[{"data":1,"prerenderedAt":898},["ShallowReactive",2],{"layer:phases-of-the-moon:understand":3},{"layer":4,"contentHash":879,"dependencyHashes":880,"approval":892,"releaseId":897},{"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":874,"reviewStatus":875,"authoring":876},1,"phases-of-the-moon","en","understand","Reading the Moon: one angle explains everything","Elongation, lit fraction, rise times, the terminator and why one face always faces us","Turn the phase picture into a tool. Learn to go from the Sun-Earth-Moon angle to the shape, the fraction lit and the rise and set times; find out why craters show best at quarter moon, what earthshine is, and why the Moon keeps one face towards Earth.",[13,14,15,16,17],"Define elongation and use it to work out the phase, the lit fraction and the rise and set times.","Explain why the lit fraction grows unevenly through the month.","Use the 15-degrees-per-hour rule to predict when a given phase is above the horizon.","Explain the terminator, earthshine and why a full moon looks flat in binoculars.","Explain synchronous rotation and distinguish the far side from the night side.",40,{"title":20,"rows":21},"Lesson plate",[22,25,28,31,34,37],{"label":23,"value":24},"Depth","Understand",{"label":26,"value":27},"Reading time","about 40 minutes",{"label":29,"value":30},"Prior knowledge","Discover: what a phase is",{"label":32,"value":33},"Chapters","9",{"label":35,"value":36},"Labs","Moon phase lab, rise-time sort, eclipse geometry",{"label":38,"value":39},"Maths used","Angles, ratios, a little cosine",[41,45,51,57,60,65,81,86,140,154,169,174,177,212,217,246,264,269,272,287,311,315,328,395,417,422,427,430,434,438,461,482,487,490,494,523,527,532,535,539,543,548,551,555,560,566,580,585,589,593,597,601,622,627,687,701,705,709,713,844,863],{"id":42,"type":43,"markdown":44},"intro-angle","prose","In Discover you met the idea: half the Moon is always lit, and the phase is how much of that lit half faces us.\n\nThat is the right picture, but it is not yet a *tool*. With it alone you cannot say what the Moon will look like on the 19th, or what time it will rise, or which craters will show up best in binoculars.\n\nThis layer turns the picture into a tool. It comes down to a single number — **the angle between the Sun and the Moon as seen from Earth** — and once you can read that angle, everything else follows: the shape, the name, the fraction lit, the rise time, the set time, and even which way up it looks.",{"id":46,"type":47,"variant":48,"title":49,"markdown":50},"intro-how-to-read","callout","try_it","How to use this lesson","Keep a pen and a circle handy. Several times you will be asked to shade a circle and then check it against the table.\n\nEverything here is testable. The times in the tables are predictions about your own sky; go out and check them. If a prediction fails badly, that is interesting — see the careful note in Chapter 5 about why your local times differ.",{"id":52,"type":53,"title":54,"eyebrow":55,"navLabel":56},"ch1","chapter","One angle runs everything: elongation","Chapter 01","1 Elongation",{"id":58,"type":43,"markdown":59},"ch1-elongation","Stand on Earth. Point one arm at the Sun. Point the other arm at the Moon. The angle between your arms is called the **elongation** of the Moon.\n\nThat single angle decides the phase completely.\n\n- Elongation **0 degrees**: Sun and Moon in the same direction. Its lit half points away from you. **New moon.**\n- Elongation **90 degrees**: a right angle. You are looking at the lit half exactly side-on, so you see half of it. **Quarter moon.**\n- Elongation **180 degrees**: Sun and Moon in opposite directions, Earth in the middle. The lit half is aimed straight at you. **Full moon.**\n\nBetween those, the phase changes smoothly. Elongation does not jump; it grows steadily, day after day, from 0 all the way round to 360 — and 360 is the same as 0, so the cycle begins again.",{"id":61,"type":47,"variant":62,"title":63,"markdown":64},"ch1-def-elongation","definition","Elongation","**Elongation** is the angle at Earth between the direction of the Sun and the direction of the Moon.\n\nIt runs from 0 degrees (new moon) up through 180 degrees (full moon) and on round to 360 degrees, which is new moon again.\n\nAstronomers often measure it eastward, so that it keeps increasing through the month. That is what we do here, because it makes the arithmetic clean: **elongation always grows, never shrinks.**",{"id":66,"type":67,"items":68},"ch1-formulas","formulas",[69,72,75,78],{"expression":70,"caption":71},"elongation = 360 x day ÷ 29.53","Degrees of elongation on a given day of the lunar month, taking the Moon's motion as steady.",{"expression":73,"caption":74},"lit fraction = (1 − cos e) ÷ 2","The fraction of the visible disc that is bright, from the elongation e. Exact for a spherical Moon.",{"expression":76,"caption":77},"elongation 90° → half lit","cos 90° = 0, so the fraction is (1 − 0) ÷ 2 = 0.5. A quarter moon looks like a half-disc.",{"expression":79,"caption":80},"elongation 180° → fully lit","cos 180° = −1, so the fraction is (1 + 1) ÷ 2 = 1. Full moon.",{"id":82,"type":47,"variant":83,"title":84,"markdown":85},"ch1-cos-note","nuance","If you have not met cosine yet","Do not worry about the middle formula. You can read every table in this lesson without it, and the shape of the answer is easy to describe in words:\n\n**The lit fraction does not grow steadily.** It creeps up slowly at first, races through the middle of the month, and then flattens out near full moon.\n\nThat is why the Moon seems to sit at \"nearly full\" for three or four nights in a row, and why a day-old crescent is so startlingly thin. Between day 0 and day 3.7 the Moon covers a quarter of its journey but gains only 15% of its brightness; between day 3.7 and day 7.4 it covers the next quarter and gains 35%.",{"id":87,"type":88,"caption":89,"columns":90,"rows":94},"ch1-fraction-table","table","Elongation, day of the month, and how much of the disc is lit",[63,91,92,93],"Day of the month","Lit fraction","What you see",[95,100,105,110,115,120,125,130,135],[96,97,98,99],"0°","0.00","0.0%","Nothing: new moon",[101,102,103,104],"30°","2.46","6.7%","A very thin crescent, hard to find",[106,107,108,109],"45°","3.69","14.6%","A clear crescent",[111,112,113,114],"60°","4.92","25.0%","A thick crescent",[116,117,118,119],"90°","7.38","50.0%","Exactly half: first quarter",[121,122,123,124],"120°","9.84","75.0%","Gibbous, clearly more than half",[126,127,128,129],"135°","11.07","85.4%","A fat gibbous",[131,132,133,134],"150°","12.30","93.3%","Almost full; a dark rim on one side",[136,137,138,139],"180°","14.77","100.0%","A complete bright disc: full moon",{"id":141,"type":142,"title":143,"problem":144,"steps":145,"help":152},"we-lit-fraction","worked_example","How much of the Moon is lit 5 days after new moon?","The Moon is 5.0 days past new moon. Work out its elongation and the fraction of the disc that is lit, and name the phase.",[146,147,148,149,150,151],"One full cycle of 29.53 days corresponds to 360° of elongation.","So elongation = 360 x 5.0 ÷ 29.53 = 61.0°.","That is between 0° and 90°, so the Moon is a **waxing crescent** — less than half lit, and growing.","Using lit fraction = (1 − cos e) ÷ 2 with e = 61.0°: cos 61.0° = 0.486.","Lit fraction = (1 − 0.486) ÷ 2 = 0.257, which is about **26%**.","Check it makes sense: day 5 is two days short of first quarter (day 7.38), and 26% is a bit under 50%. ✓",{"simplerExplanation":153},"Five days out of 29.5 is about one sixth of the month, so the Moon has swung about 61° away from the Sun. At that angle a bit over a quarter of the disc is bright: a healthy crescent, not yet a half.",{"id":155,"type":156,"itemId":157,"prompt":158,"check":159,"hints":163,"feedback":166},"practice-elongation","practice","phases-of-the-moon.understand-elongation-day10","The Moon is 10.0 days past new moon. What is its elongation, in degrees? Use elongation = 360 x day ÷ 29.53 and round to the nearest whole degree.",{"kind":160,"answer":161,"tolerance":5,"unit":162},"number",122,"degrees",[164,165],"Divide 10.0 by 29.53 first: that is the fraction of the month completed.","Then multiply by 360.",{"correct":167,"incorrect":168},"Right: 10.0 ÷ 29.53 = 0.3386 of the month, and 0.3386 x 360 = **121.9°**. Between 90° and 180°, so it is a waxing gibbous — about 76% lit.","Work out the fraction of the month first: 10.0 ÷ 29.53 = 0.3386. Multiply that by 360° to get 121.9°.",{"id":170,"type":53,"title":171,"eyebrow":172,"navLabel":173},"ch2","The eight phases, with numbers","Chapter 02","2 Eight, with numbers",{"id":175,"type":43,"markdown":176},"ch2-eight","Here is the whole month in one table. Every number in it was calculated from the elongation, not looked up.\n\nRead it across: the phase, where the Moon is in its orbit as an angle, how many days into the month that is, and how much of the disc is bright.",{"id":178,"type":88,"caption":179,"columns":180,"rows":185},"ch2-table-eight","The eight named phases, by the numbers",[181,63,182,183,184],"Phase","Day","Lit","Bright side (from India)",[186,189,192,194,196,199,204,208],[187,96,97,98,188],"New moon","—",[190,106,107,108,191],"Waxing crescent","Right",[193,116,117,118,191],"First quarter",[195,126,127,128,191],"Waxing gibbous",[197,136,137,138,198],"Full moon","All of it",[200,201,202,128,203],"Waning gibbous","225°","18.46","Left",[205,206,207,118,203],"Last quarter","270°","22.15",[209,210,211,108,203],"Waning crescent","315°","25.84",{"id":213,"type":47,"variant":214,"title":215,"markdown":216},"ch2-quarter-spacing","observation","The phases are evenly spaced in *angle*, not in *look*","Each named phase is exactly 45° of elongation from the next, and each one is 3.69 days after the last. The spacing is perfectly even.\n\nBut the *appearance* is not evenly spaced at all. From new moon to first quarter the lit fraction goes 0% → 14.6% → 50%. From first quarter to full it goes 50% → 85.4% → 100%.\n\nThe first and last quarters of the month bring dramatic changes; the middle two are gentler near full. This is why the Moon looks \"basically full\" for several nights around Purnima, and why a two-day-old crescent is so thin that many people have never knowingly seen one.",{"id":218,"type":219,"tone":220,"items":221},"ch2-key-numbers","spec","blue",[222,226,230,234,238,242],{"label":223,"big":224,"value":225},"Synodic month","29.531 d","New moon to new moon: 29 days, 12 hours, 44 minutes. The cycle of the phases.",{"label":227,"big":228,"value":229},"Sidereal month","27.322 d","One full orbit against the stars: 27 days, 7 hours, 43 minutes. Shorter — and Go deeper explains why.",{"label":231,"big":232,"value":233},"Quarter to quarter","7.38 d","A week and a fraction. Not exactly seven days, which is one reason the phases drift across our seven-day week.",{"label":235,"big":236,"value":237},"Daily elongation gain","12.19°","How much the Moon pulls ahead of the Sun each day. In one day the shape changes visibly.",{"label":239,"big":240,"value":241},"Daily moonrise delay","49 min","12.19° ÷ 15° per hour = 0.81 hours. Why the Moon is later every night.",{"label":243,"big":244,"value":245},"Moon's apparent size","0.52°","About half a degree: your little fingernail at arm's length covers it easily. The same as the Sun's apparent size, near enough.",{"id":247,"type":248,"component":249,"componentVersion":5,"config":250,"objective":257,"textAlternative":258,"help":259},"lab-moon-both","interactive","moon-phase",{"startDay":251,"views":252,"showNames":254,"showTithi":255,"quizRounds":256},7.4,[253],"both",true,false,8,"Match the Moon's position in its orbit to the shape you see, and test yourself on naming phases from either view.","Two linked panels. The **space view** looks down on Earth's orbit: the Sun far off to the left, Earth in the centre, the Moon on a circle around it. The sunlit half of Earth and of the Moon are shaded bright; the other halves are dark. The **sky view** shows the Moon as it would appear from India at that moment.\n\nIt opens at **day 7.4, first quarter**. In the space view the Moon is at a right angle: the Sun-Earth-Moon angle is 90°. In the sky view, exactly the right half is bright.\n\nDrag the Moon round and watch: at 0° the lit half points away and the sky view is black; at 45° a crescent about 15% lit; at 135° a gibbous about 85%; at 180° a full disc.\n\nTwo things are worth checking as you drag. First, **the Moon-ball is always exactly half bright in the space view** — that never changes. Second, the **terminator**, the line between the bright and dark parts, is always a half-circle seen at an angle, which is why it looks straight at the quarters and curved everywhere else.\n\nEight quiz rounds then ask you to name a phase from the sky view, or to place the Moon in the space view given a phase name.",{"simplerExplanation":260,"hints":261},"Move the Moon around Earth in the left picture; the right picture shows what you would see from the ground.",[262,263],"Watch the terminator as you drag. When is it a straight line?","Find the two places where half the disc is lit, and notice they are on opposite sides.",{"id":265,"type":53,"title":266,"eyebrow":267,"navLabel":268},"ch3","Where in the sky, and at what time","Chapter 03","3 Rise and set",{"id":270,"type":43,"markdown":271},"ch3-sky-position","Elongation tells you the shape. It also tells you **where the Moon is in the sky**, because elongation is literally the angle between the Sun and the Moon.\n\nThe sky turns 360° in 24 hours, which is **15° every hour**. So if the Moon is 15° east of the Sun, it crosses the sky exactly one hour behind the Sun. If it is 90° east, it is four hours behind. If it is 180° away, it is twelve hours behind — which is why a full moon rises as the Sun sets.\n\nThat gives a rule you can do in your head:\n\n**Hours behind the Sun = elongation ÷ 15.**\n\nA waxing crescent at 45° elongation is 3 hours behind the Sun: it rises about 3 hours after sunrise, is highest about 3 hours after noon, and sets about 3 hours after sunset. That is exactly why a young crescent is an early-evening object that disappears before your bedtime.",{"id":273,"type":67,"items":274},"ch3-formulas-time",[275,278,281,284],{"expression":276,"caption":277},"hours behind Sun = elongation ÷ 15","The sky turns 15° every hour, so every 15° of elongation is one hour of delay.",{"expression":279,"caption":280},"moonrise ≈ 6:00 + elongation ÷ 15","Rough local time of moonrise for an observer near the equator, where the Sun rises about 6 am.",{"expression":282,"caption":283},"moonset ≈ moonrise + 12 hours","The Moon is above the horizon for roughly half a day, like the Sun near the equator.",{"expression":285,"caption":286},"daily delay = 12.19° ÷ 15 ≈ 49 min","The Moon gains that much elongation each day, so it rises that much later.",{"id":288,"type":88,"caption":289,"columns":290,"rows":294},"ch3-rise-set-table","Rise, highest point and set for each phase (equatorial observer, 6 am sunrise)",[181,63,291,292,293],"Rises","Highest","Sets",[295,299,303,305,307,308,309,310],[187,96,296,297,298],"06:00","12:00","18:00",[190,106,300,301,302],"09:00","15:00","21:00",[193,116,297,298,304],"00:00",[195,126,301,302,306],"03:00",[197,136,298,304,296],[200,201,302,306,300],[205,206,304,296,297],[209,210,306,300,301],{"id":312,"type":47,"variant":48,"title":313,"markdown":314},"ch3-check-sky","Check the table against your own sky","This is a table you can falsify, which makes it a proper scientific claim.\n\nPick tonight's phase. Read off the predicted rise and set times. Now go outside at those times and look.\n\nTwo honest warnings, so a mismatch does not confuse you:\n\n- The times assume the Sun rises at 6 am and sets at 6 pm, which is true near the equator and roughly true in India. In Delhi in December the Sun sets before 5:30, so everything shifts.\n- The Moon's orbit is tilted and not a perfect circle, so real moonrise can be up to an hour or so from the simple prediction.\n\nWhat will **not** be off is the pattern: full moon at sunset, first quarter high in the south at sunset, last quarter high in the south at sunrise. If those fail, something is genuinely wrong with your identification of the phase.",{"id":316,"type":142,"title":317,"problem":318,"steps":319,"help":326},"we-rise-time","What time does a waxing gibbous rise?","A Moon is a waxing gibbous at an elongation of 135°. Roughly what time does it rise, and is it a good target for a 9 pm look from a terrace?",[320,321,322,323,324,325],"Hours behind the Sun = elongation ÷ 15 = 135 ÷ 15 = **9 hours**.","The Sun rises about 6 am, so the Moon rises about 6 am + 9 hours = **3 pm**.","The Sun is highest at noon, so the Moon is highest about noon + 9 hours = **9 pm**.","The Sun sets about 6 pm, so the Moon sets about 6 pm + 9 hours = **3 am**.","So at 9 pm the Moon is at its highest point in the sky — almost overhead, well clear of buildings and haze.","It is about 85% lit, bright enough to read by and with a good stretch of terminator for crater-hunting. **An excellent target.**",{"anotherExample":327},"Do the same for a waning crescent at 315°: 315 ÷ 15 = 21 hours behind the Sun, so it rises at 6 am + 21 h = 3 am and is highest at 9 am. A pre-dawn object, and a hard one.",{"id":329,"type":248,"component":330,"componentVersion":5,"config":331,"objective":393,"textAlternative":394},"lab-sort-risetimes","sort-game",{"prompt":332,"bins":333,"seconds":343,"items":344},"Sort each Moon sighting by the time of day when it is easiest to see.",[334,337,340],{"id":335,"label":336},"evening","Best in the evening",{"id":338,"label":339},"midnight","Best around midnight",{"id":341,"label":342},"morning","Best in the morning",0,[345,349,353,357,361,365,369,373,377,381,385,389],{"id":346,"label":347,"bin":335,"why":348},"s1","Waxing crescent (45° elongation)","Rises around 9 am, sets around 9 pm. Low in the west just after sunset, gone by mid-evening.",{"id":350,"label":351,"bin":335,"why":352},"s2","First quarter (90°)","Rises at noon, highest at 6 pm, sets at midnight. High in the south the moment the Sun goes down.",{"id":354,"label":355,"bin":335,"why":356},"s3","Waxing gibbous (135°)","Rises mid-afternoon and is highest around 9 pm — the best-placed bright Moon of the month for an early-evening look.",{"id":358,"label":359,"bin":338,"why":360},"s4","Full moon (180°)","Rises at sunset, is highest at midnight, sets at sunrise. Up all night, and only all night.",{"id":362,"label":363,"bin":338,"why":364},"s5","Waning gibbous (225°)","Rises about 9 pm and is highest around 3 am, so the late part of the night is its best time.",{"id":366,"label":367,"bin":341,"why":368},"s6","Last quarter (270°)","Rises at midnight, highest at 6 am, sets at noon. High in the south as you eat breakfast.",{"id":370,"label":371,"bin":341,"why":372},"s7","Waning crescent (315°)","Rises about 3 am, highest around 9 am. A pre-dawn sliver in the east.",{"id":374,"label":375,"bin":341,"why":376},"s8","The old crescent before Amavasya","The last thin crescent of the month rises only shortly before the Sun, so it is a difficult dawn object.",{"id":378,"label":379,"bin":335,"why":380},"s9","The new crescent that fixes Eid","A very young waxing crescent, visible for only twenty or thirty minutes in the western twilight after sunset.",{"id":382,"label":383,"bin":338,"why":384},"s10","A Moon that is highest at 3 am","Highest at 3 am means 15 hours behind the Sun, an elongation of 225°: a waning gibbous.",{"id":386,"label":387,"bin":338,"why":388},"s11","Karva Chauth's Moon","Krishna chaturthi: elongation about 222°, so it rises roughly three hours after sunset and climbs through the late evening.",{"id":390,"label":391,"bin":341,"why":392},"s12","A Moon high in the south at sunrise","Highest at 6 am means 18 hours behind the Sun, an elongation of 270°: last quarter.","Decide when each phase is best placed for observing, using its elongation and the 15-degrees-per-hour rule.","Twelve sightings are sorted into three bins: **best in the evening**, **best around midnight**, **best in the morning**.\n\nEvening: waxing crescent at 45°, first quarter at 90°, waxing gibbous at 135°, and the young Eid crescent — all of them Moons that are less than twelve hours behind the Sun.\n\nAround midnight: full moon at 180°, waning gibbous at 225°, a Moon highest at 3 am, and Karva Chauth's Moon at about 222°.\n\nMorning: last quarter at 270°, waning crescent at 315°, the old crescent before Amavasya, and a Moon high in the south at sunrise.\n\nThe rule underneath every card: **hours behind the Sun = elongation ÷ 15**. Small elongation means the Moon trails the Sun closely and is an evening object; large elongation means it trails by most of a day and is a morning object.",{"id":396,"type":156,"itemId":397,"prompt":398,"check":399,"hints":411,"feedback":414},"practice-rise-time","phases-of-the-moon.understand-rise-midnight","A Moon rises at about midnight. What phase is it?",{"kind":400,"options":401,"correct":410},"choice",[402,404,406,408],{"id":403,"label":187},"a",{"id":405,"label":193},"b",{"id":407,"label":197},"c",{"id":409,"label":205},"d",[409],[412,413],"Midnight is 18 hours after a 6 am sunrise.","Multiply 18 hours by 15° per hour to get the elongation.",{"correct":415,"incorrect":416},"Right: 18 hours behind the Sun means an elongation of 18 x 15 = 270°, which is **last quarter**. It will be highest in the south around sunrise and set about noon.","Count the hours from sunrise: midnight is 18 hours after 6 am. Each hour is 15° of elongation, so 18 x 15 = 270° — last quarter.",{"id":418,"type":47,"variant":419,"title":420,"markdown":421},"ch3-daytime-moon","aha","Why the daytime Moon is not a puzzle","Once you have the rule, the daytime Moon stops being mysterious and becomes obvious.\n\nThe Moon is above the horizon for about twelve hours out of every twenty-four, and those twelve hours slide later by about fifty minutes each day. So for roughly half of each month, part of the Moon's time above the horizon overlaps with daylight.\n\nThe two phases you will **never** see in a bright midday sky are the extremes: a new moon (too close to the Sun, lost in glare) and a full moon (exactly opposite the Sun, therefore below the horizon whenever the Sun is high).\n\nEverything else is fair game. The easiest catch is a **waxing gibbous in the late afternoon** or a **waning gibbous just after dawn**: bright, far from the Sun's glare, and high enough to clear the rooftops.",{"id":423,"type":53,"title":424,"eyebrow":425,"navLabel":426},"ch4","Which way up? Waxing and waning revisited","Chapter 04","4 Which way up",{"id":428,"type":43,"markdown":429},"ch4-hemisphere","In Discover you learned that from India a waxing Moon is bright on the right. Here is why — and why it is not true everywhere.\n\nThe Moon is always lit from the side the Sun is on. In the northern hemisphere, when you face the Moon in the evening, the Sun has just set to your **right-hand side** (to the west), so the bright edge faces right. Later in the month, when the Moon trails the Sun by more than half a day, you see it in the morning with the Sun rising on your left, and the bright edge faces left.\n\nThree consequences, all worth knowing:\n\n1. **In the southern hemisphere it is the other way round.** In Australia, a waxing crescent is bright on the *left*, and the D-O-C trick becomes C-O-D. Same Moon, same phase, upside-down view.\n2. **Near the equator the crescent lies on its back.** From Kerala or Tamil Nadu, a young crescent often looks like a smile or a boat rather than a letter D, because the Sun sets almost straight down and the lit edge ends up along the bottom.\n3. **The bright edge always points at the Sun.** This is the single reliable rule for every place on Earth. Draw a line from the middle of the dark side through the middle of the bright side, keep going, and you are pointing at the Sun — even when the Sun is below the horizon.",{"id":431,"type":47,"variant":48,"title":432,"markdown":433},"ch4-point-at-sun","Point at the buried Sun","Next time you see a crescent Moon in the evening, do this.\n\nImagine an arrow that starts at the dark edge of the Moon, passes through the middle of the disc, and comes out through the middle of the bright edge. Now stretch that arrow out past the Moon and down to the horizon.\n\nIt should land very close to where the Sun set. If you are looking in the morning, it lands where the Sun is about to rise.\n\nYou have just located a star you cannot see, by reading the shadows on a rock 384,400 km away. That is what astronomy is.",{"id":435,"type":47,"variant":83,"title":436,"markdown":437},"ch4-boat-moon","Why the crescent sometimes looks like a bowl","People in south India often describe the young Moon as a *boat* or a *bowl*, with both horns pointing up, while people further north describe a letter D tipped over. Both are correct descriptions of the same Moon.\n\nWhat changes is the angle at which the Sun's path meets your horizon. Near the equator that path is nearly vertical, so an object 45° east of the Sun sits almost straight *above* it soon after sunset, and the lit edge is the bottom. Further north, the path is slanted, and the crescent tips over.\n\nThis is also why the Eid crescent is easier to spot from some latitudes than others on the same evening.",{"id":439,"type":88,"caption":440,"columns":441,"rows":445},"ch4-hemisphere-table","The same phase from three places on the same night",[181,442,443,444],"From Delhi (28°N)","From Kochi (10°N)","From Melbourne (38°S)",[446,450,454,458],[190,447,448,449],"Bright edge on the right, tipped like a D","Horns pointing up: a bowl or boat","Bright edge on the left, like a C",[193,451,452,453],"Right half bright, straight edge vertical","Bright half low, terminator tipped","Left half bright",[197,455,456,457],"A full disc; the familiar 'rabbit' upright","A full disc, rabbit tilted","A full disc, rabbit upside down",[209,449,459,460],"Horns pointing up in the dawn sky","Bright edge on the right, like a D",{"id":462,"type":156,"itemId":463,"prompt":464,"check":465,"hints":476,"feedback":479},"practice-hemisphere","phases-of-the-moon.understand-hemisphere","Your cousin in Sydney sends a photo of tonight's Moon. It is a crescent, bright on the **left**, and she took it just after sunset. What phase is it?",{"kind":400,"options":466,"correct":475},[467,469,471,473],{"id":403,"label":468},"A waning crescent, because bright-on-the-left means waning",{"id":405,"label":470},"A waxing crescent, because in the southern hemisphere the waxing Moon is bright on the left",{"id":407,"label":472},"A first quarter moon",{"id":409,"label":474},"Impossible to tell from a photo",[405],[477,478],"The left\u002Fright rule is a northern-hemisphere rule; it flips south of the equator.","The time of day works everywhere: what kind of crescent can you see just after sunset?",{"correct":480,"incorrect":481},"Correct. In Sydney the whole sky appears flipped, so a waxing crescent is bright on the left. The time of day settles it: an evening crescent is **always** waxing, anywhere on Earth. It is the same Moon and the same phase you would see from India, viewed upside down.","Left and right depend on where you stand. Time of day does not: a crescent visible just after sunset must be close behind the Sun, which only happens just after new moon. It is waxing.",{"id":483,"type":53,"title":484,"eyebrow":485,"navLabel":486},"ch5","The terminator, and where the craters hide","Chapter 05","5 The terminator",{"id":488,"type":43,"markdown":489},"ch5-terminator","The curved line dividing the bright part of the Moon from the dark part has a name: the **terminator**. It is the Moon's sunrise-and-sunset line.\n\nAlong the terminator, the Sun is just on the horizon as seen from the lunar ground. That means shadows there are enormously long — the same way your own shadow stretches down the whole street at sunset. Every crater rim, every mountain, every ridge throws a black streak across the surface.\n\nMove away from the terminator towards the middle of the bright part and the Sun climbs higher in the lunar sky. Shadows shorten. At full moon the Sun is almost directly overhead across the whole visible face, and there are hardly any shadows at all.\n\nWhich leads to an unexpected rule for anyone with binoculars:\n\n**The full moon is the worst night of the month to look at the Moon.**\n\nIt is dazzlingly bright and almost completely flat, like a photograph taken with the flash on. The best views are around first quarter and last quarter, when the terminator runs straight down the middle of the disc and the craters along it stand out in three dimensions.",{"id":491,"type":47,"variant":62,"title":492,"markdown":493},"ch5-def-terminator","Terminator","The **terminator** is the boundary on a world between its lit side and its dark side — the line where the Sun is rising or setting.\n\nOn the Moon it sweeps slowly across the surface, taking about a fortnight to cross from one side to the other, because one lunar day lasts a whole synodic month.\n\nEarth has one too: the terminator is the curved sunrise line you can see in satellite pictures of our planet, and it moves at over 1,600 km\u002Fh at the equator.",{"id":495,"type":496,"title":497,"items":498},"ch5-crater-steps","steps","How to hunt craters with cheap binoculars",[499,503,507,511,515,519],{"title":500,"tag":501,"text":502},"Pick the right night","quarter moon","Look within two or three nights of first or last quarter. The terminator then runs down the middle of the disc, giving maximum shadow.",{"title":504,"tag":505,"text":506},"Steady the binoculars","elbows down","Rest your elbows on a wall or a windowsill, or lie back on a chair. Unsteady hands blur far more detail than cheap optics do.",{"title":508,"tag":509,"text":510},"Find the terminator","the ragged edge","Look for the boundary between bright and dark. At a quarter Moon it looks visibly rough and bitten, not smooth — those bites are crater rims catching the low Sun.",{"title":512,"tag":513,"text":514},"Look for black ovals","long shadows","Craters near the terminator show a bright rim on the sunward side and a long black shadow pooled inside. Watch the same crater on the next night and the shadow will have shrunk.",{"title":516,"tag":517,"text":518},"Find the dark seas","maria","The large smooth grey patches are *maria*, ancient lava plains. They have fewer craters because the lava flooded the old ones.",{"title":520,"tag":521,"text":522},"Come back at full moon","compare","Look at the same region at full moon and notice that the craters have almost vanished, while bright streaks called rays now dominate. Same rocks, different lighting.",{"id":524,"type":47,"variant":214,"title":525,"markdown":526},"ch5-flat-full-moon","Why the full moon looks flat","Photographers call it flat lighting: when the light comes from directly behind you, nothing casts a visible shadow, and a landscape loses all of its texture.\n\nAt full moon, the Sun is behind *you* (behind Earth, strictly), shining straight down onto the face you are looking at. From your viewpoint every shadow is hidden directly behind the thing that casts it.\n\nWhat you get instead is a map of how *reflective* different rocks are: dark grey maria, pale highlands, and the brilliant white rays splashed out from young craters such as Tycho. Beautiful, but two-dimensional.",{"id":528,"type":53,"title":529,"eyebrow":530,"navLabel":531},"ch6","Earthshine: the old Moon in the new Moon's arms","Chapter 06","6 Earthshine",{"id":533,"type":43,"markdown":534},"ch6-earthshine","Find a crescent Moon three or four days after new moon, on a clear evening, once the sky has properly darkened. Look at the part that should be black.\n\nIt is not black. There is a faint, ghostly grey glow filling the whole unlit disc, so that you can see the Moon's complete round outline with a bright crescent stuck on one edge. Sailors and poets called it **the old Moon in the new Moon's arms**.\n\nThe explanation is lovely, and it is the same idea as the whole lesson, run one step further.\n\nYou are looking at the Moon's **night side**. There is no sunlight there. But there *is* light: light from **Earth**. At that moment, an astronaut standing on that dark lunar ground would look up and see a huge, brilliant, nearly **full Earth** in the sky — four times wider than the Moon looks to us and dozens of times brighter, because Earth's clouds and oceans reflect far more sunlight than grey Moon rock.\n\nThat Earthlight falls on the lunar night side, bounces off, and comes back to your eye. It has made a round trip: **Sun → Earth → Moon → you**.",{"id":536,"type":47,"variant":419,"title":537,"markdown":538},"ch6-earthshine-timing","Why earthshine only shows near a crescent","Earth and Moon always show opposite phases to each other. When we see a thin crescent Moon, an observer on the Moon sees a nearly full Earth — maximum Earthlight.\n\nWhen we see a full moon, the Moon sees a *new* Earth: a dark disc with a thin bright rim. Almost no Earthlight at all.\n\nSo earthshine is strongest exactly when the Moon is a thin crescent, which is also when the glare of the sunlit part is smallest. The two effects work together, which is why a two- to four-day-old Moon is the perfect time to look.\n\nLeonardo da Vinci worked this out around 1510, centuries before anyone could check it from space.",{"id":540,"type":47,"variant":48,"title":541,"markdown":542},"ch6-earthshine-try","Hunt earthshine this month","**When:** two to five days after new moon, in the western sky about 45 to 75 minutes after sunset. Or two to five days before new moon, in the eastern sky before dawn.\n\n**How:** wait until the sky is genuinely dark. Block the bright crescent with a finger or a lamp-post — the glare is what stops most people seeing it. Let your eyes adapt for a couple of minutes.\n\n**What you should see:** the complete circle of the Moon, with the unlit part a dim blue-grey, like a pencil sketch.\n\n**Bonus observation:** earthshine is slightly brighter when the side of Earth facing the Moon is cloudy or full of ocean cloud, because clouds reflect more than land. Astronomers have used this to measure Earth's reflectivity over decades.",{"id":544,"type":53,"title":545,"eyebrow":546,"navLabel":547},"ch7","Why we always see the same face","Chapter 07","7 The same face",{"id":549,"type":43,"markdown":550},"ch7-same-face","Look at the full moon and you will see dark patches that people all over the world have turned into pictures: a rabbit, a man's face, a woman carrying a bundle of sticks, a buffalo.\n\nNow here is the strange part. **Your grandparents saw exactly the same patches. So did everyone who has ever lived.** The Moon keeps one face permanently turned towards Earth. The other half has never been visible from the ground, from anywhere, at any time in human history.\n\nThe usual guess is that the Moon does not spin. That guess is wrong, and the truth is more interesting: **the Moon spins exactly once for every orbit it makes**. One rotation takes 27.32 days; one orbit takes 27.32 days. The two are locked together, which is why the same side always points our way.\n\nThis is called **synchronous rotation**, or **tidal locking**, and it is not a coincidence. Earth's gravity pulls slightly harder on the near side of the Moon than on the far side, which stretches it very slightly into a non-spherical shape. Over billions of years, the friction of that stretching acted like a brake on the Moon's spin, slowing it until it settled into the one rate where the stretch stops fighting the rotation: exactly once per orbit.",{"id":552,"type":47,"variant":48,"title":553,"markdown":554},"ch7-walk-round","Prove that the Moon must be spinning","Put a chair in the middle of the room. The chair is Earth. You are the Moon.\n\nWalk in a circle around the chair, keeping your face towards it the whole time. After one lap, ask a friend watching from the doorway: did you turn around?\n\nYou did. You faced the door at the start, then the window, then the far wall, then the door again. **From the room's point of view you rotated exactly once.** But from the chair's point of view you never turned at all — it saw your face the whole way.\n\nThat is the Moon. One spin per orbit. The only way to keep one face towards Earth.",{"id":556,"type":47,"variant":557,"title":558,"markdown":559},"ch7-dark-side","misconception","There is no \"dark side of the Moon\"","The hidden half is called the **far side**, and calling it \"dark\" is a mistake that a famous album title made permanent.\n\nThe far side gets exactly as much sunlight as the near side does — about two weeks of blazing day followed by two weeks of night, over and over, just like the side we see. At **new moon**, when the near side is in total darkness, the *far* side is in full sunshine.\n\nSo the far side is not dark. It is just **hidden from us**.\n\nAnd it is not a mirror image of the near side either. When Luna 3 sent back the first blurry photographs in October 1959, astronomers got a surprise: the far side is covered in craters and has almost none of the big dark *maria* that dominate the near side. Its crust is thicker, so ancient lava mostly could not break through. The two halves of the Moon look like different worlds.",{"id":561,"type":562,"conceptId":563,"relation":564,"explanation":565},"ch7-link-tides","connection","tides","helps_understand","The same tidal stretching that raises the oceans on Earth is what slowed the Moon's spin until it matched its orbit. Tides and tidal locking are the same physics.",{"id":567,"type":568,"prompt":569,"options":570,"explanation":579},"predict-far-side","prediction","If the Moon keeps one face towards Earth, what does an astronaut standing in the middle of the **far side** see when they look up?",[571,573,575,577],{"id":403,"label":572},"Earth, permanently, in the same spot in the sky",{"id":405,"label":574},"Earth rising and setting once a month",{"id":407,"label":576},"No Earth at all — it is always below the horizon",{"id":409,"label":578},"Earth only during a full moon","**c.** From the middle of the far side, Earth is never visible. Ever. The Moon's own bulk is permanently in the way.\n\nThat is exactly why radio astronomers dream of putting a telescope there: it is the only place in the inner Solar System permanently shielded from Earth's radio chatter. It is also why every far-side mission needs a relay satellite to talk to home — China's Chang'e 4, which landed on the far side in January 2019, used a relay spacecraft parked beyond the Moon.\n\nMeanwhile an astronaut on the **near side** sees the opposite oddity: Earth hangs almost motionless in the same patch of sky, month after month, slowly going through its own phases.",{"id":581,"type":53,"title":582,"eyebrow":583,"navLabel":584},"ch8","Four mix-ups worth clearing up","Chapter 08","8 Mix-ups",{"id":586,"type":47,"variant":557,"title":587,"markdown":588},"mix-1-shadow","Mix-up 1: \"The phases are Earth's shadow\"","The big one, and worth repeating because it is so persistent.\n\nEarth's shadow always points directly away from the Sun. It can therefore only reach the Moon when the Moon is *opposite* the Sun — that is, at full moon. Yet we see a neat half-lit Moon at first quarter, when the Moon is 90° off to the side and Earth's shadow is pointing somewhere else entirely.\n\nThere is also a timing problem. A shadow event would be occasional and brief; phases are continuous and take a fortnight to unfold.\n\nEarth's shadow *does* fall on the Moon sometimes. That is a **lunar eclipse**: a few hours long, only at full moon, only a few times a year, and the shadowed part turns coppery red rather than black.",{"id":590,"type":47,"variant":557,"title":591,"markdown":592},"mix-2-dark-side","Mix-up 2: \"The dark part of the Moon is the far side\"","These are two completely different halves that happen to be dark at overlapping times.\n\nThe **far side** is fixed by the Moon's rotation: it is always the same physical hemisphere, the one turned away from Earth.\n\nThe **night side** is fixed by the Sun: it is whichever hemisphere is currently turned away from the Sun, and it sweeps right around the Moon once a month.\n\nThey line up only at full moon, when the far side and the night side happen to be the same half. At **new moon** they are opposites: the half we cannot see is in full daylight, and the half facing us is in night.",{"id":594,"type":47,"variant":557,"title":595,"markdown":596},"mix-3-different-countries","Mix-up 3: \"Different countries see different phases\"","They do not. The phase is set by the Sun-Earth-Moon geometry, which is the same for the whole planet. If it is a first quarter Moon, it is first quarter for everybody who can see the Moon at that moment.\n\nWhat *does* change from place to place is:\n\n- **The orientation.** The crescent is tipped differently, and is mirrored in the southern hemisphere.\n- **The time you can see it.** The Moon is below your horizon for half of each day, so your neighbours a few time zones away may be looking at it while you sleep.\n- **The date on the calendar**, since the instant of full moon might be 11 pm on Tuesday in Delhi and 5:30 pm on Tuesday in London and 1:30 pm on Tuesday in New York — or fall on a different calendar day altogether further east.\n\nSame Moon, same phase, different viewpoint and different clock.",{"id":598,"type":47,"variant":557,"title":599,"markdown":600},"mix-4-moon-doesnt-spin","Mix-up 4: \"The Moon doesn't rotate\"","It rotates once every 27.32 days — exactly the time it takes to go once around Earth. That equality is *why* we keep seeing the same face, not evidence against rotation.\n\nIf the Moon genuinely did not spin at all, we would see every side of it over the course of a month, because it would keep the same orientation against the distant stars while circling us.\n\nWalk around a chair keeping your face towards it and you will feel your own body turning through a full circle. The Moon does the same, once a month, very slowly: a point on its equator travels at about 4.6 metres per second, roughly the speed of a jogging human.",{"id":602,"type":156,"itemId":603,"prompt":604,"check":605,"hints":616,"feedback":619},"practice-new-moon-farside","phases-of-the-moon.understand-newmoon-farside","At **new moon**, which part of the Moon is in full sunlight?",{"kind":400,"options":606,"correct":615},[607,609,611,613],{"id":403,"label":608},"The near side, the one we always see",{"id":405,"label":610},"The far side, the one we never see",{"id":407,"label":612},"Neither — the whole Moon is dark",{"id":409,"label":614},"Half of the near side",[405],[617,618],"At new moon the Moon is roughly between Earth and the Sun.","Which half of the Moon is pointing at the Sun in that position?",{"correct":620,"incorrect":621},"Exactly. At new moon the Moon sits between us and the Sun, so the half pointing sunwards — which is the far side — is in full daylight, while the near side we are looking at is in the middle of its two-week night. The far side is never dark for long; it is only hidden.","Draw it: Sun on the left, Moon in the middle, Earth on the right. The half of the Moon facing the Sun is the half facing *away* from Earth — the far side. So at new moon the far side is fully lit.",{"id":623,"type":53,"title":624,"eyebrow":625,"navLabel":626},"ch9","The vocabulary, gathered","Chapter 09","9 Vocabulary",{"id":628,"type":629,"title":630,"terms":631},"ch9-glossary","glossary","The words you now own",[632,635,638,641,644,647,651,655,659,663,667,671,675,679,683],{"term":63,"meaning":633,"example":634},"The angle at Earth between the direction of the Sun and the direction of the Moon. It sets the phase completely.","Elongation 90° means a quarter moon, half the disc lit.",{"term":92,"meaning":636,"example":637},"The share of the Moon's visible disc that is sunlit, from 0 at new moon to 1 at full moon.","At 45° elongation the lit fraction is about 0.15.",{"term":223,"meaning":639,"example":640},"New moon to new moon: 29.53 days. The cycle of the phases.","29 days, 12 hours, 44 minutes.",{"term":227,"meaning":642,"example":643},"One orbit measured against the distant stars: 27.32 days. Shorter than the synodic month.","Also the Moon's rotation period, which is why we see one face.",{"term":492,"meaning":645,"example":646},"The sunrise-sunset line on a world: the boundary between its lit and unlit halves.","Craters show best along the Moon's terminator.",{"term":648,"meaning":649,"example":650},"Maria","The large dark smooth plains on the Moon, made of ancient hardened lava. Latin for 'seas'.","The rabbit and the man in the Moon are patterns of maria.",{"term":652,"meaning":653,"example":654},"Highlands","The pale, heavily cratered older regions of the Moon, standing above the maria.","Brighter because the rock is more reflective.",{"term":656,"meaning":657,"example":658},"Earthshine","Faint light on the Moon's night side, reflected from Earth.","'The old Moon in the new Moon's arms', best seen near a crescent.",{"term":660,"meaning":661,"example":662},"Synchronous rotation","Spinning exactly once per orbit, so the same face always points at the partner body.","The Moon spins once every 27.32 days and orbits in 27.32 days.",{"term":664,"meaning":665,"example":666},"Tidal locking","The process by which gravity's uneven pull slowly brakes a moon's spin until it becomes synchronous.","It is why the Moon keeps one face towards Earth.",{"term":668,"meaning":669,"example":670},"Far side","The hemisphere of the Moon permanently turned away from Earth. Not the same as the dark side.","First photographed by Luna 3 in October 1959.",{"term":672,"meaning":673,"example":674},"Near side","The hemisphere permanently facing Earth, dominated by dark maria.","Every crater you can see from the ground is on the near side.",{"term":676,"meaning":677,"example":678},"Waxing","Growing brighter night by night, from new moon to full moon.","Shukla paksha, the bright fortnight.",{"term":680,"meaning":681,"example":682},"Waning","Shrinking night by night, from full moon to new moon.","Krishna paksha, the dark fortnight.",{"term":684,"meaning":685,"example":686},"Lunar eclipse","Earth's shadow genuinely falling on the Moon. Only possible at full moon, and rare.","Quite different from a phase.",{"id":688,"type":248,"component":689,"componentVersion":5,"config":690,"objective":694,"textAlternative":695,"help":696},"lab-eclipse-why-not","eclipse-lab",{"modes":691,"showShadowCones":254,"tiltDegrees":693},[692],"why-not-monthly",5.1,"See why the shadow explanation of phases fails, by finding how rarely the three bodies really line up.","This lab shows the Sun, Earth and Moon from the side, with Earth's shadow drawn as a long dark cone stretching away from the Sun, and the Moon's orbit drawn as a ring tilted at **5.1°** to Earth's orbit around the Sun.\n\nStep the Moon through a month and watch the shadow cone. At most full moons the Moon passes **above** or **below** the cone, missing it completely — often by several times the Moon's own width. Only when the Moon happens to be crossing the flat plane of Earth's orbit at the same moment as full moon does it slide into the shadow, and an eclipse happens.\n\nSet the tilt slider to **0°** and everything changes: now the Moon crosses the shadow at every single full moon, and passes exactly in front of the Sun at every new moon. You would get a lunar eclipse and a solar eclipse every month.\n\nThe 5.1° tilt is the reason eclipses are special, and it is also a second proof that phases are not shadows: if phases were caused by the shadow, they would stop happening whenever the Moon misses the cone — which is most months.",{"simplerExplanation":697,"hints":698},"The Moon's path is tilted, so it usually sails over or under Earth's shadow instead of through it.",[699,700],"Set the tilt to zero and count the eclipses in one year.","Set it back to 5.1° and count again.",{"id":702,"type":562,"conceptId":703,"relation":564,"explanation":704},"ch9-link-eclipses","eclipses","Knowing the phase tells you when an eclipse is even possible: solar eclipses only at new moon, lunar eclipses only at full moon.",{"id":706,"type":562,"conceptId":707,"relation":564,"explanation":708},"ch9-link-light","light","Every phase is a story about light travelling in straight lines and bouncing off a rough grey surface.",{"id":710,"type":711,"prompt":712},"reflect-understand","reflection","You are explaining the Moon to a younger child who is convinced the dark part is a shadow. You may use one everyday object and one question you ask them. Which object would you pick, what question would you ask, and why would that combination change their mind rather than just contradict them?",{"id":714,"type":715,"title":716,"questions":717},"quiz-understand","quiz","Check yourself",[718,727,740,753,766,779,792,805,818,831],{"itemId":719,"prompt":720,"options":721,"correct":407,"why":726},"phases-of-the-moon.understand-q-elongation","What is the elongation of the Moon at first quarter?",[722,723,724,725],{"id":403,"label":96},{"id":405,"label":106},{"id":407,"label":116},{"id":409,"label":136},"First quarter is a right angle between the directions of Sun and Moon, so exactly half of the lit hemisphere shows: a half-lit disc.",{"itemId":728,"prompt":729,"options":730,"correct":403,"why":739},"phases-of-the-moon.understand-q-fraction45","At an elongation of 45°, roughly what fraction of the disc is lit?",[731,733,735,737],{"id":403,"label":732},"About 15%",{"id":405,"label":734},"Exactly 25%",{"id":407,"label":736},"Exactly 50%",{"id":409,"label":738},"About 85%","About 15%. The lit fraction does not grow in step with the angle: it creeps up slowly at first, which is why a young crescent is so thin.",{"itemId":741,"prompt":742,"options":743,"correct":405,"why":752},"phases-of-the-moon.understand-q-risetime","A Moon is at 135° elongation. Roughly what time does it rise?",[744,746,748,750],{"id":403,"label":745},"About 9 am",{"id":405,"label":747},"About 3 pm",{"id":407,"label":749},"About sunset",{"id":409,"label":751},"About midnight","135 ÷ 15 = 9 hours behind the Sun. Sunrise is about 6 am, so moonrise is about 3 pm. It is highest around 9 pm — the best-placed bright Moon of the month.",{"itemId":754,"prompt":755,"options":756,"correct":405,"why":765},"phases-of-the-moon.understand-q-terminator","Why do craters show up best near the terminator?",[757,759,761,763],{"id":403,"label":758},"The rock there is a different colour",{"id":405,"label":760},"The Sun is low there, so shadows are long",{"id":407,"label":762},"Those craters are bigger",{"id":409,"label":764},"The atmosphere is clearer there","Along the terminator the Sun sits on the lunar horizon, so every rim and ridge throws a long black shadow. At full moon the Sun is overhead and the view goes flat.",{"itemId":767,"prompt":768,"options":769,"correct":407,"why":778},"phases-of-the-moon.understand-q-earthshine","What causes earthshine, the faint glow on the unlit part of a crescent Moon?",[770,772,774,776],{"id":403,"label":771},"The Moon glowing faintly by itself",{"id":405,"label":773},"Starlight",{"id":407,"label":775},"Sunlight reflected from Earth onto the Moon",{"id":409,"label":777},"City lights on Earth","An astronaut standing there would see a nearly full Earth overhead, far brighter than our full moon. That Earthlight bounces off the lunar night side and comes back to us.",{"itemId":780,"prompt":781,"options":782,"correct":405,"why":791},"phases-of-the-moon.understand-q-rotation","How long does the Moon take to spin once on its axis?",[783,785,787,789],{"id":403,"label":784},"24 hours",{"id":405,"label":786},"About 27.3 days, the same as one orbit",{"id":407,"label":788},"It does not spin at all",{"id":409,"label":790},"About 365 days","Exactly one rotation per orbit, which is why one face stays towards us. If it did not rotate, we would see every side of it during a month.",{"itemId":793,"prompt":794,"options":795,"correct":405,"why":804},"phases-of-the-moon.understand-q-farside-new","At new moon, the far side of the Moon is:",[796,798,800,802],{"id":403,"label":797},"In complete darkness",{"id":405,"label":799},"In full sunlight",{"id":407,"label":801},"Half lit",{"id":409,"label":803},"Hidden by Earth's shadow","At new moon the Moon lies between us and the Sun, so the half pointing sunwards — the far side — is in full day, while the near side is in the middle of its two-week night.",{"itemId":806,"prompt":807,"options":808,"correct":405,"why":817},"phases-of-the-moon.understand-q-hemisphere","In the southern hemisphere, a waxing crescent Moon is bright on which side?",[809,811,813,815],{"id":403,"label":810},"The right, the same as in India",{"id":405,"label":812},"The left",{"id":407,"label":814},"The top",{"id":409,"label":816},"It has no bright side","The whole sky appears flipped south of the equator, so D-O-C becomes C-O-D. The reliable rule everywhere is the time of day: an evening crescent is waxing.",{"itemId":819,"prompt":820,"options":821,"correct":407,"why":830},"phases-of-the-moon.understand-q-full-flat","Why is a full moon the worst night for looking at craters?",[822,824,826,828],{"id":403,"label":823},"It is too far away that night",{"id":405,"label":825},"It is too low in the sky",{"id":407,"label":827},"The Sun is overhead there, so almost nothing casts a shadow",{"id":409,"label":829},"There are no craters on the full moon side","Flat lighting. With the Sun behind you, every shadow hides directly behind the object making it, so the surface loses all its texture and you see only differences in rock colour.",{"itemId":832,"prompt":833,"options":834,"correct":405,"why":843},"phases-of-the-moon.understand-q-daily-delay","Why does the Moon rise about 49 minutes later each day?",[835,837,839,841],{"id":403,"label":836},"Earth is slowing down",{"id":405,"label":838},"The Moon moves about 12° further east along its orbit each day, so the sky has to turn further to bring it up",{"id":407,"label":840},"The Moon gets further away each day",{"id":409,"label":842},"Because of the tilt of Earth's axis","The Moon gains about 12.19° of elongation daily. At 15° per hour of sky rotation, that is 0.81 hours, or about 49 minutes of delay.",{"id":845,"type":846,"title":847,"points":848},"cheat-sheet-understand","summary","Cheat sheet",[849,850,851,852,853,854,855,856,857,858,859,860,861,862],"**Elongation** is the Sun-Earth-Moon angle, and it sets everything: 0° new, 90° quarter, 180° full.","**elongation = 360 x day ÷ 29.53**, and **lit fraction = (1 − cos e) ÷ 2**.","**Lit fraction is not proportional to angle.** 45° gives only about 15%; the Moon fattens slowly, then quickly, then slowly again.","**hours behind the Sun = elongation ÷ 15**, because the sky turns 15° every hour.","**Full moon rises at sunset**, first quarter is high in the south at sunset, last quarter is high in the south at sunrise.","**Moonrise slips about 49 minutes later each day**, because the Moon gains 12.19° of elongation daily.","**The bright edge always points at the Sun** — the one orientation rule that works at every latitude.","**Left\u002Fright flips** in the southern hemisphere; near the equator the crescent lies on its back like a boat.","**The terminator** is the Moon's sunrise line. Craters show best along it, which is why quarter moons beat full moons in binoculars.","**Earthshine** is sunlight bounced off Earth onto the Moon's night side: Sun → Earth → Moon → you.","**The Moon spins once per orbit** (27.32 days), which is why one face always points at us. That is tidal locking, not a lack of rotation.","**Far side is not dark side.** At new moon the far side is in full sunlight and the near side is in night.","**Everyone on Earth sees the same phase** at the same moment; only the orientation and the clock differ.","**Phases are a viewing angle. Eclipses are a shadow.** Do not mix them up.",{"id":864,"type":865,"sourceIds":866},"sources-understand","sources",[867,868,869,870,871,872,873],"phases-of-the-moon-nasa-moon-phases","phases-of-the-moon-nasa-moon-facts","phases-of-the-moon-wikipedia-lunar-phase","phases-of-the-moon-wikipedia-tidal-locking","phases-of-the-moon-britannica-moon","phases-of-the-moon-nasa-svs-libration","phases-of-the-moon-timeanddate-moon",[867,868,869,870,871,872,873],"needs_review",{"generatedBy":877,"notes":878},"claude-code","Draft generated locally; pending owner review. Lit fractions from (1 - cos e)\u002F2 and all rise\u002Fset times computed in numbers.py.","8754872e9d258024cf3791b720d951fafd4548c51754f525582d37ce09f46e52",{"logic:practice":881,"component:moon-phase@1":882,"component:sort-game@1":883,"component:eclipse-lab@1":884,"source:phases-of-the-moon-britannica-moon":885,"source:phases-of-the-moon-nasa-moon-facts":886,"source:phases-of-the-moon-nasa-moon-phases":887,"source:phases-of-the-moon-nasa-svs-libration":888,"source:phases-of-the-moon-timeanddate-moon":889,"source:phases-of-the-moon-wikipedia-lunar-phase":890,"source:phases-of-the-moon-wikipedia-tidal-locking":891},"3d6b0fe1b15255975a32b0fcd94e8019bc959ad45cbf12e136e86149549c6878","39afeb0bba7518b8118317655457b27214a4a2a315d236762bf1d6a6a40e18f3","b164f45a2c8ca08f26c450768ff0231e113e9fe45381eddb34dc6d0548596c38","284cb1682e3994af706a6cf907dd7747c9c141577245765bceff3a44fc81cf10","7851afcfed44e83c91e1cd033e5a51b31ed72915a7f12ac117b6c8076cf99a8a","42fce1fa13db44246876ac36bd3506964da8f7fa12ecfcb17813905e4161b696","a204a49fd1f0bcc99b9c71da877e71b83931eba1bf4e931d4af08d83cabb175c","e28faaf9d6da287ce1cdb3b7a69399f9bd6eb3488f41f10fb1ad71da7869c299","bae89711f5182b93b0801064bc61ce6f84a72c54016f6369f629d9fbf2be2d7f","7d0371e53bbaa01b158d9a8d193e8b907f2089d4cb08a73f37d1328b6add3ef9","5a0004750c70296311da84b7e5d934630608574f01b5e3f87445136cf54ff892",{"state":893,"reviewer":894,"selfReview":254,"reviewedAt":895,"method":896},"approved","The library owner","2026-09-20T10:18:37.581Z","owner_bulk","preview-7e1cbbcc4f",1789899599019]