day, period of time for the earth to rotate once on its axis. The ordinary day, or solar day, is measured relative to the sun, being the time between successive passages of the sun over a stationary observer's celestial meridian. The length of a solar day varies during the course of a year, so for purposes of time measurement an average, or mean, solar day is used (see solar time), equal to exactly 24 hr. The sidereal day, used by astronomers, is measured relative to the fixed stars rather than the sun (see sidereal time); it is about 4 min shorter than the mean solar day. The term day is also used to refer to that part of each 24-hr period during which the sun's direct rays are not blocked by the earth, this period of daylight hours extending from sunrise to sunset; the remaining portion of the 24 hr is called night. If the plane of the earth's orbit about the sun coincided with the plane of the equator, day and night would each be 12 hr long everywhere on the earth all year long. However, because of the tilt of the earth's axis of rotation, the times of sunrise and sunset vary from day to day, with the result that in the Northern Hemisphere there are long days and short nights in the summer and short days and long nights in the winter. See equinox; solstice.