Natural satellite
From the Simple English Wikipedia, the free encyclopedia that anyone can change
For the Earth's Moon see The Moon
A moon is a large, usually ball-shaped object that goes around (orbits) a planet. 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. Often Earth's moon is written with a capital letter, like Moon. The Latin word for the moon is Luna.
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.
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[change] Earth's Moon
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.
Neil Armstrong and Buzz Aldrin were the first people to walk on the Moon. They did this in July 20, 1969.
[change] Orbits
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.
[change] Moons of moons
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 [1] 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,[2] something that is known with shepherd moons.
[change] Asteroid moons
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.
[change] Moons of the Solar System
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.
| Mean diameter (km) |
Satellites of planets | Dwarf planet satellites | Satellites of SSSBs |
Non-satellites for comparison |
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|---|---|---|---|---|---|---|---|---|---|---|
| Earth | Mars | Jupiter | Saturn | Uranus | Neptune | Pluto | Eris | |||
| 6000-7000 | Mars | |||||||||
| 5000-6000 | Ganymede | Titan | ||||||||
| 4000-5000 | Callisto | Mercury | ||||||||
| 3000-4000 | The Moon (Luna) |
Io Europa |
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| 2000-3000 | Triton | Eris Pluto |
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| 1500-2000 | Rhea | Titania Oberon |
(136472) 2005 FY9 90377 Sedna |
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| 1000-1500 | Iapetus Dione Tethys |
Umbriel Ariel |
Charon | (136108) 2003 EL61 90482 Orcus 50000 Quaoar |
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| 500-1000 | Enceladus | Ceres 20000 Varuna 28978 Ixion 2 Pallas, 4 Vesta many more TNOs |
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| 250-500 | Mimas Hyperion |
Miranda | Proteus Nereid |
Dysnomia | S/2005 (2003 EL61) 1 S/2005 (79360) 1 |
10 Hygiea 511 Davida 704 Interamnia and many others |
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| 100-250 | Amalthea Himalia Thebe |
Phoebe Janus Epimetheus |
Sycorax Puck Portia |
Larissa Galatea Despina |
S/2005 (2003 EL61) 2 many more TNOs |
many | ||||
| 50-100 | Elara Pasiphaë |
Prometheus Pandora |
Caliban Juliet Belinda Cressida Rosalind Desdemona Bianca |
Thalassa Halimede Neso Naiad |
Nix[3] Hydra[3] |
Menoetius[4] S/2000 (90) 1 many more TNOs |
many | |||
| 10-50 | Phobos | Carme Metis Sinope Lysithea Ananke Leda Adrastea |
Siarnaq Helene Albiorix Atlas Pan Telesto Paaliaq Calypso Ymir Kiviuq Tarvos Ijiraq Erriapus |
Ophelia Cordelia Setebos Prospero Perdita Mab Stephano Cupid Francisco Ferdinand Margaret Trinculo |
Sao Laomedeia Psamathe |
Linus[5] S/2000 (762) 1 S/2002 (121) 1 Romulus[6] Petit-Prince[7] S/2003 (283) 1 S/2004 (1313) 1 and many TNOs |
many | |||
| less than 10 | 2006 RH120 (temporary) | Deimos | at least 47 | at least 21 | many | many | ||||
[change] Planets that have moons
Planets in our Solar System that have moon(s):
[change] Dwarf planets that have moons
[change] Planets that do not have moons
Planets in our Solar System that do not have moons:
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| The Sun · Mercury · Venus · Earth · Mars · Ceres · Jupiter · Saturn · Uranus · Neptune · Pluto · Eris |
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Planets · Dwarf planets · Moons: Terrestrial · Martian · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Eridian
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Small bodies: Meteoroids · Asteroids/Asteroid moons (Asteroid belt) · Centaurs · TNOs (Kuiper belt/Scattered disc) · Comets (Oort cloud)
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| See also: List of solar system objects |
