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Sunday, October 4, 2015

Earthsky Tonight: January 12, 2010 – Cassiopeia and Big Dipper are opposite each other

Courtesy of EarthSky
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Cassiopeia and Big Dipper opposite each other

The constellation Cassiopeia the Queen is easy to recognize in the northern sky, either in the evening or before dawn. This constellation is shaped like a W or M and contains five moderately bright stars. The distinctive shape of Cassiopeia makes her very noticeable among the stars of the northern sky.

Cassiopeia is also famous in relationship to another constellation, Ursa Major, which contains the Big Dipper asterism. At nightfall this month, Cassiopeia is high in the north while the Dipper lurks low. From the southern half of the U.S., the Big Dipper sinks out of sight, since it’s partially or totally beneath the horizon. North of about 40 degrees north latitude (the latitude of Denver, Colorado), the Big Dipper always stays above the horizon. Nonetheless, the Big Dipper’s proximity to the horizon may cause haze low in the sky to obscure these stars in the evening at this time of year.

That’ll change later tonight, as the great carousel of stars wheels westward (counter-clockwise) around Polaris, the North Star. Polaris resides halfway between Cassiopeia and the Big Dipper, so these two star formations are like riders on opposite sides of a Ferris wheel. Looking northward, they rotate counter-clockwise around Polaris – the star that marks the sky’s north pole – once a day. Approximately every 12 hours, as Earth spins beneath the heavens, Cassiopeia and the Big Dipper trade places in the sky.

Thus, around 11 p.m. tonight, Cassiopeia circles directly west (left) of Polaris, whereas the Big Dipper sweeps to Polaris’ east (right). Before dawn tomorrow, the Big Dipper climbs right above the North Star, while Cassiopeia swings directly below.

Written by earthsky

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