What keeps a body in orbit

1. Every orbit is held together by gravitational attraction between the orbiting body and the much more massive body it travels around.
2. The force always points from the orbiting body towards the centre of the body it circles, so a planet is pulled towards the Sun and a moon is pulled towards its planet.
3. Orbit shapes lie between two extremes, a circle with the central body exactly in the middle, and a long thin ellipse with the central body well off to one end.
4. The distance from the central body stays fixed in a circular orbit, so the orbital speed also stays fixed.
5. The distance changes throughout an elliptical orbit, and the body moves fastest where it is closest to the central body and slowest where it is furthest away.
6. The reason is that the gravitational field is strongest close in, so the pull that speeds the body up on the way in is far larger than the pull it feels at the far end of the orbit.

Orbits of planets and moons

1. The eight planets travel around the Sun on slightly elliptical paths that are so close to circles that they are usually drawn as circles.
2. All of the planets orbit in the same direction and lie in almost the same flat plane, so the Solar System is shaped like a disc rather than a ball.
3. The time for one complete orbit is the orbital period, and it grows rapidly with distance from the Sun, from about $88$ days for Mercury to about $165$ years for Neptune.
4. A natural satellite orbits its planet rather than the Sun, and these orbits are also close to circular.
5. Our Moon takes about $27$ days to complete one orbit of the Earth at a mean distance of about $384000km$.
6. A moon is carried around the Sun as well, because it stays with the planet that holds it while that planet completes its own orbit.

Orbits of comets

1. A comet follows a highly elliptical orbit around the Sun, so its path is a long stretched oval rather than a near circle.
2. The Sun is not in the middle of that oval but close to one end, so the distance between the comet and the Sun changes enormously during a single orbit.
3. The comet accelerates as it falls in towards the Sun, reaches its greatest speed at the closest point, then slows steadily as it climbs back out.
4. It spends only a short part of each orbit near the Sun and most of it moving slowly through the outer Solar System, which is why comets are visible so rarely.
5. Comet orbits are also tilted out of the plane of the planets, and some run round the Sun in the opposite direction to the planets.
6. Halley's comet is the familiar example, returning to the inner Solar System roughly every $76$ years.

Orbits of artificial satellites

1. An artificial satellite is put into a chosen orbit by a rocket, and the orbit is chosen to suit the job the satellite has to do.
2. A low polar orbit passes over both poles at a height of a few hundred kilometres and takes about $90$ minutes for one circuit.
3. The Earth turns underneath a polar satellite, so it passes over a fresh strip of ground on every orbit and can eventually photograph the whole surface, which suits weather monitoring, mapping and spying.
4. A geostationary orbit lies directly above the equator at a height of about $36000km$ and takes exactly one day for one circuit.
5. Because the satellite and the ground turn together, the satellite stays above the same point on the equator and appears fixed in the sky.
6. That is what a satellite television dish needs, since the dish can be bolted in one position and never has to track a moving target.
7. Geostationary orbits therefore carry communication and broadcasting satellites, while polar orbits carry observation satellites.