The Solar System, Gravity and Orbits

Explained

Orbits, and the one force holding all of it together

The solar system is the Sun together with the planets, dwarf planets, moons, asteroids and comets that orbit it. The Sun is one star among hundreds of billions in the Milky Way, and the Milky Way is one galaxy among billions in the universe. Each of those scales is worth being able to state, because questions often start by asking you to place one inside the next.

Gravity

Gravity is an attractive force between masses. Two things determine its strength. Larger masses attract more strongly, and the force falls as the distance between the masses increases.

The consequences run right through the topic. The Sun holds the planets because its mass is vast. Gravitational field strength on the Moon is lower than on Earth because the Moon has less mass. Objects far from the Sun orbit slowly, because the pull on them is weaker.

Circular and elliptical orbits

In a circular orbit the distance from the central body does not change, so the strength of the pull does not change and the speed stays constant. Gravity still changes the direction of travel continuously, which is what keeps the object curving round instead of flying off in a straight line.

An elliptical orbit is a stretched circle, and the central body sits at one focus of the ellipse rather than in the middle. The distance now varies, so the pull varies. A comet moves fastest when it is closest to the Sun and slowest when it is furthest away, and comets have highly elliptical orbits, which is why they appear briefly and then disappear for decades.

Artificial satellites

A satellite in a low polar orbit travels round in roughly ninety minutes, passing over the poles while the Earth turns beneath it, so over time it sees the whole surface. That suits weather monitoring and imaging.

A geostationary satellite orbits much higher, directly above the equator, with a period of exactly twenty four hours. It therefore stays above the same point on the ground, which is what a fixed satellite dish requires. The higher orbit is slower, and that is why the period is longer.

Orbital speed for a circular orbit is given by v equals two pi r divided by T, which is simply the circumference of the orbit divided by the time to go round once.

Why orbiting objects do not fall in

They are falling, continuously. The satellite moves sideways fast enough that as gravity pulls it down, the surface it would land on curves away beneath it by the same amount. Remove the sideways motion and it would drop; remove gravity and it would fly off in a straight line. The orbit is the balance between the two.