The Big Dipper
May 27, 2009
One the most well-known constellation asterisms found in northern hemisphere skies is the Big Dipper. This bright, sauce-pan shaped group of stars is part of the much larger constellation of Ursa Major – the Great Bear. At this time of year, as the Earth’s orbital motion seems to tip our northern climes to the Sun and as spring spreads it verdant blanket across the land, the Big Dipper can easily be found high above the north celestial pole after darkness falls. In fact, sky-watchers living anywhere near 56 degrees north latitude can see the Big Dipper sprawling across their zenith at the end of evening twilight during the month of May. The astronomical term for the Big Dipper above the north celestial pole in spring is “upper culmination.” During autumn it lies near, on, or below the northern horizon, and is said to be at “lower culmination.”
Five of the seven stars that make up the Big Dipper comprise a portion of the Ursa Major Moving Group. Also designated as Collinder 285, this is part of one of the closest star clusters to our solar system. Roughly 75 light years away, the stars Merak, Phecda, Megrez, Alioth, and the Mizar/Alcor system are all moving together toward our solar system at ~ 10 km/second.
Photo details: The above photograph; is one single, three minute guided exposure, centered at 10:32 p.m. local time. It was made on the beautifully clear evening of May 11, 2009, from the driveway of my home near De Soto, Kansas. In order to highlight the Big Dipper itself, I used a Tiffen Fog 2 filter, which I removed briefly during the exposure to illuminate our fresh spring oak and hickory trees with an electronic flash. I used a Canon Rebel XTi camera set at ISO 200, with a 24mm Nikkor lens, @t f/4.