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2.2: Polaris, the Pole Star

  • Page ID
    4456
  • 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.

    Polaris
    Figure \(\PageIndex{1}\): (Not to scale) Polaris lies near the Earth's rotation axis.

    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, go to the chapter “Finding the Pole Star".

    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.