The Solar System

[steve_bank]

Sidereal and solar time.

Sidereal time - Wikipedia

en.wikipedia.org

Sidereal time ("sidereal" pronounced /saɪˈdɪəriəl, sə-/ sy-DEER-ee-əl, sə-) is a system of timekeeping used especially by astronomers. Using sidereal time and the celestial coordinate system, it is easy to locate the positions of celestial objects in the night sky. Sidereal time is a "time scale that is based on Earth's rate of rotation measured relative to the fixed stars".[1] A sidereal day (also known as the sidereal rotation period) represents the time for one rotation about the planet axis relative to the stars.[2]

Viewed from the same location, a star seen at one position in the sky will be seen at the same position on another night at the same time of day (or night), if the day is defined as a sidereal day.

A sidereal day on Earth is approximately 86164.0905 seconds (23 h 56 min 4.0905 s or 23.9344696 h). (Seconds are defined as per International System of Units and are not to be confused with ephemeris seconds.) Each day, the sidereal time at any given place and time will be about four minutes shorter than local civil time (which is based on solar time), so that for a complete year the number of sidereal "days" is one more than the number of solar days.

Synodic day - Wikipedia

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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.

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. 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 the solstices near the equator.

Earth's orbit - Wikipedia

en.wikipedia.org

Earth orbits the Sun at an average distance of 149.60 million km (92.96 million mi), or 8.317 light-minutes,[1] in a counterclockwise direction as viewed from above the Northern Hemisphere. One complete orbit takes 365.256 days (1 sidereal year), during which time Earth has traveled 940 million km (584 million mi).[2] Ignoring the influence of other Solar System bodies, Earth's orbit, also called Earth's revolution, is an ellipse with the Earth–Sun barycenter as one focus with a current eccentricity of 0.0167. Since this value is close to zero, the center of the orbit is relatively close to the center of the Sun (relative to the size of the orbit).

As seen from Earth, the planet's orbital prograde motion makes the Sun appear to move with respect to other stars at a rate of about 1° eastward per solar day (or a Sun or Moon diameter every 12 hours).[nb 1] Earth's orbital speed averages 29.78 km/s (18.50 mi/s; 107,208.00 km/h; 66,615.96 mph), which is fast enough to cover the planet's diameter in 7 minutes and the distance to the Moon in 4 hours.[3] The point towards which the Earth in its solar orbit is directed at any given instant is known as the "apex of the Earth's way".[4][5]

From a vantage point above the north pole of either the Sun or Earth, Earth would appear to revolve in a counterclockwise direction around the Sun. From the same vantage point, both the Earth and the Sun would appear to rotate also in a counterclockwise direction.