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    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.

    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 Referencing Hub articles