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Why Does the Location of the Big Dipper Change?

Ursa Major, a constellation that represents a bear and contains the famous Big Dipper star system, seems to change its location during the course of the evening. This is due to the rotation of the Earth on its own axis and not the movement of the stars themselves.
  1. Identification

    • The Big Dipper looks like a gigantic celestial ladle and consists of three stars that form a handle and four others that outline a bowl. The Dipper is one of the most prominent asterisms, a grouping of conspicuous stars that seem to form an easily recognizable pattern in the sky.

    Significance

    • To understand why the Big Dipper seems to circle a certain point in the sky, an observer must envision what a celestial sphere is. It is a giant imaginary sphere, with Earth as its center.

    Effects

    • The North Celestial Pole is that part of the inside of the celestial sphere that an imaginary line projected from the Earth's North Pole would touch. The star Polaris is within one degree of this spot, making it seem as if it never moves to an observer in the Northern Hemisphere.

    Considerations

    • Ursa Major is a circumpolar constellation, meaning that it is close enough to the North Star that it never "sets" below the horizon as the Earth rotates.

    Time Frame

    • As the night wears on, the Dipper will seem to move counter-clockwise around Polaris, completing one complete cycle around the star in a 24-hour period as the Earth finishes one revolution on its axis.


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