Synodic day

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).

See also: Rotation period

The synodic day is distinguished from the sidereal day, which is one complete rotation in relation to distant stars.[1] This is different from the duration of a synodic day because the revolution of the body around its parent star would cause a single "day" to pass relative to a star, even if the body did not rotate itself.

Earth's synodic day

Earth's synodic day is the time it takes for the Sun to pass over the same meridian (a line of longitude) on consecutive days, whereas a sidereal day is the time it takes for a given star of reference to the pass over a meridian on consecutive days.[2] So, for example in the Northern Hemisphere, a synodic day could be measured as the time taken for the sun move from exactly true south (i.e. its highest declination) on one day to exactly south again on the next day (or exactly true north in the Southern Hemisphere).

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). This length is not constant, and changes over the course of the year due to the eccentricity of the Earth's orbit around the Sun and the axial tilt of the Earth.[3] 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.[4] The longest and shortest synodic days' durations differ by about 51 seconds.[5]

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.[6] 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.[7]

The Moon's synodic day
Main article: Lunar day

The Moon's synodic day is through tidal locking the same length as the Moon's synodic orbital period.

See also

References
  1. Gerard, T. Hooft; Stefan, Vandoren (12 May 2014). Time in Powers of Ten: Natural Phenomena and Their Timescales. ISBN 9789814494939.
  2. "Sidereal vs. Synodic Motions". Astronomy Education at the University of Nebraska-Lincoln. The University of Nebraska-Lincoln. Retrieved 22 September 2020.
  3. 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 (4): 1529–35. Bibcode:1989MNRAS.238.1529H. doi:10.1093/mnras/238.4.1529. ISSN 0035-8711.CS1 maint: uses authors parameter (link)
  4. "Equation Of Time". In-The-Sky.org. Dominic Ford. Retrieved 22 September 2020.
  5. Jean Meeus (1997), Mathematical astronomy morsels (Richmond, VA: Willmann-Bell) 346. ISBN 0-943396-51-4.
  6. https://www.nasa.gov/multimedia/imagegallery/image_feature_635.html
  7. "SATELLITES AND ORBITS" (PDF).
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