How to Spot Gemini, the Heavenly Twins, in the Night Sky

Here's an interesting question to set up this week's night sky column: What do actresses Mary-Kate and Ashley Olsen, comedians Randy and Jason Sklar, glamor models Kristina and Karissa Shannon and astronauts Mark and Scott Kelly all have in common? Answer: They are all twins. 

And in our current evening sky, we can add Pollux and Castor to this list; the constellation representing the Heavenly Twin brothers known as Gemini.

Gemini is conventionally referred to as a winter constellation, yet such seasonal designations are rather loose. They simply tell the season when a star pattern is best seen during convenient evening hours. In fact, some astronomers actually think that Gemini should be considered a spring constellation. [February's Night Sky: Visible Planets, Moon Phases & Events] 

Indeed, at dusk on the first night of spring, the Twins are almost directly overhead, and by the time the month of May arrives, they are still quite evident, standing upright, side-by-side about half way up in the western sky. Meanwhile, most of the rest of the winter pageant of bright stars have either disappeared or have sunk very low in the western sky to where they are difficult, if not impossible, to see. 

At the moment, we can see Gemini oriented sideways, about halfway up in the eastern sky as darkness falls. Like brilliant Orion, which is poised just to the lower right of the Twins, Gemini is an oblong figure, somewhat longer and thinner with fewer brilliant stars than its famous neighbor.

Hopeless to visualize? (Maybe not)

On moonless nights or in those places free of significant light pollution, stargazers can locate Gemini's feet in the Milky Way. Its brightest stars, Pollux (yellow-orange) and Castor (bluish-white) mark the heads of the Twins. They are just 5 degrees apart, making for a very good celestial yardstick in the sky. At New York's Hayden Planetarium, there was a very popular lecturer in the 1940s and '50s named Henry Neely who would point to Pollux and Castor and then down to a third bright star named Alhena, which marks one of Gemini's feet. Connecting these three stars, Neely would make reference to the "Wedge of Gemini," a far easier pattern for stargazers of today to find. "Somehow or other," Neely would tell his audience, "The ancient stargazers managed to find in these stars the outlines of two heroes standing close together." 

"But," he would continue, "it is hopeless for us to try and duplicate this feat of imagery."

And yet, it’s not as hopeless as Neely would have liked us to think. In his classic book, "The Stars – A New Way to see them" (Houghton Mifflin Co., 1952), H.A. Rey turns Gemini into a plausible outline: two matchstick men holding hands — the Twins. Rey's book became quite popular after its 1952 debut. He also lived in New York and I often wondered if he ever tried to meet Neely in person just to set him straight.

Not exactly "twins"

Some historical evidence suggests that when they were first chosen to represent the Twins, Pollux and Castor actually appeared to be twin stars of equal brightness. If true, either Pollux has grown brighter or Castor had faded, for there is a noticeable difference between them now. Pollux now appears about twice as bright as Castor and is also listed as one of the 57 navigational stars. 

Pollux is approximately 34 light-years away. It is estimated to be nine times bigger than our sun and is about twice as massive. In 1993, it was suspected that a planet was revolving around Pollux — that supposed planet was confirmed in 2006. It is a giant world, calculated to have a mass that is 2.3 times that of Jupiter and orbits Pollux with a period of 590 days. Originally designated as "Pollux b," the International Astronomical Union has formally christened this planet "Thestias." 

Castor, which is farther than Pollux at 51 light-years away, has some interesting credentials as well. In fact, the point of light we see with the unaided eye as a single star, is in reality six stars. In a telescope we can see two, Castor A and B. Furthermore, both A and B are themselves doubles, though much too close to be separated optically. Finally, well off to the south of the main pair is Castor C, a pair of dim red stars. To help visualize this remarkable sextuplet stellar family, picture three waltzing couples on a giant ballroom floor. Two couples (representing Castor A and B) move around each other a few feet apart, while the third pair (representing C) dances in a far corner of the room and very, very slowly circles the first two couples. [The Brightest Stars in the Sky: A Starry Countdown]

A lot happens here!

Some consider Gemini to be the "Times Square of the sky," because it seems to entertain all sorts of interesting events. Since the ecliptic crosses its boundaries, the sun, moon and planets can cross through Gemini. Up until 1988, the summer solstice occurred within Gemini's boundaries when the sun passed near the star Eta Geminorum on June 21 (the solstice point has since shifted west into the constellation of Taurus). Not far from Eta, Sir William Herschel discovered the planet Uranus — and in 1930, right next to the star Delta Geminorum, a tiny speck of light was identified as Pluto (once the ninth planet, now classified as a dwarf planet). The best meteor display of the year — the Geminids — appear to radiate from this part of the sky in mid-December and finally, Gemini is home to M35, considered to be one of the most beautiful star clusters. Dimly visible to the eye under favorable conditions, a good telescope shows it as a striking pattern of stars forming curves and festoons.

The end of the line

A good friend of mine was the late George Lovi, who wrote the "Rambling through the Sky" column for Sky & Telescope magazine for more than two decades up until his untimely death in 1993. Along with astronomy, George had a great fascination for trains, and once, he likened the stars of winter to watching a passing train at a railroad crossing. As you look down one end of the tracks, you can see the singular headlight of the approaching train; steam locomotives can have their headlight on the smokebox door or up near the funnel. In the sky, George likened the brilliant star Capella, in Auriga, the Charioteer as the locomotive headlight. Appearing above the northeast horizon around 10 p.m. in mid-September, it reminded George of looking down one end of the tracks, announcing the impending arrival of colder nights, soon to be accompanied by a train of brilliant stars passing before us that would take us right through the winter season.

And at the end of the train, we of course have the caboose accompanied with a pair of taillights moving with the receding train off into the distance, before finally disappearing. And to George, those "taillights in the sky" were Pollux and Castor, marking the very end of the precession of winter stars, finally disappearing beyond the northwest horizon at dusk on the first night of summer. 

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmer's Almanac and other publications, and he is also an on-camera meteorologist for News 12 Westchester, N.Y. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.