A natural satellite is a natural object that goes around (orbits) a planet. It is usually called a moon and is large and ball-shaped. The Earth has only one moon. Some other planets have many moons, and some have none. When people write just "the moon", they are usually talking about the moon of the Earth. Earth's moon is written with a capital letter, Moon. The Latin word for the moon is luna, which is why the adjective used to talk about the moon is "lunar". For example, lunar eclipse.
Anything that goes around a planet is called a satellite. Moons are natural satellites. People also use rockets to send machines into orbit around the Earth. These machines are called artificial (man-made) satellites. They help us to communicate, the telephones and televisions need these satellites.
Earth's moon [change]
Moons do not make their own light. We can see the Earth's moon because it acts like a mirror, and reflects the light of the Sun. The same half of the moon faces toward Earth at all times, no matter where it moves. But different parts of the moon are lit up by the Sun, so it looks different at different times of the month. This change as seen from Earth is called the phases of the moon, or lunar phases.
A moon's cycle is the time the moon takes to change from looking very bright and round to looking very small and thin, and then back to bright and round again. In the case of the Earth's moon, this is about four weeks. It does this about 13 times in one year. The moon's cycle is about 28 days, a bit shorter than a calendar month.
The orbit of a moon or other satellite is affected by two forces: gravity, and the centripetal force. For example, the Earth's moon is kept in orbit by the gravitational pull from the Earth. This is also the way the Earth is attracted to the Sun, and stays in its orbit. The orbit of the Earth's moon actually causes the tides and waves on Earth.
Moons of moons [change]
No moons that belong to moons have been found. In most cases, the tidal effects of the main body would make such as unstable.
However, math completed after the recent finding  of a possible ring system around Saturn's moon Rhea show that Rhean orbits would be stable. Also, the rings are thought to be narrow, something that is known with shepherd moons.
Asteroid moons [change]
The finding of 243 Ida's moon Dactyl in the early 1990s was the proof that some asteroids have moons; indeed, 87 Sylvia has two. Some, such as 90 Antiope, are double asteroids with two same-sized parts.
Moons of the Solar System [change]
The biggest moons in the Solar System (those bigger than about 3000 km across) are Earth's moon, Jupiter's Galilean moons (Io, Europa, Ganymede, and Callisto), Saturn's moon Titan, and Neptune's captured moon Triton.
The following is a table grouping the moons of the solar system by diameter. The column on the right has some notable planets, dwarf planets, asteroids, and Trans-Neptunian Objects for comparing. It is normal for moons to be named after people from mythology.
Planets that have moons [change]
Planets in our Solar System that have moon(s):
Dwarf planets that have moons [change]
Planets that do not have moons [change]
Planets in our Solar System that do not have moons:
- Arnold, Brian; Woolley, Steve; Johnson, Penny (2009). Edexcel IGCSE Physics Student Book. Pearson Education. p. 51. ISBN 9780435966904.
- Holland, Simon (2001). Dorling Kindersley Eyewonder: Space. Dorling Kindersley. ISBN 9781405304726.
- Kidd, D. A. (2008). Collins Pocket Latin Dictionary. HarperCollins. p. 207. ISBN 9780007263745.
- "The Dust Halo of Saturn's Largest Icy Moon, Rhea -- Jones et al. 319 (5868): 1380 -- Science". http://www.sciencemag.org/cgi/content/abstract/319/5868/1380?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=rhea&searchid=1&FIRSTINDEX=0&resourcetype=HWCIT. Retrieved 2008-03-06.
- "Saturn satellite reveals first moon rings - 06 March 2008 - New Scientist". http://www.newscientist.com/article/dn13421-saturn-satellite-reveals-first-moon-rings.html. Retrieved 2008-03-06.
- Diameters of the new Plutonian satellites are still very poorly known, but they are estimated to lie between 44 and 130 km.
- (617) Patroclus I Menoetius
- (22) Kalliope I Linus
- (87) Sylvia I Romulus
- (45) Eugenia I Petit-Prince
☾ = moon(s) ∅ = rings
|Mercury||Venus||Earth ☾||Mars ☾|
|Jupiter ☾ ∅||Saturn ☾ ∅||Uranus ☾ ∅||Neptune ☾ ∅|
|Dwarf planets||Ceres||Pluto ☾||Haumea ☾||Makemake|
|Groups and families: Vulcanoids · Near-Earth asteroids · Asteroid belt
Jupiter Trojans · Centaurs · Neptune Trojans · Asteroid moons · Meteoroids · Pallas · Juno · Vesta · Hygiea · Interamnia · Europa
|See also the list of asteroids.|
|Kuiper belt – Plutinos: Orcus · Ixion – Cubewanos: Varuna ·
Quaoar · Huya
|Scattered disc: Sedna|
|Comets||Periodic comets and non-periodic comets
Damocloids · Oort cloud
|See also the list of solar system objects|