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An orbit is a fixed and repeating path followed by an object in space around another body.

Example: Earth has a natural satellite “The Moon” which moves around the earth in a fixed path which is called an orbit.

Sputnik -1 the first artificial satellite launched by the Soviet Union on October 4, 1957.

The Shape of an Orbit

The shape of an orbit is generally ellipse (oval shape) but sometimes it may also be circular.

Since the shape of the orbit is elliptical, satellites that move in an orbit doesn’t always stay at the same distance from the planet or star.                         

Periapsis and Apoapsis

The closest point a satellite can come near to the Earth is called Perigee.

The farthest point a satellite can be away from the Earth is called Apogee.

Since the orbit of the Moon around the Earth is elliptical, the moon is at different distances from Earth at Perigee and Apogee.

Perigee and Apogee
The difference between apogee and perigee.

On an Average, the distance from Moon to Earth is 384,400 Kilometers.

Perigee and Apogee
A Full Moon looks bigger and brighter when it occurs around perigee and smaller and dimmer at apogee.

The Moon passes through the perigee and apogee about once a month.

Perigee and Apogee are also called extreme points or apsides

Earth Revolution

Earth Travels at nearly 113,000km/hr or 70,000 miles/hr around the Sun in an elliptical orbit.

Every year on January 3rd, the Earth is about 147 million km away from Sun, which is the closest point termed as Perihelion.

Similarly, every year on July 4th, the Earth is about 152 million kilometers away from Sun, which is the farthest point termed as Aphelion.

Earth’s orbit around the Sun changes.

On an average Earth is about 150 million kilometers from Sun.

Distance from sun plays a very minor role in the seasonal temperature variation.

Orbital Period

Time taken by the satellite or a planet to move around completely in an orbit is called Orbital Period.

Orbital Period of Earth around the Sun is 365.256 Days or 365 Days.

Orbital Period of Moon around the Earth is 27.32 Days or 27 Days.

Seasons on Earth occur only due to the tilt of the Earth.

Read Introduction to Earth Orbits

Glossary

Anomalistic monthTime taken by the Moon to travel from perigee to perigee, is called the anomalistic month, and it takes around 27.55455 days to complete one anomalistic month.

Apoapsis – Furthest point on an orbit to the orbited body is called the apoapsis.

Periapsis – Closest point on an orbit to the orbited body is called the periapsis.