Themisto (moon)

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S 2000 J 1.jpg
Rediscovery images of Themisto taken by the UH88 telescope in November 2000
Discovery [1][2]
Discovered byCharles T. Kowal (1975)
Elizabeth P. Roemer (1975)
Scott S. Sheppard (2000)
David C. Jewitt (2000)
Yanga R. Fernández (2000)
Eugene A. Magnier (2000)
Discovery sitePalomar Observatory
Mauna Kea Observatory (rediscovery)
Discovery date30 September 1975
21 November 2000 (rediscovery)
Jupiter XVIII
Named after
Θεμιστώ Themistō
S/2000 J 1
S/1975 J 1
AdjectivesThemistoan /θɛməˈst.ən/[4] Themistoian /θɛməˈst.iən/
Orbital characteristics[5]
Epoch 23 March 2018 (JD 2458200.5)
Observation arc42.54 yr (15,536 days)
0.0494401 AU (7,396,130 km)
+129.95 d
2° 46m 13.369s / day
Inclination45.28121° (to ecliptic)
Satellite ofJupiter
Group(own group)
Physical characteristics[7]
Mean diameter
9 km
Albedo0.04 (assumed)

Themisto /θəˈmɪst/, also known as Jupiter XVIII, is a small prograde irregular satellite of Jupiter. It was discovered in 1975, subsequently lost, and rediscovered in 2000.

Discovery and naming[edit]

Themisto observed by the Canada-France-Hawaii Telescope on 6 August 2000, several months before its rediscovery in November 2000

Themisto was first discovered by Charles T. Kowal and Elizabeth Roemer on 30 September 1975, reported on 3 October 1975,[1] and designated S/1975 J 1. However, not enough observations were made to establish an orbit and it was subsequently lost.

Themisto appeared as a footnote in astronomy textbooks into the 1980s.[citation needed] Then, in 2000, a seemingly new satellite was discovered by Scott S. Sheppard, David C. Jewitt, Yanga R. Fernández and Eugene A. Magnier, and was designated S/2000 J 1. It was soon confirmed that this was the same as the one observed in 1975.[2] This observation was immediately correlated with an observation on 6 August 2000, by the team of Brett J. Gladman, John J. Kavelaars, Jean-Marc Petit, Hans Scholl, Matthew J. Holman, Brian G. Marsden, Philip D. Nicholson and Joseph A. Burns, which was reported to the Minor Planet Center but not published as an IAU Circular (IAUC).[8]

In October 2002 it was officially named after Themisto,[9] daughter of the river god Inachus and lover of Zeus (Jupiter) in Greek mythology.


Diagram illustrating Themisto's orbit (top left) among those of the other irregular satellites of Jupiter. The satellites above the horizontal axis are prograde, the satellites beneath it are retrograde. The yellow segments extend from the pericentre to the apocentre, showing the orbital eccentricity.

Themisto's orbit is unusual: unlike most of Jupiter's moons, which orbit in distinct groups, Themisto orbits alone. The moon is located midway between the Galilean moons and the first group of prograde irregular moons, called the Himalia group.

Themisto is about 8 kilometers (5 miles) in diameter (assuming an albedo of 0.04).[10] While its true albedo could not be measured by NEOWISE due to poor timing of observations,[11] it is known to have color index B−V=0.83, V−R=0.46, and V−I=0.94.[12]


  1. ^ a b Brian G. Marsden (3 October 1975). "IAUC 2845: Probable New Satellite of Jupiter". International Astronomical Union Central Bureau for Astronomical Telegrams.
  2. ^ a b Brian G. Marsden (25 November 2000). "IAUC 7525: S/1975 J 1 = S/2000 J 1". International Astronomical Union Central Bureau for Astronomical Telegrams.
  3. ^ Cf. 'Themista' in Noah Webster (1884) A Practical Dictionary of the English Language
  4. ^ 'Themis[t]oan' in James Hall (2015) Moons of the Solar System, p. 82.
  5. ^ a b "M.P.C. 110499" (PDF). Minor Planet Circular. Minor Planet Center. 29 May 2018.
  6. ^ Sheppard, Scott. "Scott S. Sheppard - Jupiter Moons". Department of Terrestrial Magnetism. Carnegie Institution for Science. Retrieved 26 November 2020.
  7. ^ "Planetary Satellite Physical Parameters". JPL.
  8. ^ "MPEC 2000-Y16: S/1975 J 1 = S/2000 J 1, S/1999 J 1". International Astronomical Union Minor Planet Center. 19 December 2000.
  9. ^ Daniel W. E. Green (22 October 2002). "IAUC 7998: Satellites of Jupiter". International Astronomical Union.
  10. ^ Sheppard, S. S.; Jewitt, D. C.; An abundant population of small irregular satellites around Jupiter, Nature, 423 (May 15, 2003), pp. 261–263
  11. ^ Grav, T.; Bauer, J. M.; Mainzer, A. K.; Masiero, J. R.; Nugent, C. R.; Cutri, R. M.; et al. (August 2015). "NEOWISE: Observations of the Irregular Satellites of Jupiter and Saturn" (PDF). The Astrophysical Journal. 809 (1): 9. arXiv:1505.07820. Bibcode:2015ApJ...809....3G. doi:10.1088/0004-637X/809/1/3. S2CID 5834661. 3.
  12. ^ Grav, Tommy; Holman, M. J.; Gladman, B. J.; Aksnes, K. (2003). "Photometric survey of the irregular satellites". Icarus. 166 (1): 33–45. arXiv:astro-ph/0301016. Bibcode:2003Icar..166...33G. doi:10.1016/j.icarus.2003.07.005. S2CID 7793999.

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