# Astrophysics: Gravity and Orbits

## Astrophysics: Gravity and Orbits

• Gravity is a force that pulls objects towards each other. The more massive an object is, the stronger its gravitational pull.

• The force of gravity between two objects depends on two things: the masses of the objects and the distance between them.

• Sir Isaac Newton’s Law of Universal Gravitation states that every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres.

• The gravity of the Sun keeps the planet in their orbits. The balance of the gravitation pull and the inertia (or the resistance to change in motion) of the planets results in an elliptical path around the Sun.

• The orbit of a planet around the Sun, or a satellite around a planet, is not a perfect circle. It is an ellipse, or oval shape. The Sun or planet being orbited is at one of the two foci of the ellipse.

• The closer a planet is to the Sun, the stronger the Sun’s gravitational pull and the faster the planet moves in its orbit.

• Kepler’s First Law, or the Law of Ellipses, states that each planet orbits the Sun in a path called an ellipse with the Sun at one focus.

• Kepler’s Second Law, the Law of Equal Areas, details that a line that connects a planet to the Sun sweeps out equal areas in equal times.

• Kepler’s Third Law, the Law of Harmonies, compares the orbital periods and radii of the orbits of different planets.

• Astrophysics involves measuring the gravitational forces between celestial bodies to understand their movements and interactions. This information can then be used to predict future movements and understand the history of the universe.

• When an object is in orbit, it is actually falling towards the object it’s orbiting, but it’s also moving forward fast enough that it keeps missing it. This is what’s known as freefalling.

• The Moon orbits Earth because of the pull of Earth’s gravity. The Moon’s inertia - its tendency to keep moving in a straight line at a constant speed - would carry it straight off into space if there were no gravity to hold it in its orbit.

• Tides on Earth are caused by the Moon’s gravity pulling on the Earth. This causes the sea to bulge out in the direction of the Moon. The bulge on the other side of the Earth is caused by the Earth being pulled away from the sea.

• The gravitational force between Earth and an object becomes less as the object is taken further away from Earth. This is why there is no gravitational pull in space.