soft grey border appears around the dark centre.\n3. Crouch so your eye is in the dark centre and peek at the bulb past the ball: it is completely hidden. Now shuffle sideways into the grey border and peek again: you can see a sliver of the bulb.\n\nThat sliver is the whole difference between a total and a partial eclipse. **Never do step 3 with the Sun.** A bulb is safe to glance at; the Sun is not.",{"id":139,"type":59,"title":140,"eyebrow":141,"navLabel":142},"ch2","Solar eclipse: the Moon's shadow lands on us","Chapter 02","2 Solar eclipse",{"id":144,"type":43,"markdown":145},"solar-what","Once a month, at **new moon**, the Moon passes between Earth and the Sun. The side of the Moon facing us gets no sunlight, which is why a new moon is invisible from Earth.\n\nMost months the Moon slips past a little above or below the Sun, and nothing happens. But occasionally it passes **exactly** in front, and then its shadow cone reaches all the way down and touches Earth.\n\nIf you are standing where the **umbra** touches, the Moon covers the Sun completely: a **total solar eclipse**.\nIf you are standing where the **penumbra** sweeps over, part of the Sun is still showing: a **partial solar eclipse**.\n\nThe umbra's footprint is tiny — usually a spot around 160 kilometres across, less than the distance from Delhi to Agra. The penumbra's footprint is thousands of kilometres wide. That is why partial eclipses are common and totality is so rare: the dark spot is small, Earth is enormous, and the spot races across the surface in a few hours.",{"id":147,"type":148,"title":149,"items":150},"steps-solar-order","steps","What happens during a total solar eclipse, in order",[151,155,159,163,167,171,175,178],{"title":152,"tag":153,"text":154},"First contact","hour 0","A tiny notch appears on one edge of the Sun. Only visible through a proper filter. Nothing else seems different.",{"title":156,"tag":157,"text":158},"The bite grows","next ~75 min","The Sun becomes a fat crescent. The light is still bright enough that most people walking past would not notice.",{"title":160,"tag":161,"text":162},"The world changes","last 15 min","Light turns silvery and flat. The air cools, often by about 5 °C. Shadows go strangely crisp.",{"title":164,"tag":165,"text":166},"Crescents everywhere","last 10 min","Gaps between leaves act as pinholes and scatter hundreds of little crescent Suns across the ground.",{"title":168,"tag":169,"text":170},"Diamond ring","seconds","One last blazing point of sunlight sits on the dark rim like a jewel on a ring.",{"title":172,"tag":173,"text":174},"Totality","up to 7 min 32 s","The disc goes black, the pearly corona blazes out, bright stars and planets appear, the horizon glows orange all the way round.",{"title":176,"tag":169,"text":177},"Second diamond ring","Sunlight bursts out on the other side. Filters go back on immediately.",{"title":179,"tag":180,"text":181},"It unwinds","~75 min","The crescent grows back the other way, the birds start up again, and the day returns.",{"id":183,"type":47,"variant":184,"title":185,"markdown":186},"safety-first","careful","Eye safety: read this before you watch anything","**Never look at the Sun without a certified solar filter. Not for a second. Not even when it is a thin crescent.**\n\n- Use only eclipse glasses or a viewer marked **ISO 12312-2**. They let through about 0.003% of the light — roughly one thirty-thousandth.\n- **Sunglasses are not safe**, not even several pairs stacked. They pass about 10% of the light, over 3,000 times too much.\n- **Also unsafe:** smoked glass, candle-blackened glass, exposed photographic negatives, X-ray film, CDs or DVDs, coloured plastic, welding glass below shade 12, and looking at the Sun's reflection in water or in a bucket of ink.\n- **Never** look through a camera, phone camera, binoculars or telescope, *even while wearing eclipse glasses*. The lens concentrates the heat, melts the filter and blinds you in an instant.\n- The safest method of all needs no filter: **projection**. Turn your back on the Sun and look at an image of it instead. There are three ways on the next page.\n- Damage to the retina is **painless** — there are no pain nerves there. You feel nothing while it happens, and notice the blank patch hours later. That is exactly why the rule has to be absolute.",{"id":188,"type":43,"markdown":189,"help":190},"projection-methods","Here are the three safe ways. All of them share one rule: **your back is to the Sun and you are looking at a piece of paper.**\n\n**1. The pinhole card.** Take two pieces of stiff card. Make one clean pinhole in the first with a pin (not a big ragged hole — a small neat one). Stand with the Sun behind you, hold the pinhole card up, and let the light fall on the second card held about a metre away. A little disc of light appears: that is an **image of the Sun**, about 9 mm across at one metre. During a partial eclipse it will be a crescent. Move the cards further apart and the image grows: at 5 metres it is about 47 mm across, and dimmer.\n\n**2. The kitchen colander.** Hold a colander, a slotted spoon or even a piece of card with twenty pinholes in it, and let the sunlight through onto the ground. Every hole makes its own image of the Sun. During an eclipse you get a spray of dozens of crescents. This is easily the best crowd-pleaser, and it is free.\n\n**3. The mirror.** Cover a small flat mirror with paper that has a hole about 5 mm across cut in it. Stand in a doorway or a shaded veranda, catch the sunlight on the mirror and bounce it onto a wall in the shade, at least 5 metres away. A surprisingly big image of the Sun appears on the wall. Never point the mirror at anyone's face, and never look into the mirror yourself.\n\nAnd the one that needs no equipment at all: **look under a leafy tree.** The gaps between overlapping leaves are already pinholes. During a partial eclipse the ground under a neem or a peepal tree is covered in crescents. This one always makes people gasp.",{"simplerExplanation":191,"anotherExample":192},"Do not look at the Sun. Let the Sun draw a picture of itself on a piece of paper, and look at the paper.","A biscuit tin with a pinhole in one end and greaseproof paper at the other is a ready-made projector: point the pinhole end at the Sun over your shoulder and look at the paper.",{"id":194,"type":47,"variant":195,"title":196,"markdown":197},"safety-projection","aha","Why projection is completely safe","A pinhole does not gather light — it **throws most of it away**. Only the thin pencil of rays that fits through the hole gets past the card, and that little bit is spread out over a disc of paper. The image is thousands of times fainter than the Sun itself.\n\nA lens, by contrast, **gathers** light from a wide area and squeezes it to a point. That is exactly why you can burn paper with a magnifying glass, and exactly why you must never point one at the Sun while your eye is behind it.",{"id":199,"type":103,"component":200,"componentVersion":5,"config":201,"objective":206,"textAlternative":207,"help":208},"lab-solar-eclipse","eclipse-lab",{"modes":202,"showShadowCones":204,"tiltDegrees":205},[203],"solar",true,5.1,"Put the Moon between the Sun and Earth and watch its shadow cone land, making a total eclipse in the middle and a partial one around it.","This lab shows the Sun on the left, then the Moon, then Earth, drawn from the side with the two shadow cones behind the Moon marked in.\n\nSlide the Moon up and down. When it is well above or below the Sun–Earth line, the shadow cone misses Earth entirely and nothing happens: this is what almost every new moon looks like.\n\nBring the Moon onto the line and the cones sweep across Earth. The wide grey **penumbra** cone covers a huge region — several thousand kilometres across. Anyone inside it sees a bite taken out of the Sun.\n\nThe narrow black **umbra** cone comes to a point just about where Earth is, and lands as a spot only around 160 km wide. Only people inside that little spot see **totality**. A panel beside the globe shows what the Sun looks like from a chosen spot: a full disc outside the shadow, a crescent inside the penumbra, and a black disc with a ring of corona inside the umbra.\n\nRun time forwards and the two spots race eastwards across Earth's surface at roughly 2,000 km per hour, faster than any airliner.",{"simplerExplanation":209,"hints":210},"The Moon's shadow has a small black middle and a big grey edge. The black middle gives totality; the grey edge gives a partial eclipse.",[211,212],"Move the Moon a long way off the line first, so you can see how often *nothing* happens.","Watch how much bigger the grey spot is than the black one. That ratio is why you have probably seen a partial eclipse but not a total one.",{"id":214,"type":59,"title":215,"eyebrow":216,"navLabel":217},"ch3","Lunar eclipse: the Moon walks into our shadow","Chapter 03","3 Lunar eclipse",{"id":219,"type":43,"markdown":220},"lunar-what","Now turn the whole picture around.\n\nTwo weeks after new moon comes **full moon**, when the Moon is on the opposite side of Earth from the Sun. And behind Earth, pointing straight away from the Sun, is Earth's own shadow cone — 1.38 million kilometres of darkness, more than three and a half times further out than the Moon.\n\nMost full moons, the Moon sails past a little above or below that cone. But occasionally it goes straight in. The Moon does not disappear; it turns a deep, dusty **copper red**, and stays that way for up to about an hour and three quarters.\n\nThat is a **total lunar eclipse**, and it is much easier to see than a solar one for two reasons:\n\n1. **Everybody on the night side can see it.** The Moon itself is what changes, so anyone who can see the Moon at all sees the eclipse — roughly half the planet at once.\n2. **It is completely safe to look at.** The eclipsed Moon is around ten thousand times fainter than a full moon, which is itself perfectly harmless. Use your eyes, binoculars, a telescope, a camera — anything you like.",{"id":222,"type":223,"caption":224,"columns":225,"rows":230},"table-lunar-kinds","table","The three kinds of lunar eclipse, and how obvious each one is",[226,227,228,229],"Kind","Where the Moon goes","What you see","Worth setting an alarm?",[231,236,241],[232,233,234,235],"**Total**","Completely inside Earth's umbra","The Moon turns copper, orange or blood red. Stars come out around it.","Absolutely. Up to about 1 h 40 min of red.",[237,238,239,240],"**Partial**","Part of the Moon in the umbra","A dark curved bite out of the Moon, with a reddish tinge on the inner edge.","Yes — the curve of Earth's shadow is beautifully obvious.",[242,243,244,245],"**Penumbral**","Only in the outer grey penumbra","A very slight dimming of one side. Many people look up and see nothing at all.","Only if you are keen. Easy to miss entirely.",{"id":247,"type":47,"variant":195,"title":248,"markdown":249},"lunar-red-simple","Why the eclipsed Moon turns red","If Earth's shadow were perfectly dark, an eclipsed Moon would simply vanish. It does not, because Earth has an **atmosphere**, and the atmosphere bends light.\n\nSunlight grazing the edge of Earth is bent inwards, into the shadow. On the way through all that air, the blue light is scattered away sideways (which is why our sky is blue and our sunsets are red), and mostly **red** light survives to carry on into the shadow and land on the Moon.\n\nSo the red light falling on an eclipsed Moon is the light of **every sunrise and every sunset happening on Earth at that moment, all at once**. If you were standing on the Moon looking back, you would see a black Earth with a thin ring of fiery orange all the way around it.",{"id":251,"type":47,"variant":252,"title":253,"markdown":254},"misconception-phases","misconception","\"The phases of the Moon are Earth's shadow\"","This is the single most common mix-up in all of astronomy, and it is worth killing off right now.\n\nThe Moon's **phases** — crescent, half, gibbous, full — have nothing whatever to do with Earth's shadow. They happen because we see different amounts of the Moon's sunlit half as it goes round us. Phases take a month and happen every month, like clockwork.\n\nA **lunar eclipse** is Earth's shadow. It happens only at full moon, only a few times a year at most, lasts a few hours, and turns the Moon **red**, not black.\n\nQuick test: if you can see a crescent Moon in a blue afternoon sky, it obviously is not Earth's shadow — Earth's shadow always points *away* from the Sun, and the Sun is right there.",{"id":256,"type":53,"conceptId":257,"relation":55,"explanation":258},"conn-phases","phases-of-the-moon","Eclipses can only happen at new moon or full moon, so knowing the phases tells you when to look.",{"id":260,"type":103,"component":200,"componentVersion":5,"config":261,"objective":264,"textAlternative":265,"help":266},"lab-lunar-eclipse",{"modes":262,"showShadowCones":204,"tiltDegrees":205},[263],"lunar","Send the Moon through Earth's shadow and see the difference between a penumbral, partial and total lunar eclipse.","This lab shows the Sun on the left, Earth in the middle and Earth's two shadow cones stretching away to the right, with the Moon's path crossing them.\n\nEarth's dark **umbra** where the Moon crosses it is about 2.6 Moon-widths across, so the Moon fits inside with room to spare. The grey **penumbra** there is about 4.7 Moon-widths across.\n\nDrag the Moon's path up and down and three things can happen:\n\n- **Path high or low, clipping only the grey zone**: a *penumbral* eclipse. The little Moon panel shows a barely noticeable dimming on one side.\n- **Path clipping the black zone**: a *partial* eclipse. A dark curved bite appears, and it always curves the same way, because it is Earth's round shadow.\n- **Path through the middle**: a *total* eclipse. The whole Moon slides inside the umbra and the panel turns deep copper-red. A timer shows it can stay there for up to about 1 hour 40 minutes.\n\nNotice that the whole night side of Earth is shaded in the lab, and everyone there can see the same thing at the same moment.",{"hints":267},[268,269],"Try a path that just clips the grey zone, then one that just clips the black zone. Compare the Moon panel.","Count how many Moon-widths across the black circle is.",{"id":271,"type":59,"title":272,"eyebrow":273,"navLabel":274},"ch4","Telling the two apart, instantly","Chapter 04","4 Which is which?",{"id":276,"type":43,"markdown":277},"telling-apart","People muddle these two constantly, and photographs in newspapers get captioned wrongly all the time. Here is how to never get it wrong again.\n\nAsk yourself one question: **what is the dark thing sitting in front of?**\n\nIn a **solar** eclipse the dark disc is in front of a brilliant white Sun in a **daytime** sky. Everything around it is bright. It only lasts minutes. And everyone who can see it is in the same narrow strip of the world.\n\nIn a **lunar** eclipse the Moon is hanging in a **night** sky full of stars. There is no glare. It goes red rather than black. It takes hours. And half the world can watch it together.\n\nThere is a third giveaway that never fails: **the shape of the edge.**",{"id":279,"type":223,"caption":280,"columns":281,"rows":285},"table-tell-apart","Six ways to tell a solar eclipse from a lunar eclipse",[282,283,284],"Clue","Solar eclipse","Lunar eclipse",[286,290,294,298,302,306,310],[287,288,289],"Whose shadow?","The **Moon's**, falling on Earth","The **Earth's**, falling on the Moon",[291,292,293],"Time of day","Daytime — it must be, the Sun is up","Night-time — the Moon must be up",[295,296,297],"Moon phase","**New** moon, always","**Full** moon, always",[299,300,301],"Colour","Black disc, pearly white corona around it","Deep copper-red, never fully black",[303,304,305],"How long","Totality of seconds to 7 min 32 s at most","Totality up to about 1 hour 40 minutes",[307,308,309],"Who sees it","A strip of Earth ~160 km wide","About half of Earth at once — the whole night side",[311,312,313],"Is it safe to look?","**No.** Filters or projection only","**Yes.** Completely safe with bare eyes",{"id":315,"type":316,"prompt":317,"options":318,"explanation":331},"predict-which","prediction","Your cousin sends you a photo from the terrace at 9 p.m. It shows a reddish-orange Moon with a dark curve across one side, and a few stars visible nearby. She says \"look, a solar eclipse!\"\n\nWhat should you tell her?",[319,322,325,328],{"id":320,"label":321},"a","She is right — the Sun is blocking the Moon",{"id":323,"label":324},"b","It is a lunar eclipse: Earth's shadow on the Moon",{"id":326,"label":327},"c","It is a solar eclipse, but photographed badly",{"id":329,"label":330},"d","It is just a normal moonrise looking orange","**It is a lunar eclipse (b).** Three clues, any one of which settles it:\n\n- It is 9 p.m. and there are **stars**. A solar eclipse needs the Sun to be up.\n- The Moon is **red**, which is what happens when Earth's atmosphere bends sunset light into Earth's shadow.\n- There is a **dark curve** across it — the round edge of Earth's shadow.\n\nOption (d) is a fair guess, because a Moon low on the horizon really does look orange for a similar reason. But a plain low Moon is a smooth orange all over; it does not have a dark curved bite out of it.\n\nAnd \"the Sun blocking the Moon\" (a) cannot happen at all: the Sun is not a solid thing that gets in the way, and it is 400 times further off.",{"id":333,"type":103,"component":334,"componentVersion":5,"config":335,"objective":390,"textAlternative":391},"lab-sort-eclipses","sort-game",{"prompt":336,"bins":337,"items":340,"seconds":389},"Solar or lunar? Drop each clue into the right box.",[338,339],{"id":203,"label":283},{"id":263,"label":284},[341,345,349,353,357,361,365,369,373,377,381,385],{"id":342,"label":343,"bin":203,"why":344},"i1","Happens at new moon","The Moon must be between us and the Sun, which is exactly what new moon means.",{"id":346,"label":347,"bin":263,"why":348},"i2","Happens at full moon","Earth must be between the Sun and the Moon, which is exactly what full moon means.",{"id":350,"label":351,"bin":203,"why":352},"i3","The Moon's shadow falls on Earth","Solar eclipses are the Moon's shadow landing on us.",{"id":354,"label":355,"bin":263,"why":356},"i4","Earth's shadow falls on the Moon","Lunar eclipses are our own shadow landing out there.",{"id":358,"label":359,"bin":203,"why":360},"i5","You must never look without a filter","The Sun is still dangerously bright even as a thin crescent.",{"id":362,"label":363,"bin":263,"why":364},"i6","Completely safe to look at with bare eyes","An eclipsed Moon is about ten thousand times fainter than a full moon.",{"id":366,"label":367,"bin":203,"why":368},"i7","Only a strip about 160 km wide sees totality","The Moon's umbra lands as a small spot and races across the ground.",{"id":370,"label":371,"bin":263,"why":372},"i8","Roughly half the world can watch at once","Anyone who can see the Moon sees it eclipsed — the whole night side.",{"id":374,"label":375,"bin":263,"why":376},"i9","Turns deep copper-red","Sunset light bent through Earth's atmosphere lands on the Moon.",{"id":378,"label":379,"bin":203,"why":380},"i10","A pearly white corona appears","With the bright disc hidden, the Sun's faint outer atmosphere becomes visible.",{"id":382,"label":383,"bin":203,"why":384},"i11","Totality lasts at most 7 minutes 32 seconds","The umbra spot is small and moves fast.",{"id":386,"label":387,"bin":263,"why":388},"i12","Totality can last about 1 hour 40 minutes","Earth's shadow is far wider than the Moon, so the Moon takes ages to cross it.",0,"Sort twelve clues into solar and lunar, and read why each one belongs where it does.","A card-sorting game with two boxes: **Solar eclipse** and **Lunar eclipse**. Twelve clue cards must go into the right box, and each one explains itself when you drop it.\n\nSolar cards: happens at new moon; the Moon's shadow falls on Earth; you must never look without a filter; only a strip about 160 km wide sees totality; a pearly white corona appears; totality lasts at most 7 minutes 32 seconds.\n\nLunar cards: happens at full moon; Earth's shadow falls on the Moon; completely safe to look at with bare eyes; roughly half the world can watch at once; turns deep copper-red; totality can last about 1 hour 40 minutes.\n\nThe cards come in pairs of opposites, so if you are unsure about one, find its partner and put them in different boxes.",{"id":393,"type":59,"title":394,"eyebrow":395,"navLabel":396},"ch5","So why isn't there an eclipse every month?","Chapter 05","5 Not every month",{"id":398,"type":43,"markdown":399,"help":400},"why-not-monthly","Here is a puzzle that should be bothering you.\n\nThe Moon goes round Earth once a month. So once a month it passes between us and the Sun (new moon), and once a month it passes behind us (full moon). If everything lay flat in one plane, we would get a solar eclipse **and** a lunar eclipse every single month, like clockwork.\n\nWe do not. We get somewhere between four and seven eclipses of all kinds in a whole year, and most of those are partial or penumbral. Why?\n\nBecause the Moon's path around Earth is **tilted**. It is tipped by about **5.1 degrees** compared with the path Earth takes around the Sun. So most months the Moon passes **above** the Sun, or **below** it, and its shadow misses Earth entirely — flying off into empty space.",{"simplerExplanation":401},"The Moon's orbit is tilted, so most months it passes above or below the Sun instead of in front of it, and the shadow misses.",{"id":403,"type":47,"variant":48,"title":404,"markdown":405},"tilt-feel","5.1 degrees sounds tiny. It isn't.","Five degrees is about the width of three fingers held up at arm's length. That does not sound like much — until you compare it with the thing you are trying to hit.\n\nThe Sun in our sky is only about **half a degree** wide. (Check it yourself: half a degree is roughly the width of your little fingernail at arm's length, and the Sun and the full Moon both fit neatly behind it.)\n\nSo the Moon can stray nearly **ten times the Sun's own width** away from it. At the Moon's distance, 5.1 degrees works out at about **34,600 km** — nearly three Earth-diameters off target. The shadow sails past and hits nothing.",{"id":407,"type":43,"markdown":408},"nodes","A tilted circle crossing a flat one must cut through it in exactly **two places**. Picture a hula hoop pushed through a tabletop at a slant: it crosses the table's surface at two points on opposite sides.\n\nThose two crossing points are called the **nodes** of the Moon's orbit, and they are where all eclipses live.\n\n- If the Moon happens to be at a node *and* it is new moon → **solar eclipse**.\n- If the Moon happens to be at a node *and* it is full moon → **lunar eclipse**.\n- If it is a node but the wrong phase, or the right phase but nowhere near a node → nothing at all.\n\nBoth things have to be true at once, and that is why eclipses come in bunches a few times a year rather than every month. These bunches are called **eclipse seasons**, and there are two of them a year, roughly six months apart.\n\nHere is a lovely detail for anyone who grew up hearing about **Rahu** and **Ketu**: in classical Indian astronomy those two names are given to exactly these two points, the north and south nodes of the Moon's orbit. The old stories say Rahu and Ketu swallow the Sun and Moon. The geometry says eclipses happen at the nodes. Astronomers in India worked out centuries ago that these were two descriptions of the same two places in the sky.",{"id":410,"type":103,"component":200,"componentVersion":5,"config":411,"objective":413,"textAlternative":414,"help":415},"lab-why-not-monthly",{"modes":412,"showShadowCones":204,"tiltDegrees":205},[398],"Tilt the Moon's orbit and see for yourself why most new moons produce no eclipse at all.","This lab shows Earth going round the Sun, with the Moon's tilted orbit drawn as a ring around Earth. A slider changes the tilt from 0° up to 10°.\n\nSet the tilt to **0°** and step through the months. Every single new moon gives a solar eclipse and every single full moon gives a lunar eclipse — twenty-four eclipses a year. This is the world we do *not* live in.\n\nSet the tilt to the real **5.1°** and step through again. Now the Moon's shadow flies above or below Earth almost every month, and the Moon sails above or below Earth's shadow. Eclipses only happen in the two short windows each year when the line of nodes — where the tilted ring cuts the flat one — points at the Sun.\n\nA counter keeps score of eclipses per year: at 0° tilt it reads 24; at 5.1° it settles to between 4 and 7, matching the real sky. Two markers on the ring show the north and south **nodes**, and they light up during an eclipse season.",{"simplerExplanation":416,"hints":417},"Slide the tilt to zero and you get an eclipse every month. Slide it back to 5.1 degrees and they become rare. The tilt is the whole answer.",[418,419],"Try a tilt of 1° and see how much more often eclipses happen.","Watch where the two node markers are pointing when an eclipse does happen.",{"id":421,"type":316,"prompt":422,"options":423,"explanation":432},"predict-tilt","Suppose the Moon's orbit were not tilted at all, but lay perfectly flat in the same plane as Earth's orbit. How many eclipses would we get in a year?",[424,426,428,430],{"id":320,"label":425},"Still four to seven, as now",{"id":323,"label":427},"About 24 — one solar and one lunar every month",{"id":326,"label":429},"None at all",{"id":329,"label":431},"Exactly two, one of each","**About 24 (b).** Every new moon would put the Moon squarely in front of the Sun, and every full moon would push it squarely into Earth's shadow. Twelve solar and twelve lunar eclipses a year.\n\nIt is worth pausing on how strange that world would be. Eclipses would be ordinary. Nobody would travel to see one, and nobody would have worshipped, feared or written poetry about them. The tilt is what makes them rare, and rarity is what makes them extraordinary.\n\nTry it in the lab above: slide the tilt to 0° and count.",{"id":434,"type":59,"title":435,"eyebrow":436,"navLabel":437},"ch6","The luckiest coincidence in the sky","Chapter 06","6 400 and 400",{"id":439,"type":43,"markdown":440,"help":441},"coincidence","Go outside and look at the full Moon. Now look at the Sun — no, don't. But you know how big it looks: about the same.\n\nThat is astonishing, and there is no reason for it.\n\nThe Sun is about **400 times wider** than the Moon. And it happens to be about **400 times further away**. Those two 400s cancel out almost exactly, so the two discs look almost exactly the same size from Earth — both about **half a degree** across.\n\nNothing in physics required this. The Moon is not the right size *because* of anything. It is a coincidence, and it is the reason total solar eclipses exist at all.\n\n- If the Moon looked a bit **bigger**, it would blot out the Sun easily and we would never see the delicate corona around the edge.\n- If it looked a bit **smaller**, it could never cover the Sun completely and there would be no totality at all — just a bright ring, every time.\n\nAs it is, the Moon is *just* big enough, *just*. It covers the Sun's blinding disc and no more, leaving the faint outer atmosphere exposed for a few minutes.",{"anotherExample":442},"Hold a ₹1 coin about 2.5 metres from your eye. It covers a full moon almost exactly. A ₹1 coin is about 2.2 cm across and the Moon is 3,475 km across — the coin just happens to be at the right distance to match.",{"id":444,"type":78,"tone":445,"items":446},"spec-400s","amber",[447,451,455,459,463,467,471,475],{"label":448,"big":449,"value":450},"Sun's diameter","1,392,700 km","Big enough to hold about 1.3 million Earths.",{"label":452,"big":453,"value":454},"Moon's diameter","3,475 km","Roughly the width of India from Kashmir to Kanyakumari, three times over.",{"label":456,"big":457,"value":458},"Size ratio","≈ 401×","1,392,700 ÷ 3,475 = 400.8. The Sun is about 400 times wider.",{"label":460,"big":461,"value":462},"Distance to Sun","149.6 M km","Light takes 8 minutes 20 seconds to cross it.",{"label":464,"big":465,"value":466},"Distance to Moon","384,400 km","Light takes about 1.3 seconds. Apollo took three days.",{"label":468,"big":469,"value":470},"Distance ratio","≈ 389×","149,597,871 ÷ 384,400 = 389.2. Also about 400.",{"label":472,"big":473,"value":474},"Sun looks","0.53°","32.0 arcminutes across, on average.",{"label":476,"big":477,"value":478},"Moon looks","0.52°","31.1 arcminutes on average — very slightly smaller, which matters a lot.",{"id":480,"type":47,"variant":481,"title":482,"markdown":483},"annular-intro","example","When the Moon is too far away: the ring of fire","The Moon's path around Earth is not a perfect circle. At its closest (**perigee**) it is about 363,300 km away; at its furthest (**apogee**) about 405,500 km. That is a difference of over 42,000 km, and it changes how big the Moon looks: from **32.9** arcminutes down to **29.5** arcminutes.\n\nThe Sun, meanwhile, ranges only from 32.5 down to 31.5 arcminutes.\n\nSo when an eclipse happens with the Moon near apogee, the Moon is simply **too small to cover the Sun**. A brilliant ring of sunlight stays visible all the way round the black disc. This is an **annular eclipse** — from the Latin *annulus*, a ring — and people call it the \"ring of fire\".\n\nIt is beautiful. But it is **not** dark, it has **no** corona, and — this matters — **it is never safe to look at without a filter**, not even for a moment, because a blazing ring of the Sun's surface is still there the whole time.",{"id":485,"type":486,"title":487,"prompt":488,"options":489},"explorer-three-solar","explorer","Three kinds of solar eclipse","Pick one to see what makes it happen and what you would see.",[490,503,516],{"id":491,"label":492,"chain":493,"badge":499,"note":502},"total","Total",[494,495,496,497,498],"Moon near perigee","Looks bigger than the Sun","Umbra reaches the ground","Disc fully covered","Corona appears",{"text":500,"tone":501},"Filters off ONLY during totality","yes","The Moon is close enough to look larger than the Sun, so its full-dark umbra cone reaches all the way to Earth's surface and lands as a spot around 160 km across. Inside that spot the Sun's disc is completely hidden, the sky darkens to deep twilight, bright planets and stars come out, and the pearly **corona** blazes around the black disc. The air cools, often by about 5 °C, and birds stop singing. Totality lasts from a few seconds up to a theoretical maximum of 7 minutes 32 seconds. This is the only moment in a solar eclipse when it is safe to look with bare eyes — and filters must go back on the instant any bright edge reappears.",{"id":504,"label":505,"chain":506,"badge":512,"note":515},"annular","Annular",[507,508,509,510,511],"Moon near apogee","Looks smaller than the Sun","Umbra falls short","Ring of Sun left over","No corona",{"text":513,"tone":514},"Filters on the WHOLE time","no","The Moon is near the far end of its orbit, so its disc looks only about 29.5 arcminutes across against a Sun of up to 32.5. It cannot cover the Sun. The umbra cone runs out before it reaches the ground, and the antumbra — the region beyond the cone's tip — lands instead. From there you see a black disc with a dazzling **ring of fire** around it, leaving about 18% of the Sun's face still shining. The sky barely darkens. There is no corona, no stars, and no safe moment: keep certified filters on from beginning to end.",{"id":517,"label":518,"chain":519,"badge":525,"note":526},"partial","Partial",[520,521,522,523,524],"Only the penumbra lands","Off to one side","A bite out of the Sun","Sky stays bright","Very common",{"text":513,"tone":514},"Most people who have seen an eclipse have seen this one. You are inside the Moon's wide grey **penumbra**, so part of the Sun's disc is hidden but part is not. You see a crescent Sun, best of all as hundreds of crescents projected through the gaps between leaves. Unless more than about 90% is covered, daylight looks almost normal, because our eyes adjust so well. Every total and annular eclipse is also a partial eclipse for the huge region around the central path, which is why partial eclipses are far more common. Filters or projection, the whole time, no exceptions.",{"id":528,"type":59,"title":529,"eyebrow":530,"navLabel":531},"ch7","What totality actually looks like","Chapter 07","7 Totality",{"id":533,"type":43,"markdown":534},"totality-description","People who have stood in the umbra almost all describe it the same way: not as a picture they saw, but as something that happened to them.\n\nHere is what to expect, in the last minute before and the first minute of totality.\n\n**The light goes wrong.** In the last ten minutes the daylight turns the colour of old steel. Colours drain out of clothes and walls. Your own shadow gets strangely sharp on one side, because the Sun has stopped being a disc and become a thin line.\n\n**Shadow bands.** Sometimes, in the last minute, faint rippling stripes of light and dark run across pale ground or a white sheet, like sunlight on the bottom of a swimming pool. They are caused by our own turbulent air.\n\n**Baily's beads.** As the last sliver closes, the Moon's edge is not smooth — it has mountains and valleys. Sunlight shines through the valleys and is blocked by the peaks, so the sliver breaks into a line of glittering dots. These are **Baily's beads**.\n\n**The diamond ring.** The beads wink out one by one until a single one is left: a blazing white point on a dark ring of corona. It lasts a second or two and it is the picture everyone has seen.\n\n**Totality.** The point vanishes. In its place: a black hole in the sky, ringed by the silver-white **corona**, streaming out in feathery petals. Often a few red flecks, called prominences, glow at the rim — those are fountains of gas bigger than Earth. Venus and Jupiter appear. The horizon glows orange in **every direction at once**, because you are standing in a shadow only 160 km wide and it is still sunny everywhere beyond it. That 360° sunset is the detail that people say they were not ready for.",{"id":536,"type":78,"tone":537,"items":538},"spec-totality","copper",[539,543,546,550,554,558],{"label":540,"big":541,"value":542},"Longest possible","7 min 32 s","The theoretical maximum. Most total eclipses give 2 to 3 minutes.",{"label":544,"big":95,"value":545},"Path width","Typical. The widest possible track is about 267 km.",{"label":547,"big":548,"value":549},"Shadow speed","≈ 2,000 km\u002Fh","At best, near the equator. The Moon's shadow moves at 3,679 km\u002Fh while Earth's surface runs after it at up to 1,674 km\u002Fh.",{"label":551,"big":552,"value":553},"Temperature drop","≈ 5 °C","Commonly reported. Drops of 10–15 °C have been measured in deserts.",{"label":555,"big":556,"value":557},"Corona heat","1–3 M °C","The corona is hundreds of times hotter than the Sun's visible surface at about 5,500 °C. Nobody fully knows why.",{"label":559,"big":560,"value":561},"Corona brightness","≈ full Moon","About a millionth of the Sun's disc — which is exactly why you can only see it when the disc is hidden.",{"id":563,"type":47,"variant":184,"title":564,"markdown":565},"safety-totality","The one exception — and how to get it right","There is exactly one moment in a solar eclipse when you may look with bare eyes: **during totality itself**, when the Sun's bright disc is *completely* covered and the corona is out.\n\nAnd here are the conditions attached to it:\n\n- It applies **only** to a **total** eclipse — never to an annular one, and never to a partial one. In those two, some of the Sun's blazing surface is showing the entire time.\n- It applies only **inside the narrow path of totality**. A few kilometres outside it, there is no safe moment at all.\n- Filters must go back on **the instant** the first bright bead appears at the edge. Do not wait to be sure. The return of sunlight after totality is sudden and your eyes are fully dark-adapted, which makes it far more dangerous than usual.\n- If you are in any doubt at all about whether you are in the path, or about whether totality has really begun, **keep your filters on**. You will still see something wonderful.\n\nFor anyone under about 14, the sensible rule is simpler: **watch through a projection, and let an adult who has planned the trip tell you when, if ever, to take the filters off.**",{"id":567,"type":103,"component":568,"componentVersion":5,"config":569,"objective":593,"textAlternative":594},"lab-match-eclipse","match-pairs",{"prompt":570,"mode":571,"pairs":572},"Match each eclipse word to what it means.","connect",[573,575,577,579,581,584,587,590],{"a":82,"b":574},"The fully dark middle of a shadow",{"a":86,"b":576},"The grey fringe, where part of the Sun still shows",{"a":172,"b":578},"The minutes when the Sun's disc is completely hidden",{"a":505,"b":580},"A ring of Sun left showing all the way round",{"a":582,"b":583},"Corona","The Sun's faint pearly outer atmosphere",{"a":585,"b":586},"Baily's beads","Sunlight glittering through valleys on the Moon's edge",{"a":588,"b":589},"Node","One of the two places where the Moon's tilted path crosses ours",{"a":591,"b":592},"New moon","The only phase at which a solar eclipse can happen","Connect eight eclipse words to their meanings until every pair is joined.","A matching game with eight words on the left and eight meanings on the right; you draw a line between each pair.\n\nUmbra — the fully dark middle of a shadow.\nPenumbra — the grey fringe, where part of the Sun still shows.\nTotality — the minutes when the Sun's disc is completely hidden.\nAnnular — a ring of Sun left showing all the way round.\nCorona — the Sun's faint pearly outer atmosphere.\nBaily's beads — sunlight glittering through valleys on the Moon's edge.\nNode — one of the two places where the Moon's tilted path crosses ours.\nNew moon — the only phase at which a solar eclipse can happen.\n\nEach correct join is counted, and the game reports how many moves you took.",{"id":596,"type":59,"title":597,"eyebrow":598,"navLabel":599},"ch8","What people thought, and what they found out","Chapter 08","8 Rahu and Aryabhata",{"id":601,"type":43,"markdown":602},"beliefs","For most of human history, an eclipse was terrifying. The Sun went out in the middle of the day for no reason anyone could name, and then came back. Every culture on Earth built a story around it: a dragon in China, a jaguar in the Amazon, a wolf in Norse tales.\n\nIn India, the story is about **Rahu**. In the old account, a demon sneaked a sip of the nectar of immortality; the Sun and Moon reported him; Vishnu beheaded him with a discus — but too late, because he had already drunk, so his severed head lived on as Rahu and his body as Ketu. Rahu chases the Sun and the Moon forever in revenge, and when he catches one, he swallows it. Since he has no body, it soon falls out again.\n\nThat is a good story, and it belongs to a lot of families. Many people in India today still fast during an eclipse, avoid cooking and eating, take a bath afterwards, and ask pregnant women to stay indoors.\n\nIf that happens in your family, the right response is not mockery. It is curiosity — and then evidence.",{"id":604,"type":47,"variant":605,"title":606,"markdown":607},"evidence-superstition","nuance","What the evidence actually shows","**An eclipse is the same shadow that falls on you every sunny day.** The light that reaches you during an eclipse is ordinary sunlight, just less of it. No new radiation is produced, and nothing extra reaches the ground.\n\nSo what would we expect if eclipses harmed food or unborn babies? Cooked food would spoil in a measurable way; hospitals in the eclipse path would see a rise in birth problems on eclipse days. Doctors and public-health researchers have looked. They find nothing of the kind: babies born during eclipses are no different from babies born the day before or the day after, and food is no more dangerous than it is at dusk.\n\n**One real risk does exist, and it is the one the customs do not mention: looking at the Sun.** Eye clinics genuinely do see a spike in retinal burns after every solar eclipse. The traditional advice to stay indoors accidentally protects people from that — but so does a sheet of card with a pinhole in it, and that way you get to see the eclipse.\n\nYou can hold both things at once: enjoy the story of Rahu, and also know what is really happening. Millions of people in India have done exactly that for fifteen centuries.",{"id":609,"type":610,"title":611,"items":612},"timeline-india","timeline","Eclipses explained: fifteen centuries of Indian and world astronomy",[613,617,621,625,629,633],{"time":614,"title":615,"text":616},"~600 BCE","Babylon spots the pattern","Babylonian scribes keep eclipse records for centuries and notice that similar eclipses repeat after about 18 years — the cycle now called the **Saros**.",{"time":618,"title":619,"text":620},"499 CE","Aryabhata, aged 23","In the *Aryabhatiya*, Aryabhata states plainly that a lunar eclipse is the Moon entering **Earth's shadow**, and a solar eclipse is the **Moon's shadow** falling on Earth. He gives methods for computing them.",{"time":622,"title":623,"text":624},"628 CE","Brahmagupta calculates","Brahmagupta's *Brahmasphutasiddhanta* computes eclipses in great detail — yet still argues for the traditional Rahu account on scriptural grounds. His later *Khandakhadyaka* (665 CE) quietly drops Rahu and just uses the nodes.",{"time":626,"title":627,"text":628},"1868","Helium found in India","During the total eclipse of 18 August 1868, Jules Janssen observing from **Guntur** sees a yellow line in the Sun's atmosphere matching no known element. Norman Pogson, at Masulipatam, reports it is not sodium. It was **helium**.",{"time":630,"title":631,"text":632},"1919","Einstein put to the test","Eddington at Príncipe and Crommelin at Sobral in Brazil photograph stars near the eclipsed Sun, and find their light bent by the Sun's gravity — evidence for Einstein's general relativity.",{"time":634,"title":635,"text":636},"Today","Predicted to the second","Eclipse paths are computed years ahead and printed on maps to within a few hundred metres and a few seconds.",{"id":638,"type":47,"variant":481,"title":639,"markdown":640},"aryabhata-note","One sentence, fifteen centuries ago","Aryabhata was born in **476 CE** and wrote the *Aryabhatiya* in **499 CE**, when he was 23. That is **1,527 years** before today.\n\nIn it he says that the Moon and the planets shine by reflected sunlight, that the Moon's darkness is its own shadowed half, and that eclipses are caused by shadows falling — the Moon passing into Earth's shadow, and the Moon's shadow passing over Earth.\n\nHe did not have a telescope. He did not have algebra as we write it. He had careful records, geometry, and the nerve to say that the thing everyone called a demon was in fact a shadow, and could be calculated.",{"id":642,"type":59,"title":643,"eyebrow":644,"navLabel":645},"ch9","Go and watch one","Chapter 09","9 Watch one",{"id":647,"type":43,"markdown":648},"go-watch","You do not need equipment, a trip or money to see an eclipse. You need to know when, and you need a piece of card.\n\n**Find out when.** Eclipse dates and paths are published years in advance and are free to look up. Search for the year and \"eclipse visible from India\", or use NASA's eclipse pages or timeanddate.com. Note two things for your town: the **date and local time**, and the **maximum percentage** of the Sun that will be covered where you are.\n\nThree that are already on the calendar: **31 December 2028**, a total lunar eclipse fully visible across India; **21 May 2031**, an annular \"ring of fire\" whose path crosses Kerala, northern Sri Lanka and the Andaman and Nicobar Islands; and **20 March 2034**, a total solar eclipse whose path of totality crosses northern India, including Kashmir. If you are nine today, you will be about seventeen in 2034.\n\n**For a lunar eclipse.** Nothing to prepare at all. Find a spot with a clear view of the sky in the right direction, go out about half an hour before it starts, and let your eyes adjust for ten minutes. Look for the curved edge of Earth's shadow creeping across, then the colour change. Binoculars make it better. A phone camera on night mode will usually capture the red.\n\n**For a solar eclipse.** Prepare the day before: make a pinhole card and a colander projection, and borrow or buy **ISO 12312-2** eclipse glasses from a planetarium or science centre. Check the glasses for scratches and pinholes before use, and throw them away if damaged. Then set up in an open spot with your back to the Sun, and watch the crescents appear on the paper.\n\nTake other people with you. Half the fun of an eclipse is the moment someone who was humouring you suddenly goes quiet.",{"id":650,"type":223,"caption":651,"columns":652,"rows":655},"table-how-often","How often, and how rare",[653,654],"Question","Answer",[656,659,662,665],[657,658],"Eclipses of any kind in a year","4 to 7",[660,661],"Solar eclipses in a year","2 to 5",[663,664],"Total solar eclipses somewhere on Earth, per century","about 68",[666,667],"Wait for totality to revisit one exact spot","about 375 years, on average",{"id":669,"type":670,"itemId":671,"prompt":672,"check":673,"hints":677,"feedback":680},"practice-shadow-speed","practice","eclipses.discover-p-shadow-speed","A total solar eclipse's shadow crosses the ground at about 2,000 km per hour. About how many minutes does it take to cross a 160 km wide path? Round to the nearest whole minute.",{"kind":674,"answer":675,"tolerance":5,"unit":676},"number",5,"min",[678,679],"Time = distance ÷ speed. Turn the speed into km per minute first: 2,000 ÷ 60 ≈ 33.3 km\u002Fmin.","160 ÷ 33.3 ≈ ?",{"correct":681,"incorrect":682},"Right: 160 ÷ 33.3 ≈ 4.8 minutes, which rounds to about 5 minutes.","2,000 km\u002Fh ≈ 33.3 km per minute. 160 ÷ 33.3 ≈ 4.8, which rounds to 5 minutes.",{"id":684,"type":47,"variant":184,"title":685,"markdown":686},"safety-checklist","The safety checklist, one more time","Stick this on the fridge before any solar eclipse.\n\n- **Never** look at the Sun with bare eyes, at any stage of a partial or annular eclipse, however thin the crescent, however brief the glance.\n- **Only** ISO 12312-2 filters. Check them for scratches and holes first.\n- **Not** sunglasses, not stacked sunglasses, not smoked or candle-blackened glass, not exposed film or X-ray film, not CDs, not coloured plastic, not water reflections.\n- **Never** through a camera, phone, binoculars or telescope — even with eclipse glasses on.\n- **Projection is always safe**: pinhole card, colander, tree shadows, or a covered mirror bouncing the image onto a wall.\n- Retinal burns are **painless** and can be permanent. You will not feel a warning.\n- Bare eyes are allowed **only** during totality of a **total** eclipse, inside the path, and filters go straight back on at the first bright bead.\n- **Lunar** eclipses need none of this. Look all you like.",{"id":688,"type":689,"title":690,"terms":691},"glossary-discover","glossary","Eclipse words",[692,695,698,701,703,705,708,712,716,719,722,725,729,732,735],{"term":71,"meaning":693,"example":694},"One body moving into another's shadow, or blocking our view of it.","A solar eclipse and a lunar eclipse are the only two we see well from Earth.",{"term":82,"meaning":696,"example":697},"The fully dark middle of a shadow, where the light source is completely hidden.","Stand in the Moon's umbra and you see totality.",{"term":86,"meaning":699,"example":700},"The grey outer part of a shadow, where part of the light source is still visible.","Stand in the Moon's penumbra and you see a partial eclipse.",{"term":283,"meaning":702},"The Moon's shadow falling on Earth, so the Sun is hidden. Always at new moon.",{"term":284,"meaning":704},"Earth's shadow falling on the Moon, so the Moon darkens and reddens. Always at full moon.",{"term":172,"meaning":706,"example":707},"The part of a total eclipse when the disc is completely covered.","Up to 7 min 32 s for the Sun; up to about 1 h 40 min for the Moon.",{"term":709,"meaning":710,"example":711},"Annular eclipse","A solar eclipse where the Moon looks too small, leaving a bright ring of Sun.","Also called a ring of fire. Never safe to view unfiltered.",{"term":713,"meaning":714,"example":715},"Partial eclipse","Only part of the Sun or Moon is covered.","A crescent Sun projected onto paper is the safe way to see one.",{"term":582,"meaning":717,"example":718},"The Sun's faint, pearly outer atmosphere, visible only during totality.","About as bright as a full moon, and a million times fainter than the Sun's disc.",{"term":588,"meaning":720,"example":721},"One of the two places where the Moon's tilted orbit crosses Earth's orbital plane.","Eclipses can only happen near a node. Indian astronomy calls them Rahu and Ketu.",{"term":591,"meaning":723,"example":724},"The phase when the Moon is between Earth and the Sun and its lit side faces away.","The only phase at which a solar eclipse can happen.",{"term":726,"meaning":727,"example":728},"Full moon","The phase when Earth is between the Sun and the Moon and we see a full lit disc.","The only phase at which a lunar eclipse can happen.",{"term":730,"meaning":731},"Eclipse season","A window of about a month, twice a year, when eclipses are possible.",{"term":585,"meaning":733,"example":734},"Points of sunlight shining through valleys on the Moon's ragged edge.","They appear in the last seconds before totality.",{"term":168,"meaning":736},"The last single bead of sunlight blazing on the ring of corona.",{"id":738,"type":739,"title":740,"questions":741},"quiz-discover","quiz","Check yourself",[742,753,764,777,790,803,816,829,842,855],{"itemId":743,"prompt":744,"options":745,"correct":323,"why":752},"eclipses.discover-q-whose-shadow","In a **solar** eclipse, whose shadow falls on what?",[746,747,748,750],{"id":320,"label":355},{"id":323,"label":351},{"id":326,"label":749},"The Sun's shadow falls on Earth",{"id":329,"label":751},"The Moon's shadow falls on the Sun","The Moon gets between us and the Sun and drops its shadow on us. An eclipse is named after the thing that goes dark for you: in a solar eclipse, the Sun goes dark.",{"itemId":754,"prompt":755,"options":756,"correct":326,"why":763},"eclipses.discover-q-phase-solar","At which Moon phase must a solar eclipse happen?",[757,758,760,761],{"id":320,"label":726},{"id":323,"label":759},"First quarter",{"id":326,"label":591},{"id":329,"label":762},"Any phase","The Moon must be between Earth and the Sun, which is exactly the definition of new moon.",{"itemId":765,"prompt":766,"options":767,"correct":326,"why":776},"eclipses.discover-q-red","Why does the totally eclipsed Moon look copper-red instead of black?",[768,770,772,774],{"id":320,"label":769},"The Moon is made of reddish rock",{"id":323,"label":771},"Mars is reflecting light onto it",{"id":326,"label":773},"Sunlight bent through Earth's atmosphere reaches it, and that light is red",{"id":329,"label":775},"It is glowing with its own heat","Earth's air bends sunlight into the shadow and scatters the blue out of it. The red light that lands on the Moon is every sunrise and sunset on Earth at once.",{"itemId":778,"prompt":779,"options":780,"correct":323,"why":789},"eclipses.discover-q-not-monthly","Why isn't there a solar eclipse at every new moon?",[781,783,785,787],{"id":320,"label":782},"The Moon is sometimes on the wrong side of the Sun",{"id":323,"label":784},"The Moon's orbit is tilted about 5.1°, so its shadow usually misses Earth",{"id":326,"label":786},"Clouds usually get in the way",{"id":329,"label":788},"The Moon is too small to cast a shadow","A 5.1° tilt is nearly ten times the Sun's own apparent width, so most months the shadow sails past Earth by tens of thousands of kilometres.",{"itemId":791,"prompt":792,"options":793,"correct":326,"why":802},"eclipses.discover-q-who-sees","Roughly how much of Earth can watch a given **total lunar** eclipse?",[794,796,798,800],{"id":320,"label":795},"A strip about 160 km wide",{"id":323,"label":797},"Only India",{"id":326,"label":799},"About half the planet — the whole night side",{"id":329,"label":801},"Everywhere at once","The Moon itself is what changes, so anyone who can see the Moon sees the eclipse. A 160 km strip is the solar case.",{"itemId":804,"prompt":805,"options":806,"correct":326,"why":815},"eclipses.discover-q-safe-sunglasses","Your friend has very dark sunglasses and says two pairs together will be fine for watching a partial solar eclipse. What should you say?",[807,809,811,813],{"id":320,"label":808},"Two pairs is fine, one is not",{"id":323,"label":810},"Fine as long as it is less than a minute",{"id":326,"label":812},"No — sunglasses are thousands of times too bright; use ISO 12312-2 filters or projection",{"id":329,"label":814},"Fine if the Sun is low in the sky","Sunglasses pass around 10% of the light. Certified eclipse filters pass about 0.003%. That is over three thousand times the difference, and no amount of stacking fixes it.",{"itemId":817,"prompt":818,"options":819,"correct":320,"why":828},"eclipses.discover-q-annular","What makes an eclipse **annular** rather than total?",[820,822,824,826],{"id":320,"label":821},"The Moon is near the far end of its orbit and looks too small to cover the Sun",{"id":323,"label":823},"Thin cloud lets a ring through",{"id":326,"label":825},"It happens at full moon",{"id":329,"label":827},"The Sun is unusually bright that day","Near apogee the Moon looks about 29.5 arcminutes wide against a Sun of up to 32.5, so a bright ring is left showing. The umbra cone falls short of the ground.",{"itemId":830,"prompt":831,"options":832,"correct":323,"why":841},"eclipses.discover-q-nodes","What are the **nodes** of the Moon's orbit?",[833,835,837,839],{"id":320,"label":834},"The Moon's nearest and furthest points from Earth",{"id":323,"label":836},"The two places where the Moon's tilted path crosses Earth's orbital plane",{"id":326,"label":838},"The two poles of the Moon",{"id":329,"label":840},"Sunrise and sunset on the Moon","A tilted circle cuts a flat one in exactly two places. Eclipses can only happen when the Moon is near one of them. Indian astronomy names these two points Rahu and Ketu.",{"itemId":843,"prompt":844,"options":845,"correct":323,"why":854},"eclipses.discover-q-corona","Why can we only see the Sun's corona during totality?",[846,848,850,852],{"id":320,"label":847},"It only exists during an eclipse",{"id":323,"label":849},"It is only about a millionth as bright as the Sun's disc, so the disc normally drowns it out",{"id":326,"label":851},"It is hidden behind the Moon the rest of the time",{"id":329,"label":853},"It is too far away to see normally","The corona is always there — about as bright as a full moon. It is simply swamped by the disc until the Moon covers it.",{"itemId":856,"prompt":857,"options":858,"correct":326,"why":867},"eclipses.discover-q-tell-apart","A picture shows a black disc with a pearly white halo, taken at 11 a.m. What is it?",[859,861,863,865],{"id":320,"label":860},"A total lunar eclipse",{"id":323,"label":862},"A penumbral lunar eclipse",{"id":326,"label":864},"A total solar eclipse",{"id":329,"label":866},"A moonrise","Daytime plus a corona means the Sun. A lunar eclipse happens at night and turns the Moon red, not black-with-a-halo.",{"id":869,"type":870,"prompt":871},"reflect-discover","reflection","A neighbour tells you that during the next solar eclipse nobody in the house should cook or eat, and that the children must stay inside.\n\nWrite what you would actually say — in a way that is respectful to them and still honest about the evidence. What part of the advice would you agree with, and why? What would you offer to show them instead?",{"id":873,"type":874,"title":875,"points":876},"cheat-discover","summary","Cheat sheet",[877,878,879,880,881,882,883,884,885,886,887],"**An eclipse is a shadow.** Light travels in straight lines, so Earth and the Moon each trail a cone of darkness. When one lands on the other and somebody is standing there, that is an eclipse.","**Umbra** = fully dark middle (total eclipse). **Penumbra** = grey fringe where part of the source still shows (partial eclipse).","**Solar eclipse** = the Moon's shadow on Earth, always at **new moon**, seen from a strip only about 160 km wide, lasting at most 7 min 32 s.","**Lunar eclipse** = Earth's shadow on the Moon, always at **full moon**, seen by about half the planet at once, lasting up to about 1 h 40 min.","**The eclipsed Moon is red** because sunlight bent through Earth's atmosphere loses its blue: you are seeing every sunrise and sunset on Earth at once.","**No eclipse every month** because the Moon's orbit is tilted about **5.1°** — nearly ten times the Sun's own apparent width. Eclipses happen only near the two **nodes**.","**The 400 coincidence:** the Sun is about 400× wider than the Moon and about 400× further away, so both look about **half a degree** across. That is why totality exists at all.","**Annular** = the Moon near apogee looks too small, leaving a ring of fire. No corona, no darkness, and **never** safe to view unfiltered.","**SAFETY: never look at the Sun without ISO 12312-2 filters** — not sunglasses, not smoked glass, not film, not a CD, and never through a camera, binoculars or telescope. Projection with a pinhole, a colander or tree shadows is always safe.","**Lunar eclipses are completely safe** to watch with bare eyes, binoculars or a camera.","**Aryabhata (499 CE)** wrote that eclipses are shadows, not a demon — 1,527 years ago. The nodes are still called **Rahu** and **Ketu**.",{"id":889,"type":890,"sourceIds":891},"sources-discover","sources",[892,893,894,895,896,897,898,899],"eclipses-nasa-eclipses","eclipses-nasa-safety","eclipses-timeanddate","eclipses-wiki-solar","eclipses-wiki-lunar","eclipses-britannica-kids","eclipses-mactutor-aryabhata","eclipses-ncert-curiosity",[892,893,894,895,896,897,898,899],"needs_review",{"generatedBy":903,"notes":904},"claude-code","Draft generated locally; pending owner review. Every number computed in scratchpad\u002Feclipses\u002Fnumbers.py.","2584434d52867a7caa5bd1d91a823544d6b1f71bd81e41bada4fdd05bd3822e1",{"component:shadow-lab@1":907,"component:eclipse-lab@1":908,"component:sort-game@1":909,"component:match-pairs@1":910,"logic:practice":911,"source:eclipses-britannica-kids":912,"source:eclipses-mactutor-aryabhata":913,"source:eclipses-nasa-eclipses":914,"source:eclipses-nasa-safety":915,"source:eclipses-ncert-curiosity":916,"source:eclipses-timeanddate":917,"source:eclipses-wiki-lunar":918,"source:eclipses-wiki-solar":919},"7476ef546fdb1f398a07393483e549c923f9568dc1bb1c2561191bf47862c475","284cb1682e3994af706a6cf907dd7747c9c141577245765bceff3a44fc81cf10","b164f45a2c8ca08f26c450768ff0231e113e9fe45381eddb34dc6d0548596c38","2a8ee4ac87460b4e1175a4bb13c96b03d577db06dde95670eb7fcfe4ad787899","3d6b0fe1b15255975a32b0fcd94e8019bc959ad45cbf12e136e86149549c6878","b10d074ebe9d86c884d3f9aab5d21f1e9ba85495f2d6add6a9263b96a9eb0a6a","48492b6b93e53c68ffa8073d6918178c069db9a731071b532c12890222bcca0f","c0bbcfd69fe06099e49c296bba2b105973ec1a794b69292fb65b4d81e9a2d672","f585b6bff2c52e5ed1c593b6222bc1e2805b7024b64f26d49271a5f19aa6dff7","c8e27588447f45548af86f4ac4ceb632dd451d78252b13c94d725b85ce34a24d","1508f2bc1fe7211250c3bc94beb9fff17e79cf0168bd0bd30264ee1e68d0086a","a72372b1c84fe7003a52eaf5654f6b88dabc645b405b6df7578cea1bae5d55e2","e32e7e8bfd7f8db32db243544cfc60cb96c0c7f050e964450ae3fc99bb21d344",{"state":921,"reviewer":922,"selfReview":204,"reviewedAt":923,"method":924},"approved","The library owner","2026-09-20T10:18:37.581Z","owner_bulk","preview-7e1cbbcc4f",1789899597708]