Cassiopeia and the Big Dipper in January
Cassiopeia and the Big Dipper in the night sky
Tonight, look for the northern sky’s two most prominent sky patterns – the constellation Cassiopeia the Queen and the Big Dipper. In fact, Cassiopeia and the Big Dipper circle around Polaris, the North Star, once a day, every day. That’s because they are opposite each other, one on either side of the North Star.
At nightfall, the constellation Cassiopeia the Queen is easy to recognize in the northern sky. This constellation looks 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.
And, of course, Ursa Major the Greater Bear – which contains the Big Dipper asterism – is one of the most famous star patterns. At nightfall this month, Cassiopeia shines high in the north while the Dipper lurks low. They are always on opposite sides of the North Star. From the southern half of the U.S., the Big Dipper is partly or totally beneath the horizon this month in the evening hours. North of about 40 degrees north latitude (the latitude of Denver, Colorado and Beijing, China), the Big Dipper always stays above the horizon (if your horizon is level). To see a precise view from your location, try Stellarium Online.
They circle around Polaris all night
But remember, their positions change as the night passes, as the great carousel of stars wheels westward (counterclockwise) around Polaris, the North Star. Polaris resides halfway between Cassiopeia and the Big Dipper. As a result, they are like riders on opposite sides of a Ferris wheel. Thus, looking northward, they rotate counter-clockwise around Polaris – the star that marks the sky’s north celestial 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 midnight tonight, Cassiopeia circles directly west (left) of Polaris. Whereas the Big Dipper sweeps to Polaris’ east (right). And then before dawn tomorrow, the Big Dipper climbs right above the North Star, while Cassiopeia swings directly below.
Bottom line: Watch the celestial clock and its two great big hour hands – Cassiopeia and the Big Dipper – as they swing around the North Star every night!