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June 17, 2024

Backyard Astronomy: Gemini (2004-01-03)

By Cheryl Haimann

The Summer Triangle and autumn's Great Square are not the only seasonal asterisms the night sky offers us. Six of winter's bright stars are sometimes grouped together as the Winter Hexagon. In recent weeks, we have looked at three of the six stars that mark the hexagon: Rigel, in Orion; Aldebaran, in Taurus; and Capella, in Auriga. The fourth star in the hexagon belongs to the next zodiac constellation, Gemini, the Twins.
In the early evening, Gemini (JEM-in-eye) rises in the east, to the left of Orion and below Auriga. The most noticeable feature are the two brightest stars, Castor and Pollux, which are about 5 degrees (three finger widths) apart. Many cultures have identified these two stars as a twins, or at least a pair, of men, gods, flora, or fauna. To the Chinese, they were Yin and Yang. When the US space program started sending two men into space in one spacecraft, it was only natural to name the program Gemini after this well-known pair of stars.

In legend, Castor and Pollux were the sons of mortal Leda and that rascally uber-god Zeus. Their sister was Helen, of Trojan War fame. Instead of being birthed in the normal fashion, the boys were hatched from an egg. Fine athletes, they sailed with Jason on the Argo to search for the Golden Fleece, and helped save the ship during a storm. Since that time, sailors have turned to them for protection from storms and pirates. The exclamation "Jiminy!" is actually derived from Gemini.

According to some stories, Castor was mortal and Pollux was immortal. When Castor was killed, Pollux asked Zeus to kill him as well. Since it was impossible to kill an immortal, at least in the days before Star Trek, Zeus could not honor this request. Instead, he allowed Pollux to divide his time, one day with the gods, the next day in Hades alongside his brother.

As with Auriga, you can use either the Big Dipper or Orion to point to Gemini. From the Dipper, start where the handle joins the bowl and trace a diagonal line through the bowl. Follow that line for 45 degrees to find Pollux. From Orion, follow a line from the right/top star of the belt through Betelgeuse, the left/bottom shoulder star. The line points to the area between Castor and Pollux, about 40 degrees away. Castor and Pollux are the heads of the twins. Their bodies extend to the right, or down later in the evening so that they seem to be standing upright.

Castor is the star on top, in the early evening, or to the right later at night. It is slightly less bright than Pollux, but considerably more interesting. In a telescope, Castor appears to be two or three stars. Actually, it is a sextuple star system, with three pairs of binary stars.

This week, the moon will also show you Gemini, even as its bright fullness obscures the fainter details. On January 6, in the early evening, the moon will be to the left of Saturn in the body of Castor. On the 7th, the moon will be right of Pollux.

Binocular users can see a nice open star cluster, M35, near Castor's foot and almost directly on the ecliptic. Wait another week, though, after the full moon has passed, to get the best view of this cluster.
Article © Cheryl Haimann. All rights reserved.
Published on 2004-01-03
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