The Moon, the planets and their movement
Physics · Key Stage 2 · The School
Orbits, not circles on a page
Eight planets orbit the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The further out a planet is, the longer its year — Mercury takes 88 days, Neptune about 165 of ours. The Sun, Earth and Moon are all approximately spherical, which is worth saying because diagrams flatten everything into circles on paper.
Why the Moon changes shape
The Moon orbits the Earth about every 27 days and produces no light of its own — we see the half of it lit by the Sun. As it moves round, we see that lit half from different angles, so it appears to change from new, to crescent, to half, to full and back. The Moon itself is not changing shape and is not in the Earth's shadow.
Why the Sun seems to move
The Sun does not travel across our sky; the Earth spins once about every 24 hours, and we are carried round with it. Sunrise is our part of the Earth turning to face the Sun and sunset is it turning away. Everything else — shadows swinging, the Sun's height changing — follows from that single rotation.
Mercury goes round the Sun in 88 days; Earth takes 365; Jupiter takes almost 12 years; Neptune takes 165. Two things stack up. The further out you are, the longer the path — Neptune's orbit is about 30 times the size of Earth's. And the further out you are, the SLOWER you travel, because the Sun's pull is weaker. Longer road and lower speed together, which is why the numbers grow so fast.