Sunday, December 30, 2012

The Constellation Gemini

This post is the one of a series on constellations and posted throughout the year as each constellation comes into prominence.
Gemini is an important constellation to learn for a number of reasons. Known as the twins, it is home to two of the brightest stars in the sky -- Pollux and Castor. Pollux on the left is the 12th and Castor the 16th brightest stars in the Northern hemisphere. These stars are the heads of two twin brothers standing side by side. On a good night, you can see lines of stars outlining the bodies, as shown below:
Image from Wikipedia
Gemini is one of the 12 zodiacal constellations, which means it lies along what's called the ecliptic. The ecliptic is a circle of constellations surrounding us which lies along the same plane as the planets and the solar system as a whole. Therefore, any planets you see are always somewhere along this plane. Gemini is therefore useful to know as planets are often located nearby.  For instance, the bright planet Jupiter will be approaching Gemini early 2013 and spend late 2013 and most of 2014 within the region.

Being along the ecliptic means that the moon will pass through the constellation once a month.

Being along the ecliptic also means that the sun will pass through the constellation (rendering it invisible of the brightness of the sun) once a year. The sun passes through Gemini from May 21 to June 20 each year. During the winter months, the sun is in the opposite side of the sky which makes Gemini an easy constellation to see.

Below is a map of the surrounding constellations for the evening hours in December.
2012 December skies ~ 7pm
2013 January skies ~ 6pm
Image by Skymaps
As you can see from this image, Gemini is pretty easy to spot -- being above and to the left of the familiar constellation Orion. Finding the bright head stars should be easy to do, but next time you have a clear night, see if you can identify some of the other stars making up the body. I have often seen them as forming the Greek letter Ω (Omega).

Of course, a description of Gemini would be incomplete without mentioning the Geminids, a meteor shower that is one of the best in the year. During the nights around December 13 and 14 each year, many bright meteors (sometimes around 100 per hour!) can be visible. They emanate from Gemini - which means they will go outwards away from Gemini. They don't always begin in that constellation, but their paths will tend to point away from Gemini more often than not. Most meteor showers are best observed in the wee morning hours.

A good binoculars object, M35 is a nice cluster of stars that will be my goal to find in the next month. It has the width of the moon, and contains well over 200 stars, but most are invisible to the naked eye. Castor is supposedly juggling it on his foot like a soccer ball.

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