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Polaris - 10 Fascinating Facts
1. If you can find Polaris, you know which direction is north.
If you stand facing Polaris and stretch your arms out to your sides, your right hand will point east and your left hand will point west.
2. Polaris is easy to find.
You can locate Polaris from the Big Dipper (called “the Plough” in Britain). This is the one star pattern almost everyone knows. The two stars of the bowl which are farthest from the handle are called Merak and Dubhe. If you follow their line, as shown here, they point to Polaris. This is particularly useful at latitude 41 degrees and above, because the Big Dipper is always above the horizon there. It can be seen at some times all the way down to just south of the Equator.
3. Polaris is not the brightest star in the sky.
Although it's a common belief that Polaris is the brightest star in the sky, the importance of Polaris is its position. Yet as a second magnitude star it is still fairly easy to see, and just makes it into a list of the top fifty brightest stars. (The dimmer the star appears the higher its magnitude number. The bright star Sirius actually has a negative magnitude.)
4. Polaris indicates north because it just happens to be in line with the north pole of Earth's axis of rotation.
Imagine the north pole of our axis pointing into the sky. It points to within one degree of Polaris. If you were outside trying to orient yourself, this would point you in the right direction, though navigators have to correct for the fact that it's not exactly at the pole.
5. If you look at the stars more than once over several hours, you will see the stars seeming to move during the night – except for Polaris.
You can capture this movement by pointing a camera at the sky and leaving the shutter open. As the Earth turns, the light from the stars forms star trails. In this picture taken by P-M Hedén in Sweden, you can see Polaris in the center of the circles. Since it's in line with the Earth's axis of rotation, not only does it seem to stay in the same place, all the other stars seem to revolve around it.
6. Polaris hasn't always been the pole star.
Polaris has been used for navigation since the fifth century, but when the Egyptians built the pyramids, Thuban (in the constellation Draco) was nearest to the north celestial pole. Vega (in the constellation Lyra) will be the pole star twelve thousand years from now. This is because of precession. Earth's axis has a little wobble in it and over a period of 26,000 years the axis points to different areas of the sky. [To find out more follow the link to “Ecliptic and Equinoxes” at the end of this article.]
7. Polaris has been known throughout most of the northern hemisphere for thousands of years and it has names in many languages.
For astronomers Polaris is Alpha Ursae Minoris, i.e., the alpha star of Ursa Minor (Little Bear). [The article “Naming Stars” explains this designation.] In English it's commonly called the North Star or the Pole Star, though Lodestar or Guiding Star are other old names. Cynosura was an ancient Greek name from the time when the constellation represented a dog. The name means “the dog's tail”.
8. Polaris is not a single star, but a triple star system.
The main star (Polaris A) is is a supergiant six times the mass of the Sun. It's also a variable star of the Cepheid type which, based on Henrietta Leavitt's work on Cepheid variables, can be used to measure galactic distances. [There is a link to “Henrietta Swan Leavitt” at the end of this article.] Polaris A has a sunlike companion (Polaris B) which orbits at a great enough distance that it can be distinguished with good binoculars. William Herschel discovered it in 1780. A second companion (Polaris Ab) is so close to Polaris A that it was only discovered through its spectrum.
9. In nineteenth century America, before the end of the Civil War, escaped slaves “followed the drinking gourd” to freedom in the north.
The “drinking gourd” was the Big Dipper which lets you locate Polaris. Along with instructions passed on by anti-slavery activists, the pole star helped them find their way to sympathisers in the northern states and perhaps on to Canada.
10. The southern hemisphere doesn't have an equivalent of Polaris.
Sigma Octantis (in the constellation Octans) is sometimes named as the southern pole star. However, although it's fairly close to the pole, it's very faint, nearly sixth magnitude. Viewing conditions would have to be quite good in order to locate it. For navigation in the southern hemisphere the Southern Cross (constellation Crux) is used. It isn't quite as easy as using Polaris, but ABC Astronomy explains a simple method of finding south.
Content copyright © 2013 by Mona Evans. All rights reserved.
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