Space Systems
The Earth, the Moon and the Sun
Tell rotation from revolution, explain day, night and the seasons from the Earth's turning and its tilt, and say what lines up during each kind of eclipse.
Shadows are long in the morning, short at midday and long again by evening. The obvious reading is that the Sun crawls across the sky. Ride a merry-go-round turning one way and the trees around you seem to turn the other; nothing is moving but you. The Sun's daily journey is that illusion, on a larger scale.
Two motions, and what each one gives us
Rotation is an object turning about a line through itself — its axis. The Earth rotates once in about 24 hours, from West to East, and that turning is what brings day and night. Revolution is one object traveling around another along a path called an orbit. The Earth revolves round the Sun in about 365 days and 6 hours, and the Moon revolves round the Earth.
Why the seasons — and two answers that are wrong
The Earth's axis is tilted rather than upright, and it keeps that tilt the whole way round its orbit. The hemisphere tilted towards the Sun gets the same sunlight spread over a smaller patch of curved ground, so it heats more, and it gets that sunlight for more than 12 hours a day. That is summer. Two tempting explanations are both wrong: the tilted hemisphere is not meaningfully closer to the Sun, and the Earth's changing distance over the year is far too small to matter — the Earth is in fact closest to the Sun in January.
Have a play
The Sun, the Moon and the Earth, lined up for a solar eclipse.
Tap any part of the picture.
Worked example
The Moon is far smaller than the Sun. How can it hide the Sun completely?
Try it with a thumb. Hold your thumb up at arm's length and close one eye: it covers a friend's whole head from a few paces away.
Your thumb has not grown. What made the difference is that it is far nearer to your eye than the head is.
Try it together
Work out why June is summer in one hemisphere and winter in the other.
Each step takes a word or a short phrase.
1.The Earth's axis is not upright with respect to its orbit. What is it instead?
Have a go
Have a go on your own. The path one object takes as it travels around another has a one-word name. What is it?
Ready to practice?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.