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Unlock This CourseAn orbit is the curved path one object takes around another because of gravity. The Moon orbits the Earth. The Earth orbits the Sun. Nothing is pushing them along. Gravity does the work.
Think about a ball on a string that you whirl round your head. The string pulls the ball towards your hand. The ball keeps moving, but its direction keeps changing, so it moves in a circle. If the string snapped, the ball would fly off in a straight line.
In space, gravity is the string. It is a force that always pulls towards the centre of the orbit. On a circular path this force does not speed the object up or slow it down. It keeps changing the direction of its motion, so the object curves round instead of going straight.
Key terms:
In every case below, the same idea applies: the gravitational force of the larger object pulls towards its centre and keeps changing the direction of the smaller object's motion.
| Orbiting object | Orbits | Force that keeps it there |
|---|---|---|
| Moons | A planet | The planet's gravity |
| Planets | The Sun | The Sun's gravity |
| Artificial satellites | The Earth | The Earth's gravity |
| Comets | The Sun | The Sun's gravity |
Our Moon is a natural satellite. The Earth's gravity pulls it towards the Earth and keeps it on a curved path. Weather and communication satellites are artificial satellites. They are launched into space and then orbit because of the Earth's gravity too.
The Sun has a huge mass, so its gravity is strong enough to hold every planet and comet in orbit around it. The Sun's gravity also keeps the whole solar system together.
Planets, moons and comets all obey gravity, but their orbits are not the same.
| Object | Orbits | Shape of orbit |
|---|---|---|
| Planet | The Sun | Nearly circular |
| Moon | A planet | Nearly circular |
| Comet | The Sun | Very stretched ellipse |
A planet's orbit is almost a perfect circle, so the planet stays at about the same distance from the Sun all the way round. A moon's orbit around its planet is nearly circular too.
A comet is different. Its orbit is a long, thin ellipse. To draw it, start with a circle and stretch it in one direction until it looks like a squashed oval. The Sun is not in the middle of this oval. It sits closer to one end, so the comet comes very close to the Sun at one point and travels a very long way out at the other.
The strength of gravity depends on distance. It is strongest when the comet is closest to the Sun and weaker when it is far away. This changes the comet's speed:
As a comet falls towards the Sun it speeds up. As it moves away again, the Sun's gravity pulls back on it and it slows down. A planet on a nearly circular orbit stays at nearly the same distance, so its speed hardly changes.
A comet orbits the Sun. At point A it is close to the Sun. At point B it is far from the Sun. Which point has the stronger gravitational force and the higher speed?
Gravity is stronger the closer the comet is to the Sun, so point A has the stronger force. The comet moves fastest when gravity is strongest, so it is also fastest at point A. It is slowest at point B.
The time to complete one orbit is the orbital period. It depends on how big the orbit is. A longer path takes more time, and objects further from the Sun move more slowly.
| Object | Orbits | Time for one orbit |
|---|---|---|
| International Space Station | Earth | about 90 minutes |
| Moon | Earth | about 27 days |
| Earth | Sun | 1 year (about 365 days) |
| Neptune | Sun | about 165 years |
| Halley's comet | Sun | about 76 years |
You do not need to learn these figures. They show a pattern: the planets furthest from the Sun take the longest to orbit, and comets often take many years because their paths are so long. Calculating orbital speed comes in the next lesson, Orbital Speed.
For "describe the differences" questions, give the shape of each orbit and what it goes round, and write about both objects. Do not just write "comets are different".
(a) Explain why the Earth stays in orbit around the Sun. [2 marks]
(b) Describe two differences between the orbit of a comet and the orbit of a planet. [2 marks]
(c) A comet moves faster when it is closer to the Sun. Explain why. [2 marks]
(a) The Sun's gravity pulls the Earth towards the Sun (1). This force acts towards the centre and keeps changing the direction of the Earth's motion, so it moves in a curve (1).
(b) The comet's orbit is a very stretched ellipse, but the planet's orbit is nearly circular (1). The comet gets much closer to and much further from the Sun than the planet does, so its distance from the Sun changes a lot (1).
(c) Gravity is stronger when the comet is closer to the Sun (1), so the pull speeds the comet up and it moves fastest at this point (1).
In an "explain" question, link the force to its effect: say gravity acts towards the centre and that this changes the direction of motion.