Synodic day

A synodic day is the period it takes for a planet to rotate once in relation to the body it is orbiting. For Earth, the synodic day is known as a solar day, and is about 24 hours long.

The synodic day is distinguished from the sidereal day, which is one complete rotation in relation to distant stars. A synodic day may be "sunrise to sunrise'" whereas a sidereal day can be from the rise of any star to the rise of the same star on the next day. These two quantities are not equal because of the body's movement around its parent.

Seen from Earth, the Sun appears to slowly move in front of a fixed sphere of stars along an imaginary line known as the ecliptic. In one synodic day this movement is a little less than one degree toward the east (360 degrees/year)/(365.25 days/year), in a manner known as prograde motion.

Although correct, neither term can be defined as the rising or setting of a celestial body because of variations due to the Earth's tilt. See the equation of time.

See also