Gemini is a constellation in the northern hemisphere. Its name means "twins" in Latin and refers to the mythical twins Castor and Polydeuces (Pollux in Latin), also known as the Dioscuri.
In Greek mythology, Zeus visited Queen Leda of Sparta, wife of King Tyndareus, in the form of a swan, represented by the constellation Cygnus. Leda became pregnant with both the god and her mortal husband and gave birth to four children. Polydeuces and Helen were fathered by Zeus and hence immortal, and Castor and Clytemnestra were children of Tyndareus and mortal.
Castor and Polydeuces grew up inseparable and were part of many tales, including the myth of the Golden Fleece. When they clashed with another pair of brothers over two beautiful women, Castor, the mortal twin, was killed. Polydeuces asked Zeus not to be separated from his brother and Zeus allowed the two to spend half of the time at Olympus and half in Hades. The twins were also said to have been given the power to save shipwrecked sailors by the sea god Poseidon and were considered to be the patron saints of sailors.
Greek astronomer Ptolemy, however, identified the constellation with Apollo and Heracles, who were half-brothers, both fathered by Zeus, and referred to the stars Castor and Pollux as "the star of Apollo" and "the star of Heracles" respectively.
The constellation Gemini occupies an area of 514 square degrees and contains four stars with known planets. It can be seen at latitudes between +90° and -60° and is best visible at 9 p.m. during the month of February. In sidereal astrology, the Sun passes through the constellation Gemini from June 16 to July 15. In tropical astrology, the sign Gemini still stretches roughly between the dates May 22 and June 23.
The two brightest stars in Gemini, 
alpha and 
beta Geminorum, are better known as Castor and Pollux, and represent the twins' heads. Both are among the brightest stars in the night sky. 
alpha Geminorum, Castor, is a visual binary composed of two separate binary stars. Castor also has a faint companion star, an eclipsing binary system, which makes it a sextuple system, one composed of hot, bluish-white stars and faint red dwarfs. To the Chinese, Castor represents Yin, a fundamental principle on which all things depend.
beta Geminorum, Pollux, is brighter than Castor. It is an orange giant 34 light-years distant. In 2006, an extrasolar planet was discovered orbiting the star. It was named Pollux b or Polydeuces. The Chinese associate Pollux with Yang, the other of the two fundamental principles of things. In Arabic, the star is known as Al-Ras al-Tau’am al-Mu’akar, or "the head of the second twin."
The constellation Gemini contains a number of named stars. 
gamma Geminorum, a spectroscopic binary star with a faint companion, is also known as Alhena ("the brand") and Almeisan ("the shining one"). The Chinese know it as "the Third Star of the Well."
mu Geminorum, an irregular variable star, has the traditional names Tejat Posterior ("back foot") and Tejat ("foot") as it represents Castor's foot. It is also known as Pish Pai ("foreleg") and Calx ("heel").
epsilon Geminorum, a supergiant, is also known as Mebsuta (from an Arabic phrase referring to the outstretched paw), Melboula and Melucta. It lies on Castor's left leg.
eta Geminorum is a triple star that also goes by the names Tejat Prior, Propus and Praepes.
delta Geminorum, a binary star, has the traditional name Wasat ("the middle").
Gemini contains several interesting deep sky objects. Messier 35, or NGC 2168, is an open star cluster, lying approximately 2,800 parsecs from Earth. It takes up an area almost the size of the full moon in the sky. The neighbouring NGC 2158 is another open cluster, thought to be about a billion years old.
The Medusa Nebula, also known as Abel 21 and Sharpless 274, is an old, large planetary nebula near the border with the constellation Canis Minor
. It was once believed to be a supernova remnant.
NGC 2371-2 is a magnitude 13 dual lobed planetary nebula that lies just southwest of 
alpha Geminorum, Castor.
Finally, the Eskimo Nebula, NGC 2392, got its name because it appears like a person's head wearing a hood, with bright orange streams in the outer layer. It is a bipolar planetary nebula that also goes by the name the Clownface Nebula.
Gemini belongs to the Zodiac family of constellations, along with Leo
Constellations directly bordering Gemini are Lynx
, Canis Minor