Does the Big Dipper Change Positions?

big-dipper-change-positions Credit: Andrew Holt/Photographer's Choice/Getty Images

The constellation Ursa Major, also known as "the Big Dipper," changes position continuously as seen from Earth. According to EarthSky, the Big Dipper is always visible on clear nights from North America. The apparent motion of the constellation is caused by three factors: the Earth's rotation on its axis, its revolution around the sun and, on a much longer timescale, the relative motions of the stars themselves.

The most readily apparent source of the Big Dipper's motion is the rotation of the Earth itself. Every night, the so-called "fixed stars" appear to cross the sky from east to west as the Earth turns on its axis. This same rotational motion causes the sun to appear to rise and set during the day. Another apparent source of motion is the seasonal shift in position caused by the Earth's movement around the sun. According to EarthSky, the Big Dipper is a circumpolar constellation, meaning that it appears to circle the north pole over the course of a year as the Earth moves from one position in its orbit to another.

The seven stars of the Big Dipper range in distance from 63 to 210 light years from Earth, and each has its own momentum. This causes the constellation to change shape, as seen from Earth, on a scale of hundreds of thousands of years, according to EarthSky.