Tag Archives: Ursa Minor

Polaris (The North Star)

Circumpolar star trailsPolaris’ special significance is that it is the Pole Star, located very near the point at which the Earth’s northern axis intersects the celestial sphere. As a result of this unique location in the sky, Polaris will appear to remain stationary all night while all of the other stars will appear to rotate around it. You may have seen star trail images that illustrate this effect. Another implication of Polaris’ unique location is that it is a measure of one’s latitude. From Ashton, MD for example, Polaris appears 39.15° above the point on the horizon in the direction of due north, which corresponds to Ashton’s latitude on a map or globe. Polaris marks the end of the bear’s tail in Ursa Minor, or the end of the handle of the Little Dipper.

The best place to start when looking for Polaris is the Big Dipper which is part of the constellation known as Ursa Major. The two stars at the end of the bowl farthest from the handle are pointer stars. Follow an imaginary line from the pointer star at the  bottom of the bowl, through the one at the top of the bowl. The 2nd magnitude star that is about five times the distance between the two pointer stars is Polaris.

The Big Dipper Pointer Stars, Jim Johnson, December 27, 2014.
The Big Dipper Pointer Stars, Jim Johnson, December 27, 2014.

Polaris has not always been the Earth’s Pole Star, nor will it always be the pole star. As a result of the procession of the equinox, the axis about which the Earth rotates will trace out a large circle every 26,000 years, as illustrated below. This sounds like a long time, but when the Egyptian pyramids were built 5,000 years ago, Thuban in Draco was the pole star, and thus some north facing entrances of the pyramids were aligned on this star. In 8,000 years from now, Deneb in Cygnus will be the pole star, and the Earth’s axis will be aligned with a point near Vega in about 12,000 years from now.
http://en.wikipedia.org/wiki/Polaris.

© James R. Johnson, 2014.
jim@jrjohnson.net

Ursa Minor (The lesser bear)

320px-UrsaMinorCCSimilar to Ursa Major, which contains the Big Dipper asterism, Ursa Minor contains another recognizable asterism that is often called the Little Dipper. Some of the dimmer dipper stars can be difficult to locate in city skies, and the other stars forming the bear might be impossible to see. Polaris, the constellation’s brightest star, is found at the end of the bear’s tail, or at the end of the dipper handle. This star is thought by some to be significant because it is the brightest star in the sky, which is incorrect – Sirius in Canis Major is actually the brightest star in all of the night sky. An observer can verify that Polaris is not the brightest star in the sky by locating it, and comparing it with other stars in the rest of the sky. Polaris can be located by using the Big Dipper’s pointer stars, the two stars at the end of the dipper bowl away from the handle, and drawing an imaginary line upwards from the top of the bowl.

Ursa Minor is the 56th-largest constellation, and one of the 48 constellations cataloged by the 2nd century astronomer Ptolemy that remain among the 88 modern constellations. There are no Messier objects in this constellation.
http://en.wikipedia.org/wiki/Ursa_Minor_(constellation)

IAU Ursa Minor Chart, IAU and Sky & Telescope magazine (Roger Sinnott and Rick Fienberg), June 5, 2011.

© James R. Johnson, 2014.
jim@jrjohnson.net