This zodiacal constellation is best known for the two bright stars Castor and Pollux, named for the mythical twins from which it takes its name. The association of this part of the sky with a pair of twins is a natural one, given the two bright stars that dominate the region, and is also a very ancient one, going back at least as far as the Babylonians. The Greeks associated it with Castor and Pollux, the twin sons of Leda; in most tellings, Castor was mortal but Pollux, fathered by Zeus, was immortal. After Castor's death, Pollux persuaded Zeus to immortalise both brothers in the sky.
Gemini in the Sky
Gemini's status as a zodiacal constellation means that it lies on the line of the Ecliptic; that is, it is one of the constellations through which the Sun passes on its annual journey through the sky, at least from the point of view of an observer on Earth. The Sun enters Gemini from neighbouring Taurus at midsummer, and progresses eastward through Gemini for some thirty days until it passes on into Cancer.
Because the Sun is in Gemini at the height of summer, the constellation is not visible at that time, and appears most prominently in the sky during the middle of winter (in the Earth's southern hemisphere, this sequence of seasons is reversed).
Finding Gemini in the sky is relatively easy, because it falls directly northeast of the unmistakable form of Orion. Tracing an imaginary line from Rigel at Orion's base through Betelgeuse at the Hunter's shoulder leads directly to Alhena, Gemini's brightest star after Castor and Pollux. Those two bright stars are easily identified, lying close together somewhat further to the north and east.
Most prominent among the stars of Gemini are, as one might expect, the pair named for the twins that give the constellation its name: Castor and Pollux. Far from being twins, these two stars are in fact quite different from one another, and only appear similar in the skies of Earth because they fall along the same line of sight. Pollux is the nearer of the two, an orange giant about 34 light years from the Solar System, while Castor is a multiple star some 18 light years beyond Pollux.
Spreading out to the south and west of Castor and Pollux, a series of relatively bright stars form an elongated irregular polygon reaching into the band of the Milky Way, a pattern that represents the bodies of the stellar twins. Brightest of these is Alhena, a white subgiant star about 105 light years from the Solar System. Northward from Alhena lie a pair of red stars, Propus and Tejat. These two are also called Tejat Prior and Tejat Posterior, but despite their shared name, these two stars are, like Castor and Pollux, quite unrelated to one another.
Deep Sky Objects
The western parts of Gemini intersect with the Milky Way, and in this part of the constellation is a scattering of open star clusters and nebulae. Most prominent among these is Messier 35, a cluster of about two hundred blue stars lying about 2,800 light years from the Solar System, beyond the star 1 Geminorum.
Moving out from the plane of the Milky Way, in Gemini's more easterly regions is a planetary nebula, the remnant of a supernova. This is NGC 2392, better known as the Eskimo Nebula. This complex object consists of waves of matter exploding outwards, the outer shells of a former giant star. Beyond these, a further pattern of debris stretches out in all directions, giving the nebula the appearance of a rayed halo.
From early to mid December each year, the Earth experiences a meteor shower that typically reaches its most intense towards 14 or 15 December. This shower is known as the Geminids, because its radiant point is in Gemini, not far from the star Castor. The meteors of the shower are by no means confined to the constellation - they can appear anywhere in the sky - but due to the motion of the Earth, their trails focus on this radiant point.
Almost all meteor showers are associated with the Earth passing through the debris of a comet, but for a long time the Geminids presented a mystery: no comet could be found that explained their appearance. It was eventually discovered that, almost exceptionally, these meteors were not connected with a comet but with an asteroid. That asteroid, Phaethon, follows a peculiar orbit that carries it close to the Sun and then out beyond the orbit of Mars, and the trail of dust left by this body is the source of the Geminid shower.