A synodic day is the period it takes for a planet to rotate once in relation to the star it is orbiting (its primary body). For Earth, the synodic day is known as a solar day, and its mean length is 24 hours (with fluctuations on the order of milliseconds).
The synodic day is distinguished from the sidereal day, which is one complete rotation in relation to distant stars. A synodic day is from "sunrise to sunrise", whereas a sidereal day is from one rise of a given star of reference to the next. (Thus, the word day denotes the orientation relative to the main "parent" star that the observer is orbiting.) These two quantities are not equal because the revolution of the body around its parent star would cause a single "day" to pass, even if the body did not rotate itself.
As viewed from Earth during the year, the Sun appears to slowly drift along an imaginary path coplanar with Earth's orbit, known as the ecliptic, on a spherical background of seemingly fixed stars. Each synodic day, this gradual motion is a little less than 1° eastward (360° per 365.25 days), in a manner known as prograde motion.
Certain spacecraft orbits, Sun-synchronous orbits, have orbital periods that are a fraction of a synodic day. Combined with a nodal precession, this allows them to always pass over a location on Earth's surface at the same mean solar time.
The synodic day is not constant, and changes in length slightly over the course of the year due to the eccentricity of the Earth's orbit around the Sun and the tilt of its axis. This change accounts for the difference between the mean and apparent solar time in the equation of time, which can also be seen in Earth's analemma.
- Gerard, T. Hooft; Stefan, Vandoren. Time in Powers of Ten: Natural Phenomena and Their Timescales.
- "SATELLITES AND ORBITS" (PDF).
- David W. Hughes, B.D. Yallop, C.Y. Hohenkerk (15 June 1989). "The Equation of Time". Monthly Notices of the Royal Astronomical Society. Royal Astronomical Society. 238: 1529-35. Bibcode:1989MNRAS.238.1529H. doi:10.1093/mnras/238.4.1529. ISSN 0035-8711.CS1 maint: uses authors parameter (link)