How to Find the Gemini Constellation

constellation Gemini
The constellation Gemini, containing the stars Castor and Pollux (which is part of the Winter Hexagon). Carolyn Collins Petersen

The Gemini constellation is one of the most ancient known star patterns. People have been observing it since earliest human history, and it was first charted by the Greek-Egyptian astronomer Claudius Ptolemy as part of his sky mapping activities. The name "Gemini" is the Latin word meaning "twins," and most star-chart makers depict the stars in this constellation as a pair of twin boys. 

Finding Gemini Constellation

Look for Gemini in the sky near the constellations Orion (which has some fascinating sights of its own) and Taurus. For northern hemisphere viewers, it's a winter star pattern and its two brightest stars, Castor and Pollux, are part of an unofficial asterism called the Winter Hexagon. That pattern contains six bright stars from the constellations Gemini, Orion, Canis Major, Canis Minor, and Taurus. Gemini looks like two long strings of stars extending down from Castor and Pollux, which are the heads of the twins. The easiest way to find it is to look for Castor and Pollux east of the vee-shaped Hyades cluster, which makes up the face of Taurus the Bull. The best views of this star pattern are available when it is straight overhead early in the new year. It remains visible until late spring, when it disappears into the sunset glow. 

the Winter Hexagon
The Winter Hexagon is made up of the brightest stars from the constellations Orion, Gemini, Auriga, Taurus, Canis Major and Canis Minor. Carolyn Collins Petersen

The Story of Gemini

The mythology of the ancient Greeks and the Babylonians concerned a pair of twins in the sky. For the Babylonians, these boys were in the realm of the gods, and they called them "Meshlamtea" and "Lugalirra." They were related to a more important god, named Nergal, who presided over the Underworld and was thought to bring about all kinds of misfortune, disease, and other ills. The Greeks and Romans called these stars after the twin sons of Zeus and the maiden Leda. The Chinese saw a bird and a tiger in these stars. The modern constellation of the twins was set by Ptolemy and formalized by later stargazers. The formal area of the sky that contains the twins was set by the International Astronomical Union and contains other stars beyond the main ones, plus nearby deep-sky objects. 

The Stars of Constellation Gemini

Gemini constellation is dominated by the bright stars Castor and Pollux. These are also known as α (alpha) Geminorum (Castor) and β (beta) Geminorum (Pollux). Castor may look like only one star, but in reality, it contains six stars in orbit with each other. It lies some 52 light-years from Earth. Twin brother Pollux is an orange giant star that lies about 34 light-years away from the Sun. Pollux also has at least one planet in orbit around it. 

The IAU chart for constellation Gemini.
The official chart showing the stars of the constellation Gemini, provided by the IAU. IAU/Sky & Telescope.com 

Stargazers who want to explore other stars in Gemini might find ε (epsilon) Geminorum, which is interesting since it's a binary star that can be seen through telescopes. One member of the pair is also a Cepheid variable star that brightens and dims with a period of about 10 days. 

Deep-Sky Objects in Gemini Constellation

Gemini isn't enriched with a lot of deep-sky objects. This is because it's situated away from the plane of the Milky Way, where most of the clusters and nebulae exist. However, there are a few things that observers can search out in the constellation. The first is a star cluster called M35. It's what astronomers call an "open" cluster. That means that its stars are fairly scattered through space but are still traveling together. There are about 200 stars in M35, and this cluster can be seen with the naked eye from dark-sky sights. It's also a lovely sight through binoculars or a telescope. Look for it near Castor's foot. 

Open star cluster M35 in constellation Gemini.
The open star cluster M35 (lower right) in the constellation Gemini. 2MASS/NASA. 

Skygazers up for a challenge can also search out two dim planetary nebulae in Gemini. These are clouds of gas that have formed around dying sun-like stars. The first is the Eskimo Nebula (also known as NGC 2392). It has been imaged by the Hubble Space Telescope and is about 4,000 light-years from Earth. Search it out by looking just to the left of Pollux's waist (marked 2392 on the chart). The other object is called the Medusa Nebula, and it's a real challenge to see. Search for it along the border with Canis Minor, below Pollux's knee.

Eskimo Nebula in Gemini constellation.
The Eskimo Nebula in Gemini, as seen by Hubble Space Telescope. NASA/ESA/STScI 

Finally, meteor shower fans spend each December 13-14 observing the Geminid Meteor shower. It's a shower created by a stream of material left behind by the asteroid 3200 Phaethon as it orbits the Sun. The meteors are not actually from Gemini, but they appear to "radiate" from the constellation. In a good year, observers can spot upwards of 100 or so meteors per hour from this shower. 

Gemini in Modern Culture

As a starry constellation, Gemini has appeared in both space science and astronomy, as well as science fiction. NASA's Gemini missions were named for this star pattern because they each carried two astronauts to space. The Gemini Observatory has two domes, one in Hawai'i and one in Chile, both inspired by the starry twins. Finally, science fiction writer Robert A. Heinlein named two of his teenaged characters after the two bright stars Castor and Pollux.