Synodic day
Asynodic day(orsynodic rotation periodorsolar day) is theperiodfor acelestial objectto rotate once in relation to thestarit 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 lockedplanet, 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[edit]
Earth's synodic day is the time it takes for theSunto 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 theeccentricityof Earth's orbit around the Sun and theaxial tiltof the Earth.[3]The longest and shortest synodic days' durations differ by about 51 seconds.[4]The mean length, however, is 24 hours (withfluctuationson the order ofmilliseconds), and is the basis ofsolar time.The difference between themeanandapparentsolar time is theequation of time,which can also be seen in Earth'sanalemma.[5]
As viewed from Earth during the year, the Sun appears to slowly drift along an imaginary pathcoplanarwithEarth's orbit,known as theecliptic,on aspherical backgroundof seeminglyfixed stars.[6]Each synodic day, this gradual motion is a little less than 1° eastward (360° per 365.25 days), in a manner known asprograde motion.
Certainspacecraftorbits,Sun-synchronous orbits,haveorbital periodsthat 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.[7]
The Moon[edit]
Due totidal lockingwith Earth, theMoon's synodic day (thelunar dayor synodic rotation period) is the same as itssynodic periodwith Earth and the Sun (the period of thelunar phases,thesynodic lunar month,which is the month of thelunar calendar).
Venus[edit]
Due to the slowretrograderotational speedofVenus,its synodicrotation periodof 117 Earth days is about half the length of itssiderealrotational period (sidereal day) and even its orbital period.[8]
Mercury[edit]
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.[9]
See also[edit]
- Noon § Solar noon
- Orbital period
- Rotation period
- Sidereal time
- Solar rotation
- Solar time
- Sun transit time
- Synodic month
- Day length fluctuations
References[edit]
- ^Gerard, T. Hooft; Stefan, Vandoren (12 May 2014).Time in Powers of Ten: Natural Phenomena and Their Timescales.World Scientific.ISBN9789814494939.
- ^"Sidereal vs. Synodic Motions".Astronomy Education at the University of Nebraska-Lincoln.The University of Nebraska-Lincoln.Retrieved22 September2020.
- ^David W. Hughes; B.D. Yallop; C.Y. Hohenkerk (15 June 1989)."The Equation of Time".Monthly Notices of the Royal Astronomical Society.238(4).Royal Astronomical Society:1529–35.Bibcode:1989MNRAS.238.1529H.doi:10.1093/mnras/238.4.1529.ISSN0035-8711.
- ^J. M. A. Danby; Jean Meeus (1997).Mathematical Astronomy Morsels.Willmann-Bell.ISBN978-0-943396-51-4.
- ^"Equation Of Time".In-The-Sky.org.Dominic Ford.Retrieved22 September2020.
- ^"The Plane of the Ecliptic".NASA.July 12, 2016.
- ^"SATELLITES AND ORBITS"(PDF).
- ^"How long is a day on Venus?".TE AWAMUTU SPACE CENTRE.Retrieved2021-06-03.
- ^"ESO".ESO.Retrieved2021-06-03.
