GCSE Physics · Edexcel 1PH0 · Topic 7A
The solar system and orbits
Key points
- The solar system has 8 planets orbiting the Sun, with moons orbiting planets, plus dwarf planets, asteroids and comets.
- Ideas changed from the geocentric model (Earth at the centre) to the heliocentric model (Sun at the centre), supported by observations such as Galileo's view of Jupiter's moons.
- Gravitational field strength varies between planets and moons, so weight changes: .
- Gravity provides the centripetal force for orbits. In a circular orbit the speed is constant but the velocity changes, because the direction keeps changing.
- Smaller orbits need higher speeds. Comets have very elliptical orbits and speed up near the Sun.
- Artificial satellites are used for communications, weather and GPS; geostationary satellites orbit once every 24 hours.
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The solar system is held together by gravity. This sub-topic describes the objects in it, how our picture of it changed over time, and why objects in orbit move the way they do.
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