Gemini (constellation)

This article is about the astronomical constellation. For the astrological sign, see Gemini (astrology).

Gemini

Constellation
List of stars in Gemini
Abbreviation	Gem
Genitive	Geminorum
Pronunciation	/ˈdʒɛmɨnaɪ/, genitive /ˌdʒɛmɨˈnɒrəm/
Symbolism	the Twins, Castor & Pollux
Right ascension	7 h
Declination	+20°
Quadrant	NQ2
Area	514 sq. deg. (30th)
Main stars	8, 17
Bayer/Flamsteed stars	80
Stars with planets	7
Stars brighter than 3.00m	4
Stars within 10.00 pc (32.62 ly)	4
Brightest star	Pollux (β Gem) (1.15m)
Nearest star	Gliese 251 (17.99 ly, 5.52 pc)
Messier objects	1
Meteor showers	Geminids Rho Geminids
Bordering constellations	Lynx Auriga Taurus Orion Monoceros Canis Minor Cancer
Visible at latitudes between +90° and −60°. Best visible at 21:00 (9 p.m.) during the month of February.

Gemini is one of the constellations of the zodiac. It was one of the 48 constellations described by the 2nd century astronomer Ptolemy and it remains one of the 88 modern constellations today. Its name is Latin for "twins," and it is associated with the twins Castor and Pollux in Greek mythology. Its symbol is (Unicode ♊).

Location

The constellation Gemini as it can be seen with the naked eye, with added connecting lines.

Gemini lies between Taurus to the west and Cancer to the east, with Auriga and Lynx to the north and Monoceros and Canis Minor to the south.

The Sun resides in Gemini from 21 June to 21 July each year. By mid August, Gemini will appear along the eastern horizon in the morning sky prior to sunrise. The best time to observe Gemini at night is overhead during the months of January and February. By April and May, the constellation will be visible soon after sunset in the west.

The easiest way to locate the constellation is to find its two brightest stars Castor and Pollux eastward from the familiar “V” shaped asterism of Taurus and the three stars of Orion’s belt. Another way is to mentally draw a line from the Pleiades star cluster located in Taurus and the brightest star in Leo, Regulus. In doing so, you are drawing an imaginary line that is relatively close to the ecliptic, a line which intersects Gemini roughly at the midpoint of the constellation, just below Castor and Pollux.

Notable features

Stars

The brightest stars in Gemini are Castor and Pollux. Although Castor has the Bayer designation "Alpha," it is actually the second brightest in the constellation after Pollux. Castor is a sextuple star system. Pollux has an extrasolar planet revolving around it, as do two other stars in Gemini, HD 50554, and HD 59686. Another noteworthy star is Mekbuda (ζ Gem), a supergiant with a radius that is 60 times solar, making it approximately 220,000 times the size of the Sun.

Deep-sky objects

To look at Gemini is to look away from the Milky Way; as a result, there are comparatively few deep-sky objects of note. The Eskimo Nebula and Medusa Nebula, Messier object M35, and Geminga are those that attract the most attention. The Eskimo and Medusa nebulae are both planetary nebulae, the one approximately 2,870 light years away and the other 1,500 light years distant. M35 is an open star cluster which was discovered in the year 1745 by Swiss astronomer Philippe Loys de Chéseaux. And Geminga is a neutron star approximately 550 light years from Earth. Other objects of note are NGC 2129, NGC 2158, NGC 2266, NGC 2331, NGC 2355, and NGC 2395.

Mythology

Gemini as depicted in Urania's Mirror, a set of constellation cards published in London c.1825.

In Babylonian astronomy, the stars Castor and Pollux were known as the Great Twins (MUL.MASH.TAB.BA.GAL.GAL). The Twins were regarded as minor gods and were called Meshlamtaea and Lugalirra, meaning respectively 'The One who has arisen from the Underworld' and the 'Mighty King'. Both names can be understood as titles of Nergal, the major Babylonian god of plague and pestilence, who was king of the Underworld.

In Greek mythology, Gemini was associated with the myth of Castor and Pollux, collectively known as the Dioscuri. When Castor died, because he was mortal, Pollux begged his father Zeus to give Castor immortality, and he did, by uniting them together in the heavens.

Visualizations

Diagram of H.A. Rey's alternative way to connect the stars of the constellation Gemini. A pair of twins are shown holding hands.

Gemini is dominated by Castor and Pollux, two bright stars that appear relatively close together, encouraging the mythological link between the constellation and twinship. The twin above and to the right (as seem from the Northern Hemisphere) is Castor, whose brightest star is α Geminorum (more commonly called Castor); it is a second magnitude star and represents Castor's head. The twin below and to the left is Pollux, whose brightest star is β Geminorum (more commonly called Pollux); it is of the first magnitude and represents Pollux's head. Furthermore, the other stars can be visualized as two parallel lines descending from the two main stars, making it look like two figures.

H.A. Rey has suggested an alternative to the traditional visualization that connected the stars of Gemini to show a pair of twins holding hands. Pollux's torso is represented by the star υ Geminorum, Pollux's right hand by ι Geminorum, Pollux's left hand by κ Geminorum; all three of these stars are of the fourth magnitude. Pollux's pelvis is represented by the star δ Geminorum, Pollux's right knee by ζ Geminorum, Pollux's right foot by γ Geminorum, Pollux's left knee by λ Geminorum, and Pollux's left foot by ξ Geminorum. Gamma Geminorum is of the second magnitude, while Delta and Xi Geminorum are of the third magnitude. Castor's torso is represented by the star τ Geminorum, Castor's left hand by ι Geminorum (which he shares with Pollux), Castor's right hand by θ Geminorum; all three of these stars are of the fourth magnitude. Castor's pelvis is represented by the star ε Geminorum, Castor's left foot by ν Geminorum, and Castor's right foot by μ Geminorum and η Geminorum; ε, μ, and η Geminorum are of the third magnitude. The brightest star in this constellation is Pollux.

Equivalents

In Chinese astronomy, the stars that correspond to Gemini are located in two areas: the White Tiger of the West (西方白虎, Xī Fāng Bái Hǔ) and the Vermillion Bird of the South (南方朱雀, Nán Fāng Zhū Què).

Astrology

Main article: Gemini (astrology)

As of 2008, the Sun appears in the constellation Gemini from June 20 to July 20. In tropical astrology, the Sun is considered to be in the sign Gemini from May 21 to June 20, and in sidereal astrology, from June 16 to July 15.

Namesakes

USS Gemini (AP-75) and USS Gemini (PHM-6) are United States navy ships named after the constellation.

See also

• Geminga, Gemini gamma-ray source
• Gemini (Chinese astronomy)
• IC 444, reflection nebula
• Messier 35 open cluster

References

• H. A. Rey, The Stars — A New Way To See Them. Enlarged World-Wide Edition. Houghton Mifflin, Boston, 1997. ISBN 0-395-24830-2.
• Ian Ridpath and Wil Tirion (2007). Stars and Planets Guide, Collins, London. ISBN 978-0007251209. Princeton University Press, Princeton. ISBN 978-0691135564.

External links

• The Deep Photographic Guide to the Constellations: Gemini
• Astrojan Astronomical Picture Collection: The clickable Gemini
• WikiSky: Gemini constellation
• Ian Ridpath's Star Tales: Gemini
• APOD Pictures of Gemini and Deep Sky Objects:

1. A Spring Sky Over Hirsau Abbey
2. The Eskimo Nebula from Hubble
3. The Medusa Nebula
4. Open Star Clusters M35 and NGC 2158
5. NGC 2266: Old Cluster in the NGC