Asynodic day (orsynodic rotation period orsolar day) is theperiod for acelestial object to rotate once in relation to thestar it isorbiting, and is the basis ofsolar time.
The synodic day is distinguished from thesidereal day, which is one complete rotation in relation to distant stars[1] and is the basis of sidereal time.
In the case of atidally locked planet, the same side always faces its parent star, and its synodic day is infinite. Its sidereal day, however, is equal to its orbital period.
Earth's synodic day is the time it takes for theSun to pass over the samemeridian (a line oflongitude) on consecutive days, whereas a sidereal day is the time it takes for a given distant star to pass over a meridian on consecutive days.[2] For example, in theNorthern Hemisphere, a synodic day could be measured as the time taken for the Sun to move from exactly true south (i.e. its highestdeclination) on one day to exactly south again on the next day (or exactly true north in theSouthern Hemisphere).
For Earth, the synodic day is not constant, and changes over the course of the year due to theeccentricity of Earth's orbit around the Sun and theaxial tilt of the Earth.[3] The longest and shortest synodic days' durations differ by about 51 seconds.[4] The mean length, however, is 24 hours (withfluctuations on the order ofmilliseconds), and is the basis ofsolar time. The difference between themean andapparent solar time is theequation of time, which can also be seen in Earth'sanalemma. Because of the variation in the length of the synodic day, the days with the longest and shortest period of daylight do not coincide with thesolstices near the equator.
As viewed from Earth during the year, the Sun appears to slowly drift along an imaginary pathcoplanar withEarth's orbit, known as theecliptic, on aspherical background of seeminglyfixed stars.[5] Each synodic day, this gradual motion is a little less than 1° eastward (360° per 365.25 days), in a manner known asprograde motion.
Certainspacecraft orbits,Sun-synchronous orbits, haveorbital periods that are a fraction of a synodic day. Combined with anodal precession, this allows them to always pass over a location on Earth's surface at the samemean solar time.[6]
Due totidal locking with Earth, theMoon's synodic day (thelunar day or synodic rotation period) is the same as itssynodic period with Earth and the Sun (the period of thelunar phases, thesynodic lunar month, which is the month of thelunar calendar).
Due to the slowretrograderotational speed ofVenus, its synodicrotation period of 117 Earth days is about half the length of itssidereal rotational period (sidereal day) and even its orbital period.[7]
Due toMercury's slow rotational speed and fast orbit around the Sun, its synodic rotation period of 176 Earth days is three times longer than its sidereal rotational period (sidereal day) and twice as long as its orbital period.[8]
