A synodic day is the period it takes for a planet to rotate once in relation to the body it is orbiting (as opposed to a sidereal day which is one complete rotation in relation to distant stars).
Thus, a synodic day may be "sunrise to sunrise'" whereas a sidereal day may be "star-rise to star-rise". These two quantities are not equal because the Sun, as seen from Earth, moves against the fixed sphere of stars along an imaginary line known as the ecliptic. The Sun appears to move a little less than a degree each day, (360 degrees/year)/(365.25 days/year), toward the east 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.
For Earth, the synodic day is known as a solar day, and is about 24 hours long.
Famous quotes containing the word day:
“Thanksgiving Day—A day devoted by persons with inflammatory rheumatism to thanking a loving Father that it is not hydrophobia.”
—H.L. (Henry Lewis)