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Earth and space · Sun, Moon and sky

Eclipses

If the Moon goes round Earth every month, why isn't there an eclipse every month?

An eclipse is a shadow falling exactly where it can be seen. Learn the geometry of umbra and penumbra, why the Moon's tilted orbit makes eclipses rare, and how to watch one safely.

5 depths≈ 185 min of reading68 practice questions

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Every depth is open. Start where it feels interesting — you can always move back.

  1. Discoverabout 30 minAn eclipse is a shadow that finds youMeet 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.Owner-accepted · not line-by-line reviewed
  2. Understandabout 40 minThe geometry of a shadow in spaceWork out the actual geometry: how long each shadow cone is, why the Moon's only just reaches us, why the discs match to 3%, how far from a node an eclipse can happen, why the Moon turns red, and the physics behind every solar viewing rule.Owner-accepted · not line-by-line reviewed
  3. Investigateabout 35 minBuild it, test it, try to break itHands-on layer: build a scale model of the Earth-Moon-Sun system, test the new-moon/full-moon rule and the shadow-width formula for yourself, find the tilt's hidden threshold, build a pinhole projector and check its numbers, and plan around three real upcoming eclipses.Owner-accepted · not line-by-line reviewed
  4. Go deeperabout 40 minThe Saros cycle, and two eclipses that changed physicsDeeper reasoning: rebuild the 1.474° eclipse limit term by term, derive the Saros and exeligmos cycles from three different lunar months, see how Aryabhata and Brahmagupta actually computed eclipses, and examine the two solar eclipses that discovered helium and tested general relativity.Owner-accepted · not line-by-line reviewed
  5. Extendabout 40 minThe same shadow rule, everywhere in the Solar SystemTake 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.Owner-accepted · not line-by-line reviewed

Practise and apply

Question bank · 68 questions

8 sections from Foundation to Challenge, each with hints and a worked solution — or play a 10-question round for points and stars.

Foundation 20Core 24Stretch 16Challenge 8

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The web

Connected ideas

Follow a connection when you’re curious — some lead into completely different areas.

Eclipses

  • 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.

  • Used inanother area

    Angles

    Whether an eclipse is total or partial comes down to angles: the Moon's tilted orbit and the apparent size of two discs.

Words to know

53 words are explained in these lessons

All vocabulary →

EclipseUmbraPenumbraSolar eclipseLunar eclipseTotalityAnnular eclipsePartial eclipseCoronaNodeNew moonFull moon+41 more