Polaris: As Pole Star

Contents

Introduction

Two bright constellations occupy opposite sides of the pole star--the Big Dipper and Cassiopeia. As the celestial sphere rotates (or appears to rotate), these constellations also march in circles around the pole . Depending on the hour of the night and the day of the year, one or the other may be low near the horizon where it is barely seen, or even hidden below the horizon. But when that happens the other constellation is sure to be high in the sky, where (weather permitting) it is easily seen

By pure chance, a moderately bright star is seen near the northern celestial pole--Polaris, the pole star (or north star). Polaris is not exactly at the pole, but its daily circle is very small and for many purposes one can assume it is at the pole, a pivot around which the entire sky rotates.

All this looks much clearer if one remembers that it is the Earth that rotates, not the sky. The axis around which the Earth spins points in a certain direction in the sky, and that is also the direction of the pole star (or more accurately, the northern celestial pole). As the Earth turns, even though the observer moves with it (for instance, from point B in the drawing to point A), that direction always makes the same angle with the horizon and is always to the north. Hence the pole star is always in the same spot--north of the observer, and the same height above the horizon.

If on a clear night you find yourself lost in the wilderness or at sea, the pole star can tell you where north is, and from that you easily deduce east, west and south. Any other star is unreliable for determining direction--it will move across the sky, and may even set--but not this one. For instructions on finding the pole star at night, click here.

The closer you are to the equator, the closer is the pole star to the horizon, and at the equator (point C) it is on the horizon, and probably not easy to see. Further south, at points such as D, it is no longer visible, but now you can see the southern pole of the sky. Unfortunately, no bright star comparable to Polaris marks that position. The existence of a bright star near the north celestial pole is just a lucky accident, and as will be seen, it wasn't always so, and will not be a few thousand years from now.

The Big Dipper

The Big Dipper consists of 7 bright stars, forming a dipper, a small pot with a long handle. In England it is often called "the plough" (spelled "plow" in the US), and fugitive slaves before the Civil War knew it as "the drinking gourd", a signpost in the sky pointing the way north to safety, to Canada where slavery was outlawed. Astronomers name it "Ursa Major," Latin for "the big bear," and some other languages also refer to it as the Big Bear.

When the territory of Alaska in 1926 decided to create a flag of its own, it asked citizens to submit proposed designs for the new flag. The winning design was that of Benny Benson, age 13, and is reproduced on the right. It shows the 7 stars of the Big Dipper and Polaris, the north star. When Alaska became a state, this became the state flag, and Alaska's Flag, a song about it by Marie Drake, was chosen as the state song.

The flag also shows how the north star can be found. Imagine a line connecting the two stars at the front of the "dipper", continue it on the side where the dipper is "open" to a distance 5 times that between the two stars (the flag shortens this a bit!), and you will arrive at (or very close to) the pole star. Because of their role in locating Polaris, these two stars are often called "the guides." And by the way--the last-but-one star in the handle of the "dipper", named Mizar by Arab astronomers, is a double star, whose components are readily separated by binoculars--or, some say, by very sharp eyes during good viewing conditions.

Cassiopeia

Cassiopeia was a queen in Greek mythology, and the constellation named for her is shaped like the letter W. Polaris is above the first "V" of this letter. If you draw a line dividing the angle of that "V" in half and continue along it, you will reach the vicinity of Polaris.

The name of Cassiopeia's husband, King Cepheus, goes with a nearby constellation, above the other "V" (the brighter one), but Cepheus is nowhere as striking as Cassiopeia. Her daughter Andromeda has another constellation, framed by a big undistinguished rectangle of four stars. An unremarkable constellation to the eye--but it contains a large galaxy, our nearest neighbor in space (not counting two dwarf galaxies in the southern sky), one which seems to resemble ours in size and shape.

Ursa Minor

The "Small Bear" or "Little Dipper" is a constellation somewhat resembling the Big Dipper, and Polaris is the last star in its tail. The "dipper" itself faces the tail of the Big Dipper, so that the two "tails" (or "handles") point in opposite directions. The two front stars of the "little dipper" (quite smaller and more square than the big one) are fairly bright, but other stars are rather dim and require good eyes and a dark sky.

Preview Image

An image of Polaris as seen by NASA's High-speed Imaging Photometer for Occultations (HIPO) optics through the Stratospheric Observatory for Infrared Astronomy (SOFIA), the largest airborne observatory in the world.  (Source: "NASA Airborne Observatory Sees Stars For First Time.")

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