EclipsesExtendabout 40 min

The same shadow rule, everywhere in the Solar System

Moons too small to eclipse, a moon that eclipses constantly, transits at home, and other worlds' planets

Take the eclipse geometry beyond Earth: why Phobos and Deimos only ever transit the Sun from Mars, why Io causes true eclipses on Jupiter routinely, how Mercury and Venus transit the Sun from Earth, Venus's 243-year transit rhythm, and how the same trick finds other stars' planets.

Start at chapter 1

In this part you’ll

Compare a crossing body's angular size with the Sun's to decide whether it can ever cause a total eclipse or only a transit.
Explain why Io causes routine total eclipses on Jupiter while Phobos and Deimos can only transit the Sun from Mars.
Calculate a transit's light-dip percentage from the crossing body's width ratio, and apply the same method to an exoplanet.
Explain the 8-year-pair, 243-year Venus transit cycle and its historical use for measuring the astronomical unit.
Pose and reason through an open puzzle about eclipse geometry, using computed angular sizes to support an argument.

Everything in this topic so far has been about one particular Sun, one particular Moon and one particular Earth. But the rule behind an eclipse is not special to us: any moon, orbiting any planet, lit by any star, casts a shadow — and that shadow can land on its own planet, on a neighbouring moon, or nowhere at all.

This layer takes the same geometry everywhere else in the Solar System, and a little beyond it: to two moons too small to ever cover the Sun, to a moon whose shadow makes a true eclipse on a giant planet's clouds every few days, to two planets that only ever cross the Sun's face as tiny dots, and to the same tiny-dot idea used to find planets around other stars entirely.

Need a different angle?

Chapter 01

Mars: two moons, no total eclipse ever

Mars has two moons, Phobos and Deimos, both tiny, lumpy, potato-shaped rocks captured long ago rather than formed neatly like our Moon. Phobos orbits closer to Mars than any large moon orbits any other planet in the Solar System — only about 6,000 km above the surface. Deimos is smaller and further out, at about 20,000 km.

Worked example

0 / 5 steps shown

Could Phobos ever black out the Sun from Mars?

Phobos is about 22.5 km across and orbits only 6,000 km above Mars's surface. The Sun, seen from Mars (1.524 AU out), looks about 0.350° across. How big does Phobos look from the Martian surface, and is it bigger or smaller than the Sun?

Need a different angle?

TablePhobos and Deimos: could either ever cause a total eclipse?
Moon	Width	Altitude	Angular size	vs Sun (0.350°)	Verdict
Phobos	22.5 km	≈6,000 km	≈0.215°	61% as wide	Transit only, fairly often
Deimos	12.4 km	≈20,000 km	≈0.036°	10% as wide	A barely-visible speck crossing the Sun

Predict first

The worked example found Phobos needs to be about 63% bigger across, at its real altitude, to fully cover the Sun from Mars. If a future spacecraft somehow nudged Phobos much closer to Mars instead of making it bigger, could that achieve the same result?

No — only physical size matters, not distanceYes — moving Phobos closer increases its angular size, exactly as making it bigger wouldNo — Phobos would break apart before getting close enoughYes, but only if Mars's orbit also changed

Chapter 02

Jupiter: real eclipses, every few days

Now the opposite extreme. Jupiter's big moon Io is a genuine, roomy world 3,643 km across — bigger than our own Moon — orbiting only 421,700 km from Jupiter's cloud tops, closer relative to its size than our Moon is to Earth.

Need a different angle?

Worked example

0 / 5 steps shown

Io's shadow: transit dot, or true eclipse?

Jupiter, at 5.204 AU, sees the Sun at about 0.103° across — much smaller than we do, because Jupiter is so far out. Io is 3,643 km across, orbiting 421,700 km from Jupiter. How does Io's angular size compare with the Sun's, as seen from Jupiter's cloud tops?

Need a different angle?

Lab

Sort eight Sun-crossing events from across the Solar System into true eclipses and mere transits.

Sort each Solar System body's Sun-crossing event into the right box.

8 cards, 2 bins. Tap a card, then tap its bin. You can also drag, or press a bin’s number key.

Text version of this activity

A sorting game with two boxes — true eclipse and transit only — and eight real examples: Io and Europa crossing the Sun from Jupiter (both eclipses); Phobos, Deimos, Mercury and Venus crossing from their respective vantage points (all transits); and our own Moon at perigee (eclipse) versus apogee (transit-like ring) as the two extremes of our own case.

The pattern to notice: it is always the same test — is the crossing body's angular size bigger or smaller than the Sun's, from where you are standing?

Chapter 03

Transits at home: Mercury and Venus

Earth has its own transits, caused by the two planets that orbit closer to the Sun than we do: Mercury and Venus. Neither can ever eclipse the Sun from Earth — they are too small and too far away — but both cross its face as dots, visible (with a proper solar filter) as a small, perfectly round, dark circle.

TableMercury and Venus, crossing the Sun as seen from Earth
Planet	Width	Distance at transit	Angular size	Share of Sun's width	Share of Sun's area (light dip)
Mercury	4,879 km	≈0.53 AU	≈12.7″	≈0.66%	≈0.0000437%
Venus	12,104 km	≈0.288 AU	≈58.1″	≈3.03%	≈0.092%

Worked example

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How much does Venus dim the Sun during a transit?

Venus's disc is about 3.03% of the Sun's width during a transit. If you measured the Sun's total brightness precisely throughout the transit, by how much would it dip?

Need a different angle?

It’s more subtle

Why 18th-century astronomers cared so much about Venus transits

Deepen explained that the Sun's own parallax — about 8.78 arcseconds — is one of astronomy's hardest numbers to pin down directly, because it demands measuring an almost imperceptible shift. Venus transits gave astronomers a clever workaround: by timing exactly when Venus's black dot touched the Sun's edge, from several widely separated places on Earth at once, the small differences in timing could be turned into a measurement of the Earth-Sun distance itself — without ever measuring the Sun's parallax directly.

Edmond Halley proposed the method; expeditions across the globe (including observers sent to India and the Indian Ocean) attempted it for the 1761 and 1769 transits. The results were messy — the "black drop effect", where Venus's disc seems to smear against the Sun's edge, limited the precision — but they gave the first serious, evidence-based estimate of the astronomical unit, decades before other methods caught up.

Chapter 04

The strange rhythm of Venus transit pairs

Venus transits do not come at a steady interval. They arrive in pairs, eight years apart, and then nothing for over a century.

Need a different angle?

Pair gap

8 years

Long gaps

105.5 or 121.5 years

Full cycle

8 + 105.5 + 8 + 121.5 = 243 years

Last pair

2004, 2012

Next pair

2117, 2125

Worked example

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Why 243 years, and why pairs of two?

Venus transits happen only when Venus, at inferior conjunction (passing between Earth and the Sun), is also near one of its own orbital nodes — the same two-condition test as any eclipse. Given the pattern 8, 105.5, 8, 121.5 years repeating, confirm the cycle length and explain the pairing.

Need a different angle?

Reflect

The next Venus transit is in 2117. If you are 11 years old today (2026), work out roughly how old you would be in 2117, and decide whether you think you could plausibly see it. Then research (or estimate) how many people alive today are likely to see it, and how that compares with how many will see, say, the 2034 total solar eclipse crossing India.

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Chapter 05

The same trick, used to find planets around other stars

Here is where this topic connects to one of the biggest ongoing projects in all of astronomy. If a tiny, precise dip in a star's brightness reveals a planet crossing in front of it — exactly the trick used for Venus and Mercury — then the same method, pointed at other stars, should reveal their planets too. It does, and it is now the single most productive method for discovering planets beyond our Solar System.

TableComparing transit dips: from Mercury to a whole planet-sized world
Transiting body	Width vs Sun	Light dip
Mercury	≈0.66%	≈0.0000437%
Venus	≈3.03%	≈0.092%
Earth	≈0.917%	≈0.0084%
Jupiter	≈10.27%	≈1.05%

Worked example

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Could a distant civilisation detect Earth transiting the Sun?

Earth is about 12,742 km across; the Sun is 1,392,700 km across. What fractional dip in the Sun's brightness would Earth's transit cause, and is that within reach of a sensitive space telescope, which can typically detect dips as small as about 0.001%?

Need a different angle?

Lab

Connect six ideas from across the Solar System — and beyond it — to the fact that defines each one.

Match each body or idea to the fact that goes with it.

6 pairs are hiding in two mixed-up columns. Pick one from each side to join them.

Text version of this activity

A matching game pairing Phobos, Io, the Venus transit pair pattern, the black drop effect, the transit-dip-is-squared rule, and the exoplanet transit method with the single fact that best explains each.

Chapter 06

Occultations: blocking a star instead of the Sun

Widen the idea one more step. An eclipse or a transit is really just one example of a more general event: any time a nearer body passes in front of a more distant one and dims or hides it, astronomers call it an occultation. The Moon occults stars and planets often — you can watch a bright star vanish behind the Moon's dark edge on many nights, instantly, with no fading at all, because the Moon has no atmosphere to blur the edge.

Occultations are not just pretty to watch. Timed carefully, from several locations, they have revealed things no direct picture could.

TableThe whole family, from narrowest to widest cover
Event	What crosses what	Typical cover
Total eclipse	A body that looks bigger than the light source, from where you stand	Completely hides it
Annular / partial eclipse or a big transit	A body of similar or smaller angular size	Partly hides it, or leaves a ring
Transit	A much smaller, usually distant body	A small dark dot, barely dimming the total light
Occultation	Any nearer body in front of any more distant one	The general case all the above belong to

Predict first

A star occulted by the Moon vanishes almost instantly, with no fading at all. A star occulted by Mars (which has a thin atmosphere) fades slightly before disappearing completely. What does this difference tell you?

Mars is bigger than the MoonAn atmosphere bends and scatters starlight gradually as the body's edge approaches, while an airless edge blocks it all at onceThe Moon moves faster than MarsNothing — the difference is random

Worked example

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Why does watching from several places at once matter?

A single observer sees an occultation as a dimming that starts and ends at two moments in time. Explain why astronomers deliberately spread observers across many different locations for an important occultation, rather than relying on just one telescope.

Need a different angle?

Chapter 07

Artificial eclipses: making your own corona, on demand

Discover explained why the corona is only visible during the few minutes of a natural total eclipse: the Sun's blinding disc normally drowns it out completely. For over a century, solar physicists have wanted a way around this — and two very different solutions now exist, one already flying, one very new.

Example

The coronagraph: an artificial Moon, permanently in place

A coronagraph is an instrument with a small disc built into it, positioned to block out a star's or the Sun's bright disc inside the instrument itself — a tiny, permanent, artificial 'Moon' held exactly in the right place by careful optics. Space telescopes have carried Sun-watching coronagraphs for decades, giving a view of the corona every single day rather than for a few minutes every year or two. India's own Aditya-L1 mission, launched in September 2023, carries a Visible Emission Line Coronagraph (VELC) for exactly this purpose, stationed at the L1 Lagrange point about 1.5 million km from Earth — roughly 1% of the way to the Sun, where the pull of Earth's and the Sun's gravity balance out and a spacecraft can sit with a permanently unobstructed view.

Try it

Lab

Show that a small disc held close can block the same angular size as a huge, distant one — the whole trick behind a coronagraph.

Lamp → object120 cmLamp → screen320 cm

How many times taller2.67×

Shadow height5.3 cm

Small close disc, 2 cm tall, stands 1.2 m from the lamp. The screen is 3.2 m from the lamp, which is 2.67 times further, so the shadow is 2.67 times taller: 5.3 cm. The lamp is a tiny point, so the shadow has a sharp edge.

A tiny lamp makes a sharp shadow. Every ray starts from one point, so the edge of the shadow is one clean line. Look at the two yellow rays: the lamp, the top of the small close disc and the top of the shadow all sit on one straight line. That is what makes the two triangles the same shape.

Drag the round handles on the bench, or use the sliders — or focus a handle and press the arrow keys (hold Shift for big jumps). The picture is drawn to scale.

Round 1 / 1★ 0 ptsBest: 0

Shadow challenges: move the lamp, object and screen until the shadow is exactly the size asked for. Anything within 5% counts.

Model: light travels in perfectly straight lines and the object is a flat card facing the lamp. Real shadows are also softened a little by light bouncing off walls and floors.

Text version of this activity

A lamp representing the Sun, a small disc you can slide close to a viewing point, and, further back, a much larger reference disc fixed at a great distance.

Slide the small disc closer to the viewing point and watch its angular size grow, even though its real size never changes. At the right distance, the small disc's angular size exactly matches the large, distant disc's — demonstrating that a coronagraph's tiny built-in blocker can match the Sun's own apparent size purely through careful placement, with no need to be anywhere near the Sun's actual size.

Need a different angle?

Predict first

A coronagraph's built-in disc is much closer to the camera than the real Moon is to Earth. Given that angular size = real size ÷ distance, why can a small disc a short distance inside an instrument still block the Sun's much larger, much more distant disc?

It cannot really — coronagraphs do not actually workThe disc's angular size, seen from the camera, can match the Sun's even though the disc itself is tiny, just by placing it close enoughBecause the camera is more sensitive than a human eyeBecause the disc is painted black

Chapter 08

Puzzles to take further

Worked example

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Puzzle: how much would Phobos have to grow?

Phobos's real angular size is only about 61.4% of the Sun's, as seen from Mars. If Phobos stayed at its real distance but grew until it exactly matched the Sun's angular size, how big would it need to be, and what percentage bigger is that than its real 22.5 km width?

Need a different angle?

Phobos today: 22.5 km wideAbout 61% of the Sun's angular width from Mars — always a transit.
Phobos, to fully cover the Sun: ≈36.7 km wideAbout 63% bigger, at the same altitude.
Our Moon: 3,475 km wideAlready just barely big enough, at perigee, for total eclipses.
Io: 3,643 km wideNearly 5× the Sun's tiny angular width from Jupiter — always an eclipse.

Think about it

Open questions for curious learners

Nobody has fully settled these. Pick one and dig in.

If you designed a fictional planet-moon system specifically to give the most spectacular possible total eclipses — long totality, wide path, frequent — what sizes and distances would you choose, and why can real systems rarely manage all three at once?
Saturn's rings cast a shadow on the planet itself. Is that an eclipse, by the definition used throughout this topic? Make the case either way.
If Earth had two moons instead of one, would total solar eclipses become more common, less common, or does it depend entirely on the second moon's size and orbit? Sketch out a few scenarios.
Exoplanet transits reveal a planet's size and orbit, but not directly its mass. What additional measurement, paired with a transit, could reveal the mass too — and hence the density and likely composition?
The 1919 eclipse tested gravity bending starlight. What is one prediction of modern physics that is genuinely hard to test today, the way relativity was hard to test before 1919 — and what special, rare event might one day make it possible?

Reflect

Using what you know about angular size (real size ÷ distance), sketch the numbers for a made-up moon that would give a planet unusually long totalities — longer than Earth's maximum of 7 minutes 32 seconds. Would you make the moon bigger, closer, slower-orbiting, or some combination? Explain your reasoning with at least one calculation.

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Chapter 09

Careers and citizen science built around eclipses

Eclipses are not only something to watch — for a surprising number of people, they are part of a working life or a serious hobby, and several of these paths are open to a curious student well before university.

A few ways eclipses turn into work, research or a serious hobby

Step 01Solar physicistresearcher
Studies the Sun's corona, magnetic field and eruptions, using coronagraphs, natural eclipses and spacecraft data such as Aditya-L1's.
Step 02Orbital mechanics engineerengineer
Designs and predicts spacecraft trajectories precisely enough for missions like Proba-3's formation flying, or for computing eclipse paths centuries ahead.
Step 03Eclipse chaser / astrophotographerhobbyist to professional
Travels to totality paths worldwide, and some make a living leading tours, writing guides or selling photographs and footage.
Step 04Citizen scientistvolunteer
Ordinary observers time eclipse contacts, submit eclipse photographs to projects such as NASA's Eclipse Megamovie, or time occultations from their own backyard with modest equipment, contributing real data used by professional astronomers.
Step 05Science communicator / planetarium educatoreducator
Explains eclipse safety and science to the public before major events, often the difference between a town watching safely or not.

Reflect

Pick one career or citizen-science activity from the list above that interests you. Write two questions you would want to ask someone doing it, and one small step you could take this year — even something as simple as joining an eclipse-watching event or learning to use a stopwatch precisely — to find out more.

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Chapter 10

Wrap-up

angle = size ÷ distance

Angular size, in radians (multiply by 57.3 for degrees) — the one rule behind every comparison in this layer.

dip % ≈ (width ratio)²

Light blocked in a transit, since brightness depends on area, not width.

L1 ≈ 1% of the Earth-Sun distance

Where Aditya-L1 sits, balancing Earth's and the Sun's gravity.

8 + 105.5 + 8 + 121.5 = 243 years

Venus's full transit-pair rhythm, added up.

Helps you understand

Gravity

Every orbit in this layer — Phobos, Io, Venus, an exoplanet — is held in place by the same gravity studied in that topic.

Words to know

All maths vocabulary →

Words from beyond Earth

Transit: A smaller body crossing in front of a larger, more distant one, without covering it completely.; Example: Mercury and Venus transit the Sun; Phobos transits it from Mars.
Inferior conjunction: The moment an inner planet (Mercury or Venus) passes between Earth and the Sun.; Example: A transit can only happen at inferior conjunction, near one of the planet's nodes.
Black drop effect: An optical smearing of a transiting planet's silhouette against the Sun's edge, which limited historic timing precision.
Astronomical unit (AU): The average Earth-Sun distance, about 149.6 million km, used as a convenient yardstick for the Solar System.
Exoplanet: A planet orbiting a star other than the Sun.; Example: Most known exoplanets were found by the transit method.
Light curve: A graph of a star's brightness over time.; Example: A transiting exoplanet shows up as a small, repeating dip in the light curve.
Occultation: A nearer body passing in front of a more distant one and hiding or dimming it — the general case an eclipse or transit belongs to.; Example: The Moon occulting a star has a sharp edge, since it has no atmosphere to blur it.
Coronagraph: An instrument with a small built-in disc that blocks a star's or the Sun's bright disc, revealing its faint surroundings.; Example: India's Aditya-L1 carries a coronagraph to watch the Sun's corona every day.
Lagrange point: A location in space where the gravity of two large bodies balances so a spacecraft can hold a stable position with little fuel.; Example: L1, about 1% of the way from Earth to the Sun, gives an unobstructed view of the Sun.

Keep this

Cheat sheet

The same test works everywhere: is the crossing body's angular size bigger or smaller than the Sun's, as seen from where you stand? Bigger → possible total eclipse. Smaller → transit only.
Phobos (Mars): only about 61% of the Sun's width — always a transit, however often it crosses. Deimos is even smaller, only about 10%.
Io (Jupiter): nearly 5 times the Sun's tiny, distant angular width — a true, sharp total eclipse most of its 42-hour orbits. Every large Galilean moon does the same.
Mercury and Venus transit the Sun from Earth: about 0.66% and 3.03% of its width. Light blocked depends on the square of that ratio — about 0.00004% and 0.09% respectively.
Venus transit pairs: 8 years apart, then a gap of 105.5 or 121.5 years — a 243-year full cycle. Last pair 2004/2012; next pair 2117/2125.
Historic use: 18th-century Venus transits, timed from many places on Earth (including Indian Ocean expeditions), gave the first solid estimate of the Earth-Sun distance, despite the smearing 'black drop effect'.
Exoplanets are mostly found by the same trick: a small, precise, repeating dip in a star's brightness, sized by the square of the planet-to-star width ratio — Earth would dim a distant Sun-like star by about 0.0084%.
Occultations are the general case: any nearer body passing in front of a more distant one. Timed from several locations, they revealed Uranus's rings (1977) and a ring around the small body Chariklo (2013), by catching extra dips either side of the main event.
Artificial eclipses: a coronagraph's small, close disc matches the Sun's angular size to block it — India's Aditya-L1 carries one at L1, about 1% of the way to the Sun. ESA's Proba-3, launched on an Indian rocket, flies two spacecraft in formation to fake an eclipse for hours at a time.
Careers and citizen science: solar physicists, orbital-mechanics engineers, eclipse chasers and amateur timers of occultations and transits all turn this topic into real, ongoing work — some of it open to a careful volunteer with just a clock and a clear sky.
Puzzle takeaway: our own Moon barely, but genuinely, covers the Sun — the '400 and 400' coincidence from Discover. Phobos would need to be about 63% bigger to do the same from Mars, and is shrinking towards Mars instead of growing.

Where this comes from

Sources

Eclipse Web Site (opens another website) — NASA Goddard Space Flight Centerawaiting check
Supports general solar and lunar eclipse geometry, umbra/penumbra terminology, path of totality width and duration figures, and links to eclipse predictions.
Planetary Fact Sheets (opens another website) — NASA Goddard Space Flight Center (NSSDCA)awaiting check
Supports the diameters, distances and orbital data used throughout the generators for the Sun, Earth, Moon, Mars, Jupiter, Mercury and Venus, and their moons Phobos, Deimos and Io.
Solar eclipse (opens another website) — Wikipediaawaiting check
Secondary reference supporting solar eclipse types (total, annular, partial, hybrid), the Saros cycle, and historical eclipse expeditions including 1868 and 1919.
Transit of Venus (opens another website) — Wikipediaawaiting check
Secondary reference supporting the 8/105.5/8/121.5-year Venus transit pattern, the 2004/2012 and 2117/2125 transit pairs, and the historical use of transits to measure the astronomical unit.
Solar eclipse (opens another website) — Encyclopaedia Britannicaawaiting check
Supports the plain-language description of solar eclipse types and the corona, Baily's beads and diamond ring effect used for a young audience in Discover.
Curiosity: Textbook of Science for Grade 7, Chapter 12 (Earth, Moon and the Sun) (opens another website) — NCERTawaiting check
Supports syllabus-level coverage of Earth's motion, Moon phases, and solar and lunar eclipses as taught in the current NCERT Class 7 Science (Curiosity) textbook.
Aditya-L1 (opens another website) — Indian Space Research Organisation (ISRO)awaiting check
Supports Aditya-L1's launch in September 2023, its coronagraph payload (VELC), and its station-keeping at the Sun-Earth L1 point about 1.5 million km from Earth.
About Proba-3 (opens another website) — European Space Agency (ESA)awaiting check
Supports Proba-3's two-spacecraft formation-flying design, its December 2024 launch on an ISRO PSLV rocket, and its creation of artificial solar eclipses lasting hours at a time to study the corona.

End of Extend

What you just read

Compare a crossing body's angular size with the Sun's to decide whether it can ever cause a total eclipse or only a transit.
Explain why Io causes routine total eclipses on Jupiter while Phobos and Deimos can only transit the Sun from Mars.
Calculate a transit's light-dip percentage from the crossing body's width ratio, and apply the same method to an exoplanet.
Explain the 8-year-pair, 243-year Venus transit cycle and its historical use for measuring the astronomical unit.
Pose and reason through an open puzzle about eclipse geometry, using computed angular sizes to support an argument.

The web

Explore a connection

Builds onanother area
Light
An eclipse is a shadow, and shadows need light that travels in straight lines.
Builds onanother area
Gravity
Eclipses happen only because the Sun, Earth and Moon move on fixed gravitational paths we can predict.
Builds on
Phases of the Moon
Eclipses can only happen at new moon or full moon — the two phases where the three bodies line up.

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Revision 1 · release preview-7e1cbbcc4f · accepted 20/09/2026

Could Phobos ever black out the Sun from Mars?

Io's shadow: transit dot, or true eclipse?

How much does Venus dim the Sun during a transit?

Why 243 years, and why pairs of two?

Could a distant civilisation detect Earth transiting the Sun?

Why does watching from several places at once matter?

Puzzle: how much would Phobos have to grow?

A few ways eclipses turn into work, research or a serious hobby

Words from beyond Earth

Check what you found beyond Earth

Cheat sheet

Sources

What you just read

Explore a connection