Satellite (natural)

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Earth's Moon

A natural satellite is a natural object that goes around (orbits) a planet.[1][2] 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,[3] 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 | edit source]

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 Apollo 11 mission helped Neil Armstrong and Buzz Aldrin become the first people to walk on the Moon. They did this in July 20, 1969.

Orbits[change | edit source]

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.

Selected moons, with the Earth to scale. Nineteen moons are large enough to be round, and one, Titan, has a substantial atmosphere.

Moons of moons[change | edit source]

Artist impression of Rhea's rings

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 [4] 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,[5] something that is known with shepherd moons.

Asteroid moons[change | edit source]

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 | edit source]

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
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
2000-3000 Triton Eris
Pluto
1500-2000 Rhea Titania
Oberon
(136472) 2005 FY9
90377 Sedna
1000-1500 Iapetus
Dione
Tethys
Umbriel
Ariel
Charon (136108) 2003 EL61
90482 Orcus
50000 Quaoar
500-1000 Enceladus Ceres
20000 Varuna
28978 Ixion
2 Pallas4 Vesta
many more TNOs
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
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[6]
Hydra[6]
Menoetius[7]
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[8]
S/2000 (762) 1
S/2002 (121) 1
Romulus[9]
Petit-Prince[10]
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


Planets that have moons[change | edit source]

Planets in our Solar System that have moon(s):

Dwarf planets that have moons[change | edit source]

Planets that do not have moons[change | edit source]

Planets in our Solar System that do not have moons:

References[change | edit source]

  1. Arnold, Brian; Woolley, Steve; Johnson, Penny (2009). Edexcel IGCSE Physics Student Book. Pearson Education. p. 51. ISBN 9780435966904.
  2. Holland, Simon (2001). Dorling Kindersley Eyewonder: Space. Dorling Kindersley. ISBN 9781405304726.
  3. Kidd, D. A. (2008). Collins Pocket Latin Dictionary. HarperCollins. p. 207. ISBN 9780007263745.
  4. "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.
  5. "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.
  6. 6.0 6.1 Diameters of the new Plutonian satellites are still very poorly known, but they are estimated to lie between 44 and 130 km.
  7. (617) Patroclus I Menoetius
  8. (22) Kalliope I Linus
  9. (87) Sylvia I Romulus
  10. (45) Eugenia I Petit-Prince