Orbital Motion

An orbit is the curved path of a celestial object or spacecraft around a star, planet, or moon. This includes the Earth orbiting the Sun and the Moon orbiting the Earth, which are both almost circular orbits. It also includes the orbits of comets around the Sun, which are highly elliptical orbits (a stretched circle).

Newton’s first law states that if an object is moving at a constant velocity, it will continue moving at that velocity unless it is acted upon by a resultant force. This suggests that if a planet or satellite travels at a given velocity, it should maintain this direction and speed indefinitely.

What force, then, causes planets to move in elliptical orbits? The force is gravity.

In a circular orbit, gravity acts towards the centre of the orbit. At the same time, the planet or satellite must be travelling at the right speed. This is because the direction of the planet or satellite must keep changing to keep it in a circle. So, its velocity changes, but there is no change in its speed.

  • If the object is moving too fast, this will increase the radius of the orbit. So the gravitational pull will not be great enough to keep it in orbit, causing the object to spiral out into space.
  • If the object is moving too slow, then the radius of the orbit will decrease. So gravitational pull will be too strong, causing the object to be pulled inwards.

The orbit of the Moon

The Earth is very large, so it exerts a strong gravitational pull on the Moon. Due to the Moon’s large size and the high speed at which it travels, it has a lot of forward momentum. This means that it will not drift off into space or spiral out of control and collide with the Earth.

As the Earth’s gravitational pull is not strong enough to pull the Moon completely inwards, it will travel around a fixed path, in a stable orbit. The gravitational force acting on the Moon means that it is constantly changing directions, which allows it to stay in orbit.

  • The velocity changes; however, the speed does not

If the Moon’s orbit is pulled inwards, it will be closer to the Earth, therefore increasing the gravitational force. So, it will take a shorter time to orbit the Earth and it will travel at a greater speed in orbit.

Comets

As comets get close to the Sun, their ice vaporises. This can be seen as a tail when the comet travels through space. Comets travel to the depths of the solar system, so the time it takes to complete their orbits is much longer than that of moons and planets.

The image below shows the motion of a comet around the Sun. The orbit of a comet is highly elliptical (like a stretched circle).

The radius of a stable orbit must change if the speed of the orbiting object changes. The speed of the comet increases as it passes the Sun. This is because the Sun exerts a gravitational pull, giving its orbit a smaller radius. They also slow down as they get further from the Sun, giving their orbit a larger radius.

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