Gemini, the twins

Gemini is two parallel lines of stars, the twins, with their heads marked by two bright stars close together: Castor and Pollux.

Where is it from where I am?

Lines and labels Show in my sky

About this picture

NASA/Goddard Space Flight Center Scientific Visualization Studio. Gaia DR2: ESA/Gaia/DPAC. Constellation figures based on those developed for the IAU by Alan MacRobert of Sky and Telescope magazine (Roger Sinnott and Rick Fienberg). NASA's page (opens in a new tab).

The map is a composite of 1.7 billion catalogued stars, not a photograph from one place: it shows stars far too faint for the eye, and no weather, Moon or city light. It is drawn here in this chart's projection, with each star where it was in 2000; the fastest-moving bright star has shifted about a minute of arc since, far less than a pixel here.

How to spot it: Two bright stars side by side, Castor and Pollux, the Twins' heads. Source (opens in a new tab)

The twins

Two lines of stars make the twins of Greek myth, with their heads at Castor and Pollux.

The constellation represents the Greek twins Kastor and Polydeukes (opens in a new tab), Castor and Pollux in Latin, whose stars mark their heads. In the most common version Pollux was a son of Zeus and immortal while Castor was mortal; when Castor was killed, Pollux asked Zeus that they share immortality, and Zeus put them both in the sky.

Gemini is the 30th (opens in a new tab) constellation by size.

Sources for this section Ian Ridpath, Star Tales: Gemini (opens in a new tab)

Alpha is not the brighter

Pollux, the beta star, outshines Castor, the alpha, and is the more orange of the two.

Castor is alpha Geminorum, yet in ESA's list (opens in a new tab) Pollux, beta, is the brighter, magnitude 1.16 against Castor's 1.58. Pollux is also the more orange: the same table gives it a colour index of 0.99 against Castor's 0.03, and a larger number means a redder star.

Greek letters often follow brightness, but not always: for Orion, Jim Kaler (opens in a new tab) suggests that Johann Bayer may simply have gone by position.

Sources for this section ESA, Hipparcos: The 50 Brightest Stars (opens in a new tab) · ; Jim Kaler, Rigel, STARS (University of Illinois) (opens in a new tab)

Patron saints of sailors

Twin glows in a ship's rigging were called Castor and Pollux, and meant a safe voyage.

Apollonius of Rhodes told how the Argonauts owed their safety on one voyage to Castor and Pollux (opens in a new tab), who were saviours of sailors on others too.

Pliny the Elder (opens in a new tab) wrote that when glows alight on a ship's yards, two of them mean safety and a successful voyage and are called Castor and Pollux; a single glow, called Helena, was a sign of disaster. The glows are the electrical phenomenon known as St Elmo's fire.

Sources for this section Ian Ridpath, Star Tales: Gemini (opens in a new tab)

How to find it

Two bright stars side by side, north-east of Orion.

By this site's calculation (opens in a new tab), the twins' middle stands about 76° up in the south at its highest from 40°N and about 29° up in the north from 35°S, and it is highest at about 9 p.m. local time in mid-February.

NASA's December guide (opens in a new tab) places the twins to the left of Orion and Taurus as they rise in the east on northern winter evenings.

Sources for this section Bright Star Catalogue, 5th Revised Ed. (Hoffleit & Warren 1991), as computed by this site's star engine (opens in a new tab) · ; NASA Science, What's Up: December 2024 (opens in a new tab)

A cluster for binoculars

At Castor's foot, an open cluster that is a haze to the eye and a field of stars in binoculars.

Near Castor's foot is Messier 35 (opens in a new tab), an open cluster about 2,800 light-years away: a haze to the unaided eye on very dark nights, a haze with some separate stars in binoculars, and a group of bright stars in a telescope. NASA gives the northern winter months as the best time.

Each December the Geminid meteor shower appears to come from Gemini, which is how meteor showers get their names (opens in a new tab).

Sources for this section NASA Hubble, Messier 35 (opens in a new tab) · ; NASA Space Place, What Are Constellations? (updated 2022) (opens in a new tab)

Where on Earth, and when

The whole key pattern rises from anywhere north of about 58°S.

South of about 74°S, none of it rises.

North of about 74°N, the whole key pattern never sets.

Its middle climbs at least 20° high, the finder's threshold for a suggestion, from anywhere north of about 44°S.

It is highest in the evening sky, at about 9 p.m. local time, in mid-February, from either hemisphere, wherever it rises. With daylight saving time, that is nearer 10 p.m. on the clock.

Worked out by this site from the Bright Star Catalogue, for a flat, clear horizon. Hills, buildings, haze and city light all cut into this; the air also lifts objects near the horizon by about half a degree. For your own place and night, use the sky finder.