Gemini
Gemini is easy to recognize in the evening sky as winter turns to spring, thanks to two stars shining side by side: Castor and Pollux.
| Latin name | Gemini |
|---|---|
| Abbreviation | Gem |
| Brightest star | Pollux (mag 1.1) |
| Best seen | Winter–early spring evenings (Northern Hemisphere) |
| Astrological dates | May 21 – June 20 |
| Sun actually passes | June 22 – July 20 |
| Stars used by Planetica | 1,297 |
How to find it
Gemini is visible in the evening sky from December through April and is highest in the early evening from January to March. From mid-northern latitudes it climbs from the south to nearly overhead at that time of year, so it's easy to find.
First find Orion, then look up and to its left (northeast). You'll see two stars of similar brightness shining side by side, about two or three finger-widths apart. The upper white star is Castor, and the lower, slightly brighter, orange-tinged star is Pollux. A line drawn from Orion's Rigel through Betelgeuse and extended onward also lands near this pair.
The two stars are the heads of the twin brothers, and from there two parallel lines of stars run toward Orion, forming their bodies and legs. The twins' feet sit near the horns of Taurus, and next to them is M35, an open cluster you can see with binoculars. Pollux is also one of the stars of the "Winter Hexagon," along with Sirius, Procyon, Capella, Aldebaran and Rigel.
The myth: inseparable brothers
In Greek mythology, Gemini is the brothers Castor and Pollux (Polydeuces in Greek), born to Leda, queen of Sparta. They were twins, but Pollux was a son of Zeus and therefore immortal, while Castor was the son of the mortal king Tyndareus and destined to die one day. Castor was a skilled horseman and Pollux a champion boxer, and the two sailed together with Jason on the voyage of the Argo.
When Castor was killed in a fight, the grieving Pollux begged Zeus, saying he did not want to live forever without his brother. Zeus let the two spend alternate days together, one among the gods and one in the underworld, and in honor of their bond placed them side by side in the sky. Sailors of old regarded the brothers as protectors at sea.
In the East Asian 28 lunar mansions, many of Gemini's stars belong to the Well mansion (井宿). True to its name, the stars link up in a square, frame-like shape.
Main stars
| Name | Apparent magnitude | Distance (approx., ly) | Notes |
|---|---|---|---|
| Pollux (Beta) | 1.1 | ~34 | An orange giant; a planet orbiting it was discovered in 2006. Though it's the Beta star, it's brighter than the Alpha star. |
| Castor (Alpha) | 1.6 | ~51 | Looks like one star, but it is really a system of six stars in three pairs orbiting each other. |
| Alhena (Gamma) | 1.9 | ~110 | A white star at the feet of Pollux's twin. |
| Tejat (Mu) | 2.9 | ~230 | A red giant at Castor's feet whose brightness varies slightly. |
| Wasat (Delta) | 3.5 | ~60 | Sits right next to the ecliptic, so the Moon and planets often pass close by. |
Near the twins' feet, M35 is an open cluster about 2,800 light-years away; from a dark site, binoculars show it as a hazy patch of stars. With a telescope you can also find NGC 2392, the Eskimo Nebula, a planetary nebula said to look like a face in a fur hood.
Birthday sign dates: two different answers
In astrology, Gemini runs from May 21 to June 20. But for most of that period, the real Sun is passing through Taurus. The Sun is actually within the Gemini constellation from about June 22 to July 20.
The first cause is precession. As Earth's axis slowly changes direction over a cycle of about 26,000 years, reference points like the vernal equinox and the summer solstice shift along the ecliptic by about 1 degree every 72 years. The astrological signs divide the ecliptic into 30-degree slices from the equinox point based on the reference of about 2,000 years ago, so today they are almost one constellation out of step with the real sky. In fact, on the June solstice the Sun is now near the border between Taurus and Gemini.
The second cause is the constellation boundaries set by the International Astronomical Union (IAU) in 1930. Each constellation covers a different width of the ecliptic, so the time the Sun spends in each also differs. For Gemini it's about 29 days. The dates can vary by about a day from year to year.
In Planetica
If your birthday's astrological sign is Gemini, Planetica places your planetary system next to a real star inside the Gemini constellation. The candidates are the 1,297 stars in the HYG catalog that lie within about 3,000 light-years of Earth and inside the IAU boundaries of Gemini. One of them is picked at random, and your system sits about 1–6 light-years from it.
"Twin systems" from people with the same name and birthday live together in Gemini, true to the name, but each is placed next to a different star. Just like Castor and Pollux: the same sky, but a place of their own.
Planetica also tells you which constellation the Sun was really in on your birthday. For example, someone born on June 10 is a Gemini in astrology, but on that day the Sun was actually in Taurus.
Fun facts
- When William Herschel discovered Uranus in 1781, it was in Gemini.
- The photographs in which Clyde Tombaugh discovered Pluto in 1930 were also taken near Wasat (Delta Geminorum).
- The Geminid meteor shower, which peaks around December 13–14 each year, comes from debris left not by a comet but by the asteroid 3200 Phaethon.
Other constellations
Create your own planetary system
Enter a name and a birthday to get a one-of-a-kind planetary system, placed next to a real star in your birthday constellation.