Science & history · Constellation guide
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.
How to spot it: Two bright stars side by side, Castor and Pollux, the Twins' heads. Source (opens in a new tab)
The twins
The constellation represents the Greek twins Kastor and Polydeukes (opens in a new tab), known by their Latin names Castor and Pollux, whose stars mark their heads. In the most common version of the myth Pollux was immortal and Castor not; when Castor died, Pollux asked to share his immortality, and Zeus put them both in the sky.
Alpha is not the brighter
Castor is alpha Geminorum, yet Pollux, beta, is the brighter of the two, as the Bright Star Catalogue's magnitudes show. Pollux is also the more orange, as its colour index in the same catalogue shows.
Patron saints of sailors
Apollonius of Rhodes made the twins saviours of sailors (opens in a new tab), and Pliny wrote that twin glows of St Elmo's fire in a ship's rigging were called Castor and Pollux, a sign of a safe voyage.
A cluster for binoculars
Near Castor's foot is Messier 35 (opens in a new tab), an open cluster about 2,800 light-years away, a fuzzy spot to the naked eye on a very dark night. The Geminid meteor shower radiates from Gemini each December.
In another tradition
In Vedic astrology the twins' heads are the stars of Punarvasu.
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.
Sources
- History Ian Ridpath, Star Tales: Gemini (opens in a new tab) for the twins, their myth and the sailors
- Astronomy NASA Hubble, Messier 35 (opens in a new tab) for Messier 35
- Astronomy Bright Star Catalogue, 5th Revised Ed. (Hoffleit & Warren 1991), as computed by this site's star engine (opens in a new tab) for the magnitudes and colours of Castor and Pollux