Natural satellites

A satellite is anything that orbits around a larger object. A natural satellite is any celestial body in space that orbits around a larger body. Moons are called natural satellites because they orbit planets.

Satellites that are made by people and launched into orbit using rockets are called artificial satellites. There are thousands of artificial satellites orbiting the Earth.

The Moon

Any large object that orbits around a planet is called a moon (small ‘m’). The Earth has one moon called the Moon (capital ‘M’). The Moon takes 27.3 days to orbit the Earth once, moving at an orbital speed of 1 km/s.

Find out more about our Moon here.

Moons around other planets

Galileo was the first person to discover that other planets can have moons. He saw that Jupiter had four moons with his newly invented telescope in 1610 AD. At first, he thought they were stars, but he noticed that, each night, the four points of light appeared to change positions slightly. He realised they were actually moons orbiting around Jupiter. Another astronomer of the time, Simon Marius, named them Io, Europa, Ganymede and Callisto after the lovers of Zeus, the ancient Greek mythological King of the Gods and Men. We now know that Jupiter has at least 64 moons.

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Jupiter’s largest moons

Galileo discovered Jupiter’s largest moons in 1610. Another astronomer of the time, Simon Marius, named them Io, Europa, Ganymede and Callisto after the lovers of Zeus, the ancient Greek mythological King of the Gods and Men. Currently Jupiter has 53 named moons and another 26 awaiting official names. Combined, scientists now think Jupiter has 79 moons.

Image courtesy of NASA.

All except two of the planets (Venus and Mercury) in our Solar System have natural satellites called moons.

Other natural satellites in our Solar System

Planets, asteroids and comets orbit around stars such as our Sun and so can also be thought of as natural satellites. Our Solar System has eight official planets as well as millions of minor planets, asteroids, comets and other objects orbiting around the Sun. All of these can be thought of as natural satellites.

All of these natural satellites are held in orbit by the attraction of gravity between the satellite and the object it is orbiting.

For elliptical orbits, perihelion means closest orbital approach to the Sun, and aphelion means furthest orbital distance from the Sun.

Earth’s natural satellite: the Moon

The Moon orbits the Earth once every 27.3 days. This time period is called the orbital period or sidereal period. However, the time from one full moon to the next is 29.5 days (called the synodic period). This extra time is because of the change in angle as the Earth revolves around the Sun.

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Moon months

The diagram is an explanation of the moon’s sidereal and synodic months. One full orbit of the Earth takes 27.32 days – the sidereal month. From one full moon to the next takes 29.53 days – the synodic month.

The Moon appears to move across the sky from east to west, in the same direction as the Sun moves. However, this motion is apparent and not true. The Moon is in fact orbiting the Earth in a west to east direction. The reason that it appears to rise in the east and set in the west is because of the Earth’s very fast axial rotation. The Earth rotates once each day, and the Moon orbits the Earth once every 27.3 days. This means that the Moon’s true orbital motion around the Earth can be seen only indirectly. The distance moved by the Moon in 1 day can be observed by comparing its position in the sky at one time with its new position exactly 24 hours later.

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Apparent motion of the Moon

The apparent motion of the Moon from night to night. Each night, it moves around 13° to the east, which is approximately 26 Moon diameters.