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This page answers questions about moons. The questions are:
You can also find information about moons in general on the Moons Page from the Universe Family Tree, and about the Moon of the Earth in particular on the Moon Page of the AnswerBook.
There is no official definition of a moon, but if you look at celestial objects that are called moons, then you see that in practice a moon is a celestial object that orbits directly around a bigger celestial object that is not a star (such as the Sun).
In the same fashion, a satellite is more generally a thing that orbits around a bigger thing but is not a part of that bigger thing. That bigger thing can be a star, or even something like a galaxy.
Moons are made up of rocks and ice. Moons that are further away from the Sun tend to have more ice than moons that are closer to the Sun. Our Moon and the moons of Mars have no ice at all (except perhaps in deep craters near the poles), but the moon Europa of Jupiter has a layer of ice that is many kilometers thick.
It is not easy to say how many moons there are around the Sun, because that depends a lot on what you consider to be a moon around the Sun.
If you keep to the strict definition that I gave earlier, then the Sun has no moons at all, because then something that orbits directly around the Sun is by definition not a moon.
At the moment, at least 60 moons orbit around the planets with an official name. You could say that there are at least 60 named moons around the Sun.
You can also, less strictly, view the planets, comets, and asteroids as "moons" of the Sun. In that case, the number of officially named moons around the Sun is counted in the tens of thousands, of which most are asteroids.
The number of moons that has not yet been named or even discovered is most likely must greater still. There is no official least size for a moon, so with larger telescopes and better equipment you'll find ever smaller rocks and pebbles and clumps of ice that orbit around the Sun or the planets.
Here is a list of moons of all planets. New small moons are still being discovered, so the list is probably not complete. I updated the list on 2006-05-22. The information came mostly from //ssd.jpl.nasa.gov/?sat_discovery and //ssd.jpl.nasa.gov/?sat_elem. The information provided for each moon is: the name of the planet; the name of the moon; the distance \(d\) to the planet, measured in thousands of kilometers (Mm; 1 km = 0.62 mi); the radius \(R\) of the moon, measured in kilometers; the strength \(g\) of the gravity at the surface, compared to that on Earth; the orbital period \(P\) in hours (h), days (d), and years (a); the year in which the moon was discovered; the alternative name; the provisional name. Newly discovered moons get a provisional name right away. When their orbit becomes sufficiently well known, then they receive an official name. See the page about names for more information about the origin of the names.
Planet | Moon | \({d}\) | \({R}\) | \({g}\) | \({P}\)/h | \({P}\)/d | \({P}\)/a | Discovered | Name 2 | Provisional Name |
---|---|---|---|---|---|---|---|---|---|---|
Earth | Moon | 384.4 | 1737.07 | 0.1658 | 658.7 | 27.4 | 0.1 | |||
Mars | Phobos | 9.38 | 11.14 | 0.0006 | 7.7 | 0.3 | 1877 | MI | ||
Mars | Deimos | 23.46 | 6.2 | 0.0003 | 30.3 | 1.3 | 1877 | MII | ||
Jupiter | Metis | 128 | 21.5 | 0.0018 | 7.1 | 0.3 | 1979 | JXVI | S/1979 J3 | |
Jupiter | Adrastea | 129 | 10.13 | 0.0005 | 7.2 | 0.3 | 1979 | JXV | S/1979 J1 | |
Jupiter | Amalthea | 181.4 | 86.21 | 0.0019 | 12.0 | 0.5 | 1892 | JV | ||
Jupiter | Thebe | 221.9 | 49.75 | 0.0041 | 16.2 | 0.7 | 1979 | JXIV | S/1979 J2 | |
Jupiter | Io | 421.8 | 1821.6 | 0.1832 | 42.5 | 1.8 | 1610 | JI | ||
Jupiter | Europa | 671.1 | 1560.8 | 0.1341 | 85.2 | 3.6 | 1610 | JII | ||
Jupiter | Ganymede | 1070.4 | 2631.2 | 0.1457 | 171.7 | 7.2 | 1610 | JIII | ||
Jupiter | Callisto | 1882.7 | 2410.3 | 0.1261 | 400.4 | 16.7 | 1610 | JIV | ||
Jupiter | Themisto | 7284 | 4 | 0.0003 | 127.0 | 0.3 | 2000 | JXVIII | S/1975 J1 = S/2000 J1 | |
Jupiter | Leda | 11165 | 10 | 0.0007 | 241.0 | 0.7 | 1974 | JXIII | ||
Jupiter | Himalia | 11461 | 85 | 0.0064 | 250.6 | 0.7 | 1904 | JVI | ||
Jupiter | Lysithea | 11717 | 18 | 0.0013 | 259.1 | 0.7 | 1938 | JX | ||
Jupiter | Elara | 11741 | 43 | 0.0032 | 259.8 | 0.7 | 1905 | JVII | ||
Jupiter | S/2000 J11 | 12560 | 2 | 0.0000 | 287.5 | 0.8 | 2000 | S/2000 J11 | ||
Jupiter | S/2003 J12 | 15912 | 0.5 | 0.0000 | 410.0 | 1.1 | 2003 | S/2003 J12 | ||
Jupiter | Carpo | 16989 | 1.5 | 0.0001 | 452.3 | 1.2 | 2003 | JXLVI | S/2003 J20 | |
Jupiter | Euporie | 19304 | 1 | 0.0001 | 547.8 | 1.5 | 2001 | JXXXIV | S/2001 J10 | |
Jupiter | S/2003 J3 | 20221 | 1 | 0.0001 | 587.3 | 1.6 | 2003 | S/2003 J3 | ||
Jupiter | S/2003 J18 | 20514 | 1 | 0.0001 | 600.1 | 1.6 | 2003 | S/2003 J18 | ||
Jupiter | Orthosie | 20720 | 1 | 0.0001 | 609.2 | 1.7 | 2001 | JXXXV | S/2001 J9 | |
Jupiter | Euanthe | 20797 | 1.5 | 0.0001 | 612.6 | 1.7 | 2001 | JXXXIII | S/2001 J7 | |
Jupiter | Harpalyke | 20858 | 2.2 | 0.0002 | 615.3 | 1.7 | 2000 | JXXII | S/2000 J5 | |
Jupiter | Praxidike | 20907 | 3.4 | 0.0003 | 617.4 | 1.7 | 2000 | JXXVII | S/2000 J7 | |
Jupiter | Thyone | 20939 | 2 | 0.0002 | 618.9 | 1.7 | 2001 | JXXIX | S/2001 J2 | |
Jupiter | S/2003 J16 | 20963 | 1 | 0.0001 | 619.9 | 1.7 | 2003 | S/2003 J16 | ||
Jupiter | Iocaste | 21061 | 2.6 | 0.0002 | 624.3 | 1.7 | 2000 | JXXIV | S/2000 J3 | |
Jupiter | Mneme | 21069 | 1 | 0.0001 | 624.6 | 1.7 | 2003 | JXL | S/2003 J21 | |
Jupiter | Hermippe | 21131 | 2 | 0.0002 | 627.4 | 1.7 | 2001 | JXXX | S/2001 J3 | |
Jupiter | Thelxinoe | 21162 | 1 | 0.0000 | 628.8 | 1.7 | 2004 | JXLII | S/2003 J22 | |
Jupiter | Helike | 21263 | 2 | 0.0002 | 633.3 | 1.7 | 2003 | JXLV | S/2003 J6 | |
Jupiter | Ananke | 21276 | 14 | 0.0010 | 633.9 | 1.7 | 1951 | JXII | ||
Jupiter | S/2003 J15 | 22627 | 1 | 0.0001 | 695.2 | 1.9 | 2003 | S/2003 J15 | ||
Jupiter | Eurydome | 22865 | 1.5 | 0.0001 | 706.2 | 1.9 | 2001 | JXXXII | S/2001 J4 | |
Jupiter | Arche | 22931 | 1.5 | 0.0001 | 709.2 | 1.9 | 2002 | JXLIII | S/2002 J1 | |
Jupiter | S/2003 J17 | 23001 | 1 | 0.0001 | 712.5 | 2.0 | 2003 | S/2003 J17 | ||
Jupiter | Pasithee | 23004 | 1 | 0.0001 | 712.6 | 2.0 | 2001 | JXXXVIII | S/2001 J6 | |
Jupiter | S/2003 J10 | 23042 | 1 | 0.0001 | 714.4 | 2.0 | 2003 | S/2003 J10 | ||
Jupiter | Chaldene | 23100 | 1.9 | 0.0001 | 717.1 | 2.0 | 2000 | JXXI | S/2000 J10 | |
Jupiter | Isonoe | 23155 | 1.9 | 0.0001 | 719.7 | 2.0 | 2000 | JXXVI | S/2000 J6 | |
Jupiter | Erinome | 23196 | 1.6 | 0.0001 | 721.6 | 2.0 | 2000 | JXXV | S/2000 J4 | |
Jupiter | Kale | 23217 | 1 | 0.0001 | 722.6 | 2.0 | 2001 | JXXXVII | S/2001 J8 | |
Jupiter | Aitne | 23229 | 1.5 | 0.0001 | 723.1 | 2.0 | 2001 | JXXXI | S/2001 J11 | |
Jupiter | Taygete | 23280 | 2.5 | 0.0002 | 725.5 | 2.0 | 2000 | JXX | S/2000 J9 | |
Jupiter | S/2003 J9 | 23384 | 0.5 | 0.0000 | 730.4 | 2.0 | 2003 | S/2003 J9 | ||
Jupiter | Carme | 23404 | 23 | 0.0017 | 731.3 | 2.0 | 1938 | JXI | ||
Jupiter | Sponde | 23487 | 1 | 0.0001 | 735.2 | 2.0 | 2001 | JXXXVI | S/2001 J5 | |
Jupiter | Megaclite | 23493 | 2.7 | 0.0002 | 735.5 | 2.0 | 2000 | JXIX | S/2000 J8 | |
Jupiter | S/2003 J5 | 23495 | 2 | 0.0002 | 735.6 | 2.0 | 2003 | S/2003 J5 | ||
Jupiter | S/2003 J19 | 23533 | 1 | 0.0001 | 737.4 | 2.0 | 2003 | S/2003 J19 | ||
Jupiter | S/2003 J23 | 23563 | 1 | 0.0001 | 738.8 | 2.0 | 2003 | S/2003 J23 | ||
Jupiter | Kalyke | 23566 | 2.6 | 0.0002 | 738.9 | 2.0 | 2000 | JXXIII | S/2000 J2 | |
Jupiter | S/2003 J14 | 23614 | 1 | 0.0001 | 741.2 | 2.0 | 2003 | S/2003 J14 | ||
Jupiter | Pasiphae | 23624 | 30 | 0.0023 | 741.6 | 2.0 | 1908 | JVIII | ||
Jupiter | Eukelade | 23661 | 2 | 0.0002 | 743.4 | 2.0 | 2003 | JXLVII | S/2003 J1 | |
Jupiter | S/2003 J4 | 23930 | 1 | 0.0001 | 756.1 | 2.1 | 2003 | S/2003 J4 | ||
Jupiter | Sinope | 23939 | 19 | 0.0014 | 756.5 | 2.1 | 1914 | JIX | ||
Jupiter | Hegemone | 23947 | 1.5 | 0.0001 | 756.9 | 2.1 | 2003 | JXXXIX | S/2003 J8 | |
Jupiter | Aoede | 23981 | 2 | 0.0002 | 758.5 | 2.1 | 2003 | JXLI | S/2003 J7 | |
Jupiter | Kallichore | 24043 | 1 | 0.0001 | 761.5 | 2.1 | 2003 | JXLIV | S/2003 J11 | |
Jupiter | Autonoe | 24046 | 2 | 0.0002 | 761.6 | 2.1 | 2001 | JXXVIII | S/2001 J1 | |
Jupiter | Callirrhoe | 24103 | 4.3 | 0.0003 | 764.3 | 2.1 | 1999 | JXVII | S/1999 J1 | |
Jupiter | Cyllene | 24349 | 1 | 0.0001 | 776.0 | 2.1 | 2003 | JXLVIII | S/2003 J13 | |
Jupiter | S/2003 J2 | 29541 | 1 | 0.0001 | 1037.0 | 2.8 | 2003 | S/2003 J2 | ||
Saturn | Pan | 133.6 | 12.8 | 0.0002 | 13.8 | 0.6 | 1990 | SXVIII | S/1981 S13 | |
Saturn | S/2005 S1 | 136.5 | 3 | 0.0000 | 14.3 | 0.6 | 2005 | S/2005 S1 | ||
Saturn | Atlas | 137.7 | 16.26 | 0.0001 | 14.5 | 0.6 | 1980 | SXV | S/1980 S28 | |
Saturn | Prometheus | 139.4 | 50.11 | 0.0005 | 14.8 | 0.6 | 1980 | SXVI | S/1980 S27 | |
Saturn | Pandora | 141.7 | 42.18 | 0.0006 | 15.1 | 0.6 | 1980 | SXVII | S/1980 S26 | |
Saturn | Epimetheus | 151.4 | 59.32 | 0.0010 | 16.7 | 0.7 | 1980 | SXI | S/1980 S3 | |
Saturn | Janus | 151.5 | 89.19 | 0.0016 | 16.7 | 0.7 | 1966 | SX | S/1980 S1 | |
Saturn | Mimas | 185.54 | 198.52 | 0.0065 | 22.7 | 0.9 | 1789 | SI | ||
Saturn | Methone | 194 | 3 | 0.0000 | 24.2 | 1.0 | 2004 | SXXXII | S/2004 S1 | |
Saturn | Pallene | 211 | 4 | 0.0000 | 27.5 | 1.1 | 2004 | SXXXIII | S/2004 S2 | |
Saturn | Enceladus | 238.04 | 249.29 | 0.0118 | 32.9 | 1.4 | 1789 | SII | ||
Saturn | Tethys | 294.67 | 529.98 | 0.0150 | 45.4 | 1.9 | 1684 | SIII | ||
Saturn | Telesto | 294.71 | 11.2 | 0.0004 | 45.4 | 1.9 | 1980 | SXIII | S/1980 S13 | |
Saturn | Calypso | 294.71 | 9.86 | 0.0003 | 45.4 | 1.9 | 1980 | SXIV | S/1980 S25 | |
Saturn | Dione | 377.42 | 562.5 | 0.0236 | 65.7 | 2.7 | 1684 | SIV | ||
Saturn | Helene | 377.42 | 16.29 | 0.0007 | 65.7 | 2.7 | 1980 | SXII | S/1980 S6 | |
Saturn | Polydeuces | 377.42 | 4 | 0.0000 | 65.7 | 2.7 | 2004 | SXXXIV | S/2004 S5 | |
Saturn | Rhea | 527.07 | 764.5 | 0.0269 | 108.5 | 4.5 | 1672 | SV | ||
Saturn | Titan | 1221.87 | 2575.5 | 0.1381 | 382.9 | 16.0 | 1655 | SVI | ||
Saturn | Hyperion | 1500.88 | 143.04 | 0.0018 | 521.3 | 21.7 | 0.1 | 1848 | SVII | |
Saturn | Iapetus | 3560.84 | 734.5 | 0.0228 | 79.4 | 0.2 | 1671 | SVIII | ||
Saturn | Kiviuq | 11111 | 8 | 0.0004 | 437.5 | 1.2 | 2000 | SXXIV | S/2000 S5 | |
Saturn | Ijiraq | 11124 | 6 | 0.0002 | 438.3 | 1.2 | 2000 | SXXII | S/2000 S6 | |
Saturn | Phoebe | 12947.8 | 109.92 | 0.0047 | 550.4 | 1.5 | 1898 | SIX | ||
Saturn | Paaliaq | 15200 | 11 | 0.0005 | 700.1 | 1.9 | 2000 | SXX | S/2000 S2 | |
Saturn | Skathi | 15541 | 4 | 0.0001 | 723.7 | 2.0 | 2000 | SXXVII | S/2000 S8 | |
Saturn | Albiorix | 16182 | 16 | 0.0006 | 769.0 | 2.1 | 2000 | SXXVI | S/2000 S11 | |
Saturn | S/2004 S11 | 17119 | 3 | 0.0000 | 836.7 | 2.3 | 2005 | S/2004 S11 | ||
Saturn | Erriapo | 17343 | 5 | 0.0002 | 853.2 | 2.3 | 2000 | SXXVIII | S/2000 S10 | |
Saturn | Siarnaq | 17531 | 20 | 0.0007 | 867.1 | 2.4 | 2000 | SXXIX | S/2000 S3 | |
Saturn | Tarvos | 17983 | 7.5 | 0.0003 | 900.9 | 2.5 | 2000 | SXXI | S/2000 S4 | |
Saturn | S/2004 S13 | 18403 | 3 | 0.0000 | 932.6 | 2.6 | 2005 | S/2004 S13 | ||
Saturn | Mundilfari | 18685 | 3.5 | 0.0001 | 954.1 | 2.6 | 2000 | SXXV | S/2000 S9 | |
Saturn | Narvi | 19007 | 3.5 | 0.0002 | 978.9 | 2.7 | 2003 | SXXXI | S/2003 S1 | |
Saturn | S/2004 S15 | 19338 | 3 | 0.0000 | 1004.6 | 2.8 | 2005 | S/2004 S15 | ||
Saturn | S/2004 S17 | 19447 | 2 | 0.0000 | 1013.1 | 2.8 | 2005 | S/2004 S17 | ||
Saturn | Suttungr | 19459 | 3.5 | 0.0001 | 1014.0 | 2.8 | 2000 | SXXIII | S/2000 S12 | |
Saturn | S/2004 S14 | 19856 | 3 | 0.0000 | 1045.2 | 2.9 | 2005 | S/2004 S14 | ||
Saturn | S/2004 S12 | 19878 | 2.5 | 0.0000 | 1046.9 | 2.9 | 2005 | S/2004 S12 | ||
Saturn | S/2004 S18 | 20129 | 3.5 | 0.0000 | 1066.8 | 2.9 | 2005 | S/2004 S18 | ||
Saturn | S/2004 S9 | 20390 | 2.5 | 0.0000 | 1087.7 | 3.0 | 2005 | S/2004 S9 | ||
Saturn | Thrymr | 20474 | 3.5 | 0.0001 | 1094.4 | 3.0 | 2000 | SXXX | S/2000 S7 | |
Saturn | S/2004 S10 | 20735 | 3 | 0.0000 | 1115.4 | 3.1 | 2005 | S/2004 S10 | ||
Saturn | S/2004 S7 | 20999 | 3 | 0.0000 | 1136.7 | 3.1 | 2005 | S/2004 S7 | ||
Saturn | S/2004 S16 | 22453 | 2 | 0.0000 | 1256.8 | 3.4 | 2005 | S/2004 S16 | ||
Saturn | Ymir | 23040 | 9 | 0.0004 | 1306.4 | 3.6 | 2000 | SXIX | S/2000 S1 | |
Saturn | S/2004 S8 | 25108 | 3 | 0.0000 | 1486.2 | 4.1 | 2005 | S/2004 S8 | ||
Uranus | Cordelia | 49.8 | 20.1 | 0.0008 | 8.1 | 0.3 | 1986 | UVI | S/1986 U7 | |
Uranus | Ophelia | 58.8 | 21.4 | 0.0008 | 10.3 | 0.4 | 1986 | UVII | S/1986 U8 | |
Uranus | Bianca | 59.2 | 25.7 | 0.0010 | 10.4 | 0.4 | 1986 | UVIII | S/1986 U9 | |
Uranus | Cressida | 61.8 | 39.8 | 0.0015 | 11.1 | 0.5 | 1986 | UIX | S/1986 U3 | |
Uranus | Desdemona | 62.7 | 32 | 0.0012 | 11.4 | 0.5 | 1986 | UX | S/1986 U6 | |
Uranus | Juliet | 64.4 | 46.8 | 0.0017 | 11.9 | 0.5 | 1986 | UXI | S/1986 U2 | |
Uranus | Portia | 66.1 | 67.6 | 0.0025 | 12.3 | 0.5 | 1986 | UXII | S/1986 U1 | |
Uranus | Rosalind | 69.9 | 36 | 0.0013 | 13.4 | 0.6 | 1986 | UXIII | S/1986 U4 | |
Uranus | Cupid | 74.8 | 12 | 0.0000 | 14.8 | 0.6 | 2003 | UXXVII | S/2003 U2 | |
Uranus | Belinda | 75.3 | 40.3 | 0.0015 | 15.0 | 0.6 | 1986 | UXIV | S/1986 U5 | |
Uranus | Perdita | 76.42 | 13 | 0.0000 | 15.3 | 0.6 | 1999 | UXXV | S/1986 U10 | |
Uranus | Puck | 86 | 81 | 0.0030 | 18.3 | 0.8 | 1985 | UXV | S/1985 U1 | |
Uranus | Mab | 97.7 | 16 | 0.0000 | 22.1 | 0.9 | 2003 | UXXVI | S/2003 U1 | |
Uranus | Miranda | 129.9 | 235.68 | 0.0081 | 33.9 | 1.4 | 1948 | UV | ||
Uranus | Ariel | 190.9 | 578.9 | 0.0275 | 60.5 | 2.5 | 1851 | UI | ||
Uranus | Umbriel | 266 | 584.7 | 0.0233 | 99.5 | 4.1 | 1851 | UII | ||
Uranus | Titania | 436.3 | 788.9 | 0.0386 | 209.0 | 8.7 | 1787 | UIII | ||
Uranus | Oberon | 583.5 | 761.4 | 0.0354 | 323.2 | 13.5 | 1787 | UIV | ||
Uranus | Francisco | 4276 | 6 | 0.0003 | 267.2 | 0.7 | 2001 | UXXII | S/2001 U3 | |
Uranus | Caliban | 7231 | 49 | 0.0021 | 587.5 | 1.6 | 1997 | UXVI | S/1997 U1 | |
Uranus | Stephano | 8004 | 10 | 0.0004 | 684.2 | 1.9 | 1999 | UXX | S/1999 U2 | |
Uranus | Trinculo | 8504 | 5 | 0.0002 | 749.3 | 2.1 | 2001 | UXXI | S/2001 U1 | |
Uranus | Sycorax | 12179 | 95 | 0.0041 | 1284.2 | 3.5 | 1997 | UXVII | S/1997 U2 | |
Uranus | Margaret | 14345 | 5.5 | 0.0002 | 1641.5 | 4.5 | 2003 | UXXIII | S/2003 U3 | |
Uranus | Prospero | 16256 | 15 | 0.0006 | 1980.3 | 5.4 | 1999 | UXVIII | S/1999 U3 | |
Uranus | Setebos | 17418 | 15 | 0.0006 | 2196.3 | 6.0 | 1999 | UXIX | S/1999 U1 | |
Uranus | Ferdinand | 20901 | 6 | 0.0003 | 2887.0 | 7.9 | 2001 | UXXIV | S/2001 U2 | |
Neptune | Naiad | 48.23 | 33.45 | 0.0012 | 7.1 | 0.3 | 1989 | NIII | S/1989 N6 | |
Neptune | Thalassa | 50.08 | 41.25 | 0.0015 | 7.5 | 0.3 | 1989 | NIV | S/1989 N5 | |
Neptune | Despina | 52.53 | 75.26 | 0.0025 | 8.0 | 0.3 | 1989 | NV | S/1989 N3 | |
Neptune | Galatea | 61.95 | 87.75 | 0.0033 | 10.3 | 0.4 | 1989 | NVI | S/1989 N4 | |
Neptune | Larissa | 73.55 | 97.45 | 0.0035 | 13.3 | 0.6 | 1989 | NVII | S/1989 N2 | |
Neptune | Proteus | 117.65 | 209.87 | 0.0078 | 26.9 | 1.1 | 1989 | NVIII | S/1989 N1 | |
Neptune | Triton | 354.8 | 1353.4 | 0.0795 | 141.1 | 5.9 | 1846 | NI | ||
Neptune | Nereid | 5513.4 | 170 | 0.0073 | 360.1 | 1.0 | 1949 | NII | ||
Neptune | S/2002 N1 | 15728 | 24 | 0.0011 | 1734.8 | 4.7 | 2002 | S/2002 N1 | ||
Neptune | S/2002 N2 | 22422 | 24 | 0.0011 | 2952.9 | 8.1 | 2002 | S/2002 N2 | ||
Neptune | S/2002 N3 | 23571 | 24 | 0.0011 | 3182.8 | 8.7 | 2002 | S/2002 N3 | ||
Neptune | S/2003 N1 | 46695 | 14 | 0.0005 | 8874.5 | 24.3 | 2003 | S/2003 N1 | ||
Neptune | S/2002 N4 | 48387 | 30 | 0.0012 | 9361.2 | 25.6 | 2002 | S/2002 N4 | ||
Pluto | Charon | 19.6 | 593 | 0.0313 | 164.5 | 6.9 | 1978 | PI | S/1978 P1 | |
Pluto | Nix | 48.7 | 25 | 0.0007 | 635.2 | 24.9 | 0.1 | 2005 | S/2005 P2 | |
Pluto | Hydra | 64.8 | 25 | 0.0011 | 975.2 | 38.2 | 0.1 | 2005 | S/2005 P1 |
Our Moon is large, but the moons Ganymede (of Jupiter), Titan (of Saturn), Callisto (of Jupiter), and Io (of Jupiter) are larger than our Moon is. The first two are larger than even the planet Mercury. Compared to its planet, the moon Charon (of Pluto) is larger than our Moon. See the Moons Page in the UniverseFamilyTree.
Moons and planets are formed from numerous large and small fragments that collide with each other and sometimes partially stick together. It seems that our Moon was formed when the Earth was hit by another object that was about the size of Mars. Because of the impact, lots of material was thrown into orbit around the Earth, and some of that material clumped together to form the Moon. If the Earth hadn't been struck by such a large object in just that way, then the Earth might not have had such a relatively large Moon. See also question 189.
Jupiter has four great moons and a large number of small moons. The four great moons of Jupiter (Io, Europa, Ganymede, Callisto) were discovered by Galileo Galilei and are therefore sometimes called the Galilean moons. There is an orbital resonance between the four moons that prevents all four moons to ever be on the same side of Jupiter at the same time.
De large satellites of Jupiter orbit in a plane that is perpendicular to the rotation axis of Jupiter, i.e., in the equatorial plane of the planet. Most large satellites of all planets orbit in or near the equatorial plane of their planet, just like most planets orbit near the equatorial plane of the Sun. This suggests that the satellites were formed together with the planets, and the planets together with the Sun.
A moon of Jupiter returns to about the same position relative to Jupiter after on average one synodical period. The answer to question 461 explains how you can calculate the synodical period from sidereal periods. You might think that you can predict when the moons of Jupiter will have occultations and transits, from the synodical periods.
However, predictions based on synodical periods won't be very accurate, because Jupiter does not move at constant speed along a circular orbit around the Earth, but at a constantly varying speed along an elliptical orbit around the Sun. Jupiter travels slower than average in one part of its orbit, and faster than average in another part, which makes phenomena involving its moons appear to be later or earlier than average, depending on where Jupiter is in its orbit.
In addition, we do not observe Jupiter from the Sun but from Earth, which itself orbits around the Sun, but that motion is not taken into account in the calculation of the synodical period, so that, too, causes a delay or advance of the phenomena (up to about 1/30th part of the period of the moon).
Moreover, the distance between Jupiter and the Earth is not constant. Because light has a finite speed, images of a far-away occultation take longer to reach Earth than images of a nearby occultation, so a far-away occultation seems to occur later than average, and a nearby occultation earlier than average (up to about 11 minutes).
Also, the moons of Jupiter do not orbit around Jupiter in the exact same plane as the one in which Jupiter orbits around the Sun, and the orbit of Jupiter around the Sun is not in the same plane as the orbit of the Earth around the Sun, and these facts have an effect on the calculation of phenomena where the exact position is very important (such as for the beginning of an occultation of a moon), but these effects are not taken into account in the calculation of the synodical period, either. The further a moon is from Jupiter, the harder it is to get an occultation. It is possible that such a moon only gets occulted by Jupiter if Jupiter is in a particular small part of its orbit around the Sun.
All in all, synodical periods are not enough to be able to accurately predict occultations and other phenomena. They can be used to show when such a phenomenon will definitely not happen, but not every prediction of such a phenomenon using synodical periods will come true. This is similar to how it goes with solar eclipses and lunar eclipses involving the Earth's Moon. One can tell, using synodical periods, when there is a chance for such an eclipse (namely, a chance for a solar eclipse at every New Moon, and a chance for a lunar eclipse at every Full Moon -- no eclipse will occur at any other phase of the Moon), but one can't tell exactly whether those eclipses will occur, because not every Full Moon has a lunar eclipse, and not every New Moon has a solar eclipse, and the details of such eclipses are not very easy to calculate.
If you do want to be able to make accurate predictions for the occultations and transits of the moons of Jupiter, then you'll have to calculate the positions of the moons and Jupiter to great accuracy, which is a lot more difficult than just using the synodical periods. The book [Meeus] devotes 12 pages to this problem and refers to some other chapters for some of the calculations, so those should be added in as well.
Regular moons are moons with roughly circular orbits (i.e., with small eccentricity) roughly above the planet's equator (i.e., with a small inclination), that traverse their orbit in the same direction as the rotation of the planet around its axies. Such moons were likely formed together with their planet.
Of all regular moons in our Solar System, only moon Hyperion of planet Saturn does not always show the same face to its planet. All other regular planets do always show the same face to their planet. This is called bound rotation, and means that the rotational period of those moons (around their axis of rotation) is equal to their orbital period (around their planet). The rotation of Hyperion around its axis is chaotic, and predictable for only a short time. There appears to be a combination of causes for this: Hyperion has a very irregular shape (clearly not spherical), has a moderately varying distance to its planet (an eccentricity of 0.12, quite high for a regular moon), and its orbit is near to that of a far larger Moon (Titan).
Irregular moons (with a strongly inclined orbit and/or a strongly eccentric orbit and/or orbiting counter to the direction in which the planet orbits around its axis) are often very far from their planet, where tidal forces are much weaker and haven't had enough time to lock the rotation of the Moon around its axies to its motion around the planet.
Our Moon is a relatively large Moon near a not-so-large planet, so it is relatively more difficult for the tidal forces of the Earth upon the Moon to change the rotation of the Moon around its axis than it is for a moon that is smaller or near a larger planet ― which means most moons. Yet even the rotational period of the Moon is already equal to its orbital period. I don't consider it remarkable that nearly all regular Moons always show the same face to their planets.
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Last updated: 2021-07-19