8.1.7Space Physics
Non-Circular Orbits
5 min read 4 parts Lesson 7 of 10
Section 1 of 4Shapes of Orbits
Orbits are not always circular, and gravity determines the shape of every orbit around a central body.
Circular orbit
- Circular orbit Gravity acts at right angles to the velocity, so the distance from the central body and the speed both stay constant.
Elliptical orbit
- Elliptical orbit An oval shape in which the distance from the central body changes during each orbit.
Comet orbit
- Comet orbit Many comets follow highly elliptical orbits, spending most of their time far from the Sun.
Gravity
- Gravity The same gravitational attraction that holds a circular orbit together also shapes an elliptical one.
Section 2 of 4Changing Speed on an Elliptical Orbit
The speed of an orbiting body changes as it moves through its orbit, because gravity acts along the path at different points.
1
As an orbiting body moves towards the Sun, gravity has a component along its path, which speeds it up.
2
At the closest point to the Sun, called perihelion, the body moves fastest in its orbit.
3
As the body moves away from the Sun, gravity has a component opposing its motion, which slows it down.
4
At the furthest point from the Sun, called aphelion, the body moves most slowly in its orbit.
Section 3 of 4Comparing Orbits
Circular and elliptical orbits differ in shape, speed and the kinds of body that follow them.
Comparing Orbits
Shape
- Shape A circular orbit has a constant radius, while an elliptical orbit has a changing distance from the central body.
Speed
- Speed A circular orbit has a constant speed, while an elliptical orbit is fastest at its closest point and slowest at its furthest.
Examples
- Examples Earth's orbit is very nearly circular, while comets follow elliptical orbits that take them close to the Sun and then far away.
| Feature | Circular orbit | Elliptical orbit |
|---|---|---|
| Shape | Constant distance from the central body | Changing distance from the central body |
| Speed | Constant | Fastest at the closest point, slowest at the furthest |
| Example | Earth's orbit, which is very nearly circular | Comets |
Section 4 of 4Exam Checklist & Tips
8.1.7 – Key Points for the Exam
- Elliptical orbit An oval-shaped orbit in which the distance to the central body changes.
- Perihelion The point in an orbit around the Sun where the body is closest to the Sun.
- Aphelion The point in an orbit around the Sun where the body is furthest from the Sun.
- Comet An icy body that often follows a highly elliptical orbit around the Sun.
- Central body The large object that a smaller object orbits, such as the Sun for planets.
Common Mistakes
- Saying a body on an elliptical orbit moves at a constant speed, when its speed changes around the orbit.
- Claiming the body is weightless at its furthest point because gravity is zero there, when gravity is still acting.
- Stating that a comet's orbit is circular because the path is curved.
- Drawing the gravitational force pointing away from the central body, when it always points towards it.