Synodic day

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Not to be confused with Synodic period."Solar day" redirects here. For the measurement of a solar day, see Solar time.

A synodic day (or synodic rotation period or solar day) is the period for a celestial object to rotate once in relation to the star it is orbiting, and is the basis of solar time.

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The synodic day is distinguished from the sidereal day, which is one complete rotation in relation to distant stars [1] and is the basis of sidereal time.

In the case of a tidally 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

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 distant star to pass over a meridian on consecutive days. [2] For example, in the Northern Hemisphere, a synodic day could be measured as the time taken for the Sun to 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 not constant, and changes over the course of the year due to the eccentricity of Earth's orbit around the Sun and the axial 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 (with fluctuations on the order of milliseconds), and is the basis of solar time. The difference between the mean and apparent solar time is the equation of time, which can also be seen in Earth's analemma. [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 1.2° eastward (360° per 280.25 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

Due to tidal locking with Earth, the Moon's synodic day (the lunar day or synodic rotation period) is the same as its synodic period with Earth and the Sun (the period of the lunar phases, the synodic lunar month, which is the month of the lunar calendar).

Venus

Due to the slow rotational speed of Venus, its synodic rotation period of 117 Earth days is about half the length of its sidereal rotational period (sidereal day) and even its orbital period. [8]

Mercury

Due to Mercury'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

Related Research Articles

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In precise timekeeping, ΔT is a measure of the cumulative effect of the departure of the Earth's rotation period from the fixed-length day of International Atomic Time. Formally, ΔT is the time difference ΔT = TT − UT between Universal Time and Terrestrial Time. The value of ΔT for the start of 1902 was approximately zero; for 2002 it was about 64 seconds. So Earth's rotations over that century took about 64 seconds longer than would be required for days of atomic time. As well as this long-term drift in the length of the day there are short-term fluctuations in the length of day which are dealt with separately.

<span class="mw-page-title-main">Lunar phase</span> Shape of the Moons sunlit portion as viewed from Earth

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<span class="mw-page-title-main">Sidereal time</span> Timekeeping system on Earth relative to the celestial sphere

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<span class="mw-page-title-main">Axial precession</span> Change of rotational axis in an astronomical body

In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's rotational axis. In the absence of precession, the astronomical body's orbit would show axial parallelism. In particular, axial precession can refer to the gradual shift in the orientation of Earth's axis of rotation in a cycle of approximately 26,000 years. This is similar to the precession of a spinning top, with the axis tracing out a pair of cones joined at their apices. The term "precession" typically refers only to this largest part of the motion; other changes in the alignment of Earth's axis—nutation and polar motion—are much smaller in magnitude.

<span class="mw-page-title-main">Solar time</span> Calculation of elapsed time by the apparent position of the sun

Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Traditionally, there are three types of time reckoning based on astronomical observations: apparent solar time and mean solar time, and sidereal time, which is based on the apparent motions of stars other than the Sun.

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<span class="mw-page-title-main">Lunar node</span> Where the orbit of the Moon intersects the Earths ecliptic

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<span class="mw-page-title-main">Lunar precession</span>

Lunar precession is a term used for three different precession motions related to the Moon. First, it can refer to change in orientation of the lunar rotational axis with respect to a reference plane, following the normal rules of precession followed by spinning objects. In addition, the orbit of the Moon undergoes two important types of precessional motion: apsidal and nodal.

<span class="mw-page-title-main">Equation of time</span> Apparent solar time minus mean solar time

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<span class="mw-page-title-main">Rotation period (astronomy)</span> Time that it takes to complete one rotation relative to the background stars

In astronomy, the rotation period or spin period of a celestial object has two definitions. The first one corresponds to the sidereal rotation period, i.e., the time that the object takes to complete a full rotation around its axis relative to the background stars. The other type of commonly used "rotation period" is the object's synodic rotation period, which may differ, by a fraction of a rotation or more than one rotation, to accommodate the portion of the object's orbital period around a star or another body during one day.

<span class="mw-page-title-main">Solar rotation</span> Pattern of rotation of components of the Sun

Solar rotation varies with latitude. The Sun is not a solid body, but is composed of a gaseous plasma. Different latitudes rotate at different periods. The source of this differential rotation is an area of current research in solar astronomy. The rate of surface rotation is observed to be the fastest at the equator and to decrease as latitude increases. The solar rotation period is 24.47 days at the equator and almost 38 days at the poles. The average rotation is 28 days.

<span class="mw-page-title-main">Earth's rotation</span> Rotation of Earth around its axis

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<span class="mw-page-title-main">Orbit of the Moon</span> The Moons circuit around Earth

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<span class="mw-page-title-main">Orbit of Venus</span>

Venus has an orbit with a semi-major axis of 0.723 au, and an eccentricity of 0.007. The low eccentricity and comparatively small size of its orbit give Venus the least range in distance between perihelion and aphelion of the planets: 1.46 million km. The planet orbits the Sun once every 225 days and travels 4.54 au in doing so, giving an average orbital speed of 35 km/s (78,000 mph).

<span class="mw-page-title-main">Lunar month</span> Time between successive new moons

In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month.

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.
  4. ↑ J. M. A. Danby; Jean Meeus (1997). Mathematical Astronomy Morsels. Willmann-Bell. ISBN   978-0-943396-51-4.
  5. ↑ "Equation Of Time". In-The-Sky.org. Dominic Ford. Retrieved 22 September 2020.
  6. ↑ "The Plane of the Ecliptic". NASA. July 12, 2016.
  7. ↑ "SATELLITES AND ORBITS" (PDF).
  8. ↑ "How long is a day on Venus?". TE AWAMUTU SPACE CENTRE. Retrieved 2021-06-03.
  9. ↑ "ESO". ESO. Retrieved 2021-06-03.
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