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About Polaris

Ursa Minor Lesser Bear - Polaris (North Star) - Ursa Major (Greater Bear)How Do I Find Polaris?
Polaris is located in the constellation Ursa Minor, which contains the group of stars that make up the “Little Dipper.” Polaris is the star in the end of the Little Dipper handle. Often, however, the Little Dipper is not very bright and can be challenging to find. Polaris is easiest to find by locating the seven stars of the Big Dipper in the constellation Ursa Major, or Big Bear. These stars form a small bowl with a long handle. Follow the stars of the Big Dipper from the handle to the side of the bowl, to the bowl bottom, and up the other side; the two stars forming the second side, Dubhe and Merak, point to Polaris. Take the distance between Dubhe and Merak; Polaris is the bright star that sits about five times that distance away.

Polaris actually is part of a binary (two) star system. Of the stars nearest to our Sun, about half are known to be in multiple systems (two or more stars). Between 5 and 10 percent of the stars we can observe are “visible binaries” — the star companions can be observed with a telescope. These systems reveal a great deal of information. Because of their interactions, astronomers can determine the gravitational pull exerted by the companions and calculate the mass of the individual stars. Polaris is helpful to astronomers in another way; it is a variable star. Polaris pulsates, a change we detect as a pattern of dimming and brightening. This pulsation helps astronomers calculate celestial distances.

Why Doesn't Polaris Move?
Polaris is very distant from Earth, and located in a position very near Earth's north celestial pole.

Earth rotates once a day on its axis, an imaginary line that passes through Earth from its north pole to its south pole. If that imaginary line — the axis — is projected into space above the north pole, it points to Earth's north celestial pole. As the Earth spins on its axis, it also “spins around” the north celestial pole. Polaris, located almost exactly at the north celestial pole, the center of spin, stays in the same place, while stars farther away from the north celestial pole can be seen to move in a wider circle around Polaris as viewed from Earth during its daily rotation.

Polaris actually lies just a short distance away from where Earth's axis points. Polaris is located about 1 degree off to the side of the north celestial pole, so Polaris does move a little, tracing a very small arc in the night sky, around which the other visible stars make wider circles.

Earth rotates once a day on its axis, an imaginary line that passes through Earth from its north pole to its south pole.

Night sky above Hawaii

This picture of the night sky above Hawaii was taken by leaving the camera shutter open for a long time. The picture captures the apparent movement of the stars caused by Earth's rotation on its axis. Polaris is the star in the center of the star field; it shows essentially no movement. Earth's axis points almost directly to Polaris, so this star is observed to show the least movement. The other stars appear to trace arcs of movement because of Earth's spin on its axis.

http://www.wainscoat.com/astronomy/
Photograph by Richard J. Wainscoat

 

 

 

 

 

 

 

 

 

Is There a South Star?
While there are some distant stars close to the south celestial pole, there are no easily observable ones. The south celestial pole lies in the constellation Octans, and the closest stars in that constellation are very, very dim to the unaided human eye.

Why is Polaris Important?
For thousands of years, navigators have used Polaris to guide them to and from their homeports. Sailors still use Polaris for celestial navigation, and modern astronomers often use Polaris to help them align their telescopes. For any given viewing location on Earth north of the equator, Polaris' position does not change through the night or the year. It marks Earth's north celestial pole.

If you stand at Earth's north pole, Polaris is directly above you. If you stand at the equator, Polaris is on the horizon. If you are at a location between the north pole and the equator, Polaris occupies an intermediate position above the horizon. In other words, Polaris is a useful marker to help you determine where north is, and at what latitude you are located. Your latitudinal position is equal to the elevation above the horizon. At the north pole (90 degrees latitude ) Polaris is overhead at an altitude of 90 degrees. At the equator (zero degrees latitude) Polaris is at an altitude of zero degrees. The same is true for any location on Earth. For example, Cleveland, Ohio, is at a latitude of about 42 degrees north of the equator; Polaris can be observed at an altitude 42 degrees above the horizon.

Will Polaris Always Be “The Pole Star?”
No. Today, Earth's axis points just a little away from Polaris; Polaris is conveniently close to the north celestial pole and can be used in the context of the “Pole Star.” However, Earth's axis wobbles; it points to different places in the sky. This wobble is small and slow, with a cycle of about 23,000 years. Earth's pole is actually slowly moving closer to pointing directly at Polaris. It will be closest to Polaris in approximately 100 years and then will begin to move away from the star. About 4500 years ago, the north celestial pole was located in the constellation Draco. Calculations suggest that in about 10,000 years Earth's axis will point to Vega, in the constellation Lyra.

Why Do Most Stars and Constellations Move?
The stars are distant objects. Their distances vary, but they are all very far away. Excluding our Sun, the nearest star, Proxima Centauri, is more than 4 light years, or about 23 trillion miles, away. Polaris and its companion are about 390 light years away from us! As Earth spins on its axis, we, as Earth-bound observers, spin past this background of distant stars. As Earth spins, the stars appear to move across our night sky from east to west, for the same reason that our Sun appears to “rise” in the east and “set” in the west.

Stars close to the celestial poles, the imaginary points where Earth's north and south axes point in space, have a very small circle of spin. The farther from the celestial pole, the wider the circle the stars trace. Stars that make a full circle around a celestial pole, like those in the Big and Little Dippers in the northern hemisphere, are called “circumpolar stars.” They travel across the night sky and do not set. At the equator, however, there are no circumpolar stars because the celestial poles are located at the horizon. All stars observed at the equator rise in the east and set in the west.

Why Do We See Different Constellations During the Year?
If observed through the year, the constellations shift gradually to the west. This is caused by Earth's orbit around our Sun. In the summer, night sky viewers are looking in a different direction in space than they see during the winter.

To learn more about constellations, visit the SkyTellers Constellations site.

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Last updated
May 21, 2007