# Orbits

Satellites orbit planets, which orbit the Sun, which in turn orbits the centre of the Milky Way. But why?

Until Isaac Newton formulated his Laws of Motion it was generally thought that to keep an object moving it was necessary to use a force to push or pull it. This agreed with everyday experience where things came to a halt after the moving force was removed.

Galileo made a set of experiments that led him to believe that movement was an intrinsic property that needed no maintenance and Newton based his theory of motion on this concept.

## Newton's Laws of Motion

• A body in motion tends to remain in motion, or remain stopped, unless acted on by a force.
• The rate of change of a body's momentum is equal to the force acting on it. This is nowadays rephrased as the force on a body equals the product of its mass and its acceleration.
• For every action there is always an equal and opposite reaction.

From these three simple laws all the motions of bodies acted on by forces can be derived. Newton himself applied his laws to the motion of the Moon and the planets. He realised that the force of gravity was the force that controlled the planets and the Moon in their orbits around the Sun and the Earth.

## The Sun and the planets

The closer a planet is to the Sun the greater the gravitational pull and so the faster the planet must move to stay in an orbit.

The planets do not, however, move in circular orbits. Newton indicated that the planets move in elliptical orbits about the Sun.

Nowadays, we can observe the orbits very exactly and we can take into account not only the gravitational pull of the Sun but also the much smaller pulls of each of the other planets. These effects make the paths of the planets depart slightly from the classical ellipses described by the simplification of considering only the pull of the Sun.

## Halley’s Comet

One of the great achievements of Newton's theory was the idea by Halley that comets moved in either parabolic or elliptical orbits. He calculated the orbits of many comets and showed that several supposed different comets followed the same orbit. He conjectured that they were one and the same object and further predicted that it would reappear in a stated year. We now call this comet by Halley's name in honour of the fact that his analysis and his prediction were right.

## Satellites

For satellites in orbit at about 100 miles above the surface the period of revolution is close to 90 minutes. As the height of the orbit above the Earth is increased so the strength of the Earth's gravitational pull decreases, the speed of the satellite slows and its period of revolution increases. At a height of 35,900 km the orbital period of the satellite in a circular orbit is one day and so it remains above the same point on the Earth's equator. We call this a geostationary or geosynchronous orbit.

## The Milky Way

In the same way as the Earth is in orbit around the Sun, the Sun is in an orbit around the centre of our galaxy, the Milky Way. The time taken for one revolution is very long, about 200,000,000 years.