(1a) Finding the Pole Star

Lesson Plan for (1a) : Finding the Pole Star

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.

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.

Finding the North Star Lesson Plan


Next Stop: #2 The Path of the Sun, the Ecliptic



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Author and curator: David P. Stern
Last updated 9 May 1999


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