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Earth, Moon, and the Sun

When three line up, one shades another

A tumbler stands in front of a lamp. A matchbox behind the tumbler goes dark. Why?

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The idea

The Sun, a ball in the way, and another ball.
Step 1 of 4
  1. The Sun, a ball in the way, and another ball.
  2. Light leaves the Sun in straight lines.
  3. The ball in the way blocks some of it. A shadow forms.
  4. If the shadow reaches the other ball, that is an eclipse.

The rule

The tumbler blocks the light. So the matchbox is in its shadow.

A body is any object, like a tumbler or the Moon.

A body in the way makes a shadow. If it lands on another body, that is an eclipse.

A shadow on a table, and the same shadow between three worlds
What a shadow needsOn a tableOut in space
Something making lightA lampThe Sun
Something in the wayA tumblerEarth, or the Moon
Something behind, to land onA matchboxThe Moon, or Earth

Real sizes and gaps cannot fit in one picture. The pictures in this track are not to scale.

Jump to lesson

This track

  1. 0 Start here
  2. 1 Earth is doing two things at once
  3. 2 The Moon keeps one face turned to us
  4. 3 When three line up, one shades another
  5. 4 The Moon shades a patch of Earth
  6. 5 Earth shades the Moon
  7. 6 Why it is not every month
  8. 7 One is safe to watch, the other never is
  9. 8 Checkpoint: motions, the Moon and eclipses
  10. 9 Why the Sun seems to move
  11. 10 Day here, night there
  12. 11 The star that stays still
  13. 12 Checkpoint: the turning sky
  14. 13 The night sky changes through the year
  15. 14 The tilt makes the seasons
  16. 15 The longest and shortest days
  17. 16 Checkpoint: the year, the tilt and the seasons
  18. 17 The big picture
  19. 18 Track quiz

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