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Corona Borealis overhead. Moon, Mars southwest at nightfall

2014-july-4-arcturus-alphecca-corona-borealis-night-sky-chart

Tonight for July 4, 2014

To see this famous C-shaped assemblage of stars, you have to look high overhead on these July evenings for Corona Borealis, also known as the Northern Crown. This constellation looks like a half-circle, in the middle of which is a white jewel of a star called Alphecca or Gemma. Plus the moon and Mars adorn the southwest sky at nightfall. See the sky chart at the bottom of the post.

Alphecca is the jewel in the Northern Crown

Brilliant blue-white Vega, the Summer Triangle’s brightest star, is found high in the east at nightfall in June and July

The Crown is located more or less along a line between two bright stars: Arcturus in the constellation Bootes the Herdsman and Vega in the constellation Lyra the Harp. Arcturus has already passed its highest point in the evening at this time of year and is slowly descending to the west. Vega is still high in the east on July evenings. With dark skies you will notice the orange color of Arcturus, and Vega’s bright blue-white tinge. Corona Borealis can be found between these two stars, but you will need a dark sky to see the faint semi-circle of stars composing this constellation.

Arcturus cuts through galaxy’s disk

Vega: Blue-white Harp Star

Corona Borealis

Jan Wojcik, Director of Reynolds Observatory, shows Corona Borealis with laser pointer; photo credit: Kyle Foley

The meaning of the Latin star name Gemma should be obvious. This star is the gem of the Northern Crown. But the star is also called Alphecca, from an Arabic phrase meaning the bright one of the dish. Gemma, aka Alphecca, is an eclipsing binary system. It consists of a smaller sun-like star that passes in front of a brighter star every 17.4 days, as seen from our earthly vantage point.

Alphecca: Northern Crown’s brightest star

On these sultry July evenings, look for Corona Borealis’ graceful semi-circle of stars to crown the top of the sky as soon as darkness falls, in between summertime’s two brightest stars: Arcturus and Vega. And in the southwest sky, look for the moon and Mars.

The moon is closing in on the planet Mars and star Spica. As seen from North America, the moon will move in between Mars and Spica tomorrow, on July 5.

The moon is closing in on the planet Mars and star Spica. As seen from North America, the moon will move in between Mars and Spica tomorrow, on July 5.