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Because of the retrograde rotation, the length of a solar day on Venus is significantly shorter than the sidereal day, at 116.75 Earth days (making the Venusian solar day shorter than Mercury's 176 Earth days — the 116-day figure is close to the average number of days it takes Mercury to slip underneath the Earth in its orbit [the number of ...
A mean solar day (what we normally measure as a "day") is the average time between local solar noons ("average" since this varies slightly over a year). Earth makes one rotation around its axis each sidereal day; during that time it moves a short distance (about 1°) along its orbit around the Sun.
Venus was 0.7205 au from the Sun on the day of transit, decidedly less than average. [ 9 ] Moving far backwards in time, more than 200,000 years ago Venus sometimes passed by at a distance from Earth of barely less than 38 million km, and will next do that after more than 400,000 years.
A table of hours is shown for a sequence of seven days, with the day of the week indicated both for the sunrise (hour 1) and the sunset (hour 13) naming conventions. Day hours are calculated by adding up the amount of minutes from sunrise and sunset, then dividing by 12.
Comparison of the ephemeris positions with time measurements expressed in Barycentric Dynamical Time (TDB) leads to a value for the speed of light in astronomical units per day (of 86 400 s). By 2009, the IAU had updated its standard measures to reflect improvements, and calculated the speed of light at 173.144 632 6847 (69) au/d (TDB).
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Medieval unit of time used by astronomers to compute astronomical movements, length varies with the season. [4] Also colloquially refers to a brief period of time. centiday 0.01 d (1 % of a day) 14.4 minutes, or 864 seconds. One-hundredth of a day is 1 cd (centiday), also called "kè" in tradidional Chinese timekeeping.