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The Summer Triangle and the Southern Cross

December 24, 2011; 5:56 AM ET

First of all, this blog and the two sky maps below are courtesy of a favorite astronomy site of mine, EarthSky.org. This site is downright terrific and was the inspiration for this blog, and has graciously allowed me to use many skymaps in previous blogs as well as this one. This site is an absolutely fantastic source for all sorts of information about the field of science in general. I suggest and plead you to browse the site.

In the west just after sunset this time of year is a pattern of stars called the "summer triangle". This group of stars is called an "asterism". What is the difference between an asterism and a constellation you ask? The simple answer to that question is that in 1930 the International Astronomical Union (IAU) divided the sky into 88 official constellations based on what had been historically recognized as constellations. Anything outside these 88 are called asterisms.

Anyways, according to earthsky.org, "The Triangle consists of three bright stars in three different constellations. They are Vega in the constellation Lyra the Harp, Deneb in the constellation Cygnus the Swan, and Altair in the constellation Aquila the Eagle. It's called the "summer" triangle, because - for us in the Northern Hemisphere - summer is the season in which these stars soar overhead. Still, if you look for this pattern this month, you'll find that, around the time of the winter solstice, the Summer Triangle is descending in the west in early evening. It's getting closer each evening to disappearing into the sunset glare. "

Those of you who are lucky enough to spend the holiday season in Hawaii, I suggest you take advantage of a rather rare sight in the Northern Hemisphere, the Southern Cross. You can see these stars, also known as the constellation "Crux", low in the sky towards dawn near the stars Rigel Kentaurus (the alpha star in the constellation Centaurus and more widely known as Alpha Centauri and also famous as being the closest star to us besides the sun) and Hadar.

According to earthsky.org, "There is one surefire way to know if the Southern Cross is visible in your sky or not. When the M or W-shaped constellation Cassiopeia the Queen is out in the sky, the Southern Cross is below the horizon. Although Cassiopeia lights up Hawaiian skies on winter evenings, it sets beneath Hawaii's northern horizon several hours before sunrise. As Cassiopeia sets, the Southern Cross rises. For all but the southernmost parts of the United States, Cassiopeia never sets. Therefore, the Southern Cross never rises. The Southern Cross marks the southern terminus of the glowing band of stars that we call the Milky Way, whereas Cassiopeia lodges at the Milky Way's northern terminus."

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Astronomy Blog
The AccuWeather.com astronomy blog, by Dave Samuhel, discusses stargazing, including how weather will affect viewing conditions of astronomical phenomenon.