IGCSE Physics / Motion in the universe
Orbits of moons, planets, satellites and comets revision notes
Edexcel International GCSE Physics (4PH1), chapter 26: Motion in the universe, specification point 8.4, 8.5. Free lesson.
Watch the video lesson · IGCSE Physics past papers
Orbits are curved paths caused by gravitational force pulling an object towards a larger body. Knowing what each object orbits, and how comet orbits differ from planet orbits, helps you describe and draw them accurately.
What causes each orbit
Every orbit is caused by gravitational force pulling an object towards a larger body. The object being orbited depends on the type of object. Moons orbit planets. The Moon orbits Earth. Planets orbit the Sun, for example Earth and Mars. Artificial satellites orbit Earth, for example a weather satellite. Comets orbit the Sun, for example Halley's Comet.
Why an orbit is curved
An orbiting object moves sideways as gravity pulls it towards the centre of the larger body. With no force, it would continue in a straight line. Gravity continually changes its direction, but in this explanation not its speed, so its path curves. The Moon is always falling towards Earth, but its sideways motion means it keeps missing Earth.
How comet orbits differ from planet orbits
Planet and moon orbits are nearly circular. Comet orbits are highly elliptical, meaning they are long, stretched ovals. This shape means a comet's distance from the Sun changes greatly, while a planet's distance hardly changes. For a planet, the Sun is at the centre of the orbit. For a comet, the Sun is near one end of the oval, well off centre. A planet moves at almost the same speed all the way round. A comet moves fastest when closest to the Sun and slowest when furthest away. As a comet falls towards the Sun, the gravitational pull gets stronger and the comet speeds up. As it moves…
How to draw an orbit
Draw an orbit flat on the page, as if seen from above. Add a force arrow to show that gravity pulls the orbiting object towards the larger body.