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
A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
it is
orbiting, and is the basis of
solar time.
The synodic day is distinguished from the
sidereal day, which is one complete rotation in relation to distant stars, which is the basis of sidereal time. 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
Meridian or a meridian line (from Latin ''meridies'' via Old French ''meridiane'', meaning “midday”) may refer to
Science
* Meridian (astronomy), imaginary circle in a plane perpendicular to the planes of the celestial equator and horizon
* ...
(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. For example, in the
Northern Hemisphere
The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...
, a synodic day could be measured as the time taken for the Sun to move from exactly true south (i.e. its highest
declination
In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the ...
) 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. The longest and shortest synodic days' durations differ by about 51 seconds. The mean length, however, is 24 hours (with
fluctuations on the order of
millisecond
A millisecond (from '' milli-'' and second; symbol: ms) is a unit of time in the International System of Units (SI) equal to one thousandth (0.001 or 10−3 or 1/1000) of a second and to 1000 microseconds.
A unit of 10 milliseconds may be called ...
s), 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.
As viewed from Earth during the year, the Sun appears to slowly drift along an imaginary path
coplanar with
Earth's orbit
Earth orbits the Sun at an average distance of 149.60 million km (92.96 million mi) in a counterclockwise direction as viewed from above the Northern Hemisphere. One complete orbit takes days (1 sidereal year), during which time Earth ...
, known as the
ecliptic, on a
spherical background of seemingly
fixed stars
In astronomy, fixed stars ( la, stellae fixae) is a term to name the full set of glowing points, astronomical objects actually and mainly stars, that appear not to move relative to one another against the darkness of the night sky in the backgro ...
. 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
Retrograde motion in astronomy is, in general, orbital or rotational motion of an object in the direction opposite the rotation of its primary, that is, the central object (right figure). It may also describe other motions such as precession or ...
.
Certain
spacecraft orbits,
Sun-synchronous orbit
A Sun-synchronous orbit (SSO), also called a heliosynchronous orbit, is a nearly polar orbit around a planet, in which the satellite passes over any given point of the planet's surface at the same local mean solar time. More technically, it is ...
s, have
orbital periods that are a fraction of a synodic day. Combined with a
nodal precession Nodal precession is the precession of the orbital plane of a satellite around the rotational axis of an astronomical body such as Earth. This precession is due to the non-spherical nature of a rotating body, which creates a non-uniform gravitational ...
, this allows them to always pass over a location on Earth's surface at the same
mean solar time.
The Moon's synodic day
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's synodic day
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.
Mercury's synodic day
Due to
Mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
'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.
See also
*
*
Orbital period
*
Rotation period
The rotation period of a celestial object (e.g., star, gas giant, planet, moon, asteroid) may refer to its sidereal rotation period, i.e. the time that the object takes to complete a single revolution around its axis of rotation relative to the ...
*
Sidereal time
Sidereal time (as a unit also sidereal day or sidereal rotation period) (sidereal ) is a timekeeping system that astronomers use to locate celestial objects. Using sidereal time, it is possible to easily point a telescope to the proper coord ...
*
Solar rotation
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. ...
*
Solar time
*
Sun transit time
*
Synodic month
References
{{DEFAULTSORT:Synodic Day
Units of time
Astronomy