# Orbits of Planets and Satellites

## Orbits of Planets and Satellites

Basics of Orbit

• An orbit is the curved path that an object, such as a planet or a satellite, follows around another object due to the force of gravity.
• This gravitational force provides the necessary centripetal force to keep the object in the curved path of orbit.

Newton’s Law of Universal Gravitation

• 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.
• This law underpins the concept of orbit. The gravitational force between the two objects keeps them orbiting around the center of mass.

Elliptical Orbits

• Both planets and satellites typically move in elliptical orbits, not perfect circles.
• Johannes Kepler first discovered this and it was later proven by Newton’s law of gravitation.
• An elliptical orbit means that the distance between the orbiting body and the body it’s orbiting changes as it follows its path.
• The closest point to the body being orbited is called perigee, and the furthest point is called apogee.

Kepler’s Laws

• Kepler’s First Law (The Law of Orbits): All planets move in elliptical orbits with the sun at one focus.
• Kepler’s Second Law (The Law of Areas): An imaginary line drawn from the centre of the sun to the centre of the planet will sweep out equal areas in equal intervals of time.
• Kepler’s Third Law (The Law of Periods): The square of the period of any planet is proportional to the cube of the semi-major axis of its orbit.

Gravity and Orbital Speed

• Orbital speed is the speed at which an object needs to travel to stay in orbit and is determined by the gravitational force acting upon it.
• Less gravity requires a lower speed; more gravity requires a higher speed.
• The closer an object is to the body it is orbiting, the stronger the gravitational force and the faster the required orbital speed.

Geostationary Satellites

• Geostationary satellites orbit around the Earth’s equator at the same rate the Earth rotates. This means they almost stand still relative to an observer on Earth’s surface.
• These are typically positioned 35,786 kilometers above the Earth’s equator. At this height, one orbit takes 24 hours.
• They are commonly used for weather monitoring, telecommunications, and satellite TV.