Introduction to astronomy. The daily motion of the sky

At night the stars rise in the east, set in the west, and the whole sky seems to turn about an invisible axis; in fact this is the apparent result of the Earth rotating on its axis from west to east. One full turn of the sky relative to the stars takes 23 hours 56 minutes 4 seconds, one sidereal day, so in an hour the stars shift by about 15° (360° : 24). The Pole Star (Polaris, in Ursa Minor), lying close to the north celestial pole, hardly moves, which is why it helps to find north. A constellation is a region of the sky with sharply defined boundaries; the International Astronomical Union approved 88 of them in 1922–1930. At mid-latitudes star groups such as Ursa Minor, Cassiopeia and the Big Dipper (part of Ursa Major) never dip below the horizon all night, while Orion is well seen in winter.