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.

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.

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.

Natural satellite

Satellite of

Orbital speed (average)

Time for one orbit

Orbital radius (average)

The Moon

Earth

1.0 km/s

27.3 days

384 000 km

Io

Jupiter

17.33 km/s

1.77 days

421 700 km

Europa

Jupiter

13.74 km/s

3.55 days

670 900 km

Ganymede

(Solar System’s largest moon)

Jupiter

10.88 km/s

7.16 days

1.07 million km

Callisto

Jupiter

8.20 km/s

16.69 days

1.88 million km

Phobos

Mars

2.14 km/s

0.32 days

9400 km

Ceres

(largest asteroid

Sun

17.88 km/s

4.6 years

4.14 million km

Halley’s Comet

Sun

Aphelion ~1 km/s

Perihelion ~71 km/s

75–76 years

(next seen in 2061)

At perihelion – 90 000 km

At aphelion – 5.25 million km

Earth

Sun

29.8 km/s

365.26 days

149.6 million km

Jupiter

Sun

13.0 km/s

11.9 years

778.55 million km

Venus

Sun

35.0 km/s

224.7 days

108.21 million km

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.

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.

Nature of science

Galileo was able to view only four of Jupiter’s 64 moons. He was limited by the quality and power of the telescopes available to him at the time. Nowadays with far more powerful and high-quality telescopes, we can see further and with more detail. Gains in scientific knowledge and understanding are often connected to technological advances in the equipment used to aid our powers of observation.

Useful links

Learn more about the Moon from NASA.

Visit the Stardome Observatory and Planetarium website for resources and activities about comets.

    Published 27 March 2013, Updated 20 May 2015