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The planetary hours are an ancient system in which one of the seven classical planets is given rulership over each day and various parts of the day. Developed in Hellenistic astrology , it has possible roots in older Babylonian astrology , and it is the origin of the names of the days of the week as used in English and numerous other languages.
Unequal hours were the result of dividing up the period of daylight into 12 equal hours and nighttime into another 12. There is more daylight in the summer, and less night time, so each of the 12 daylight hours is longer than a night hour. Similarly in winter, daylight hours are shorter, and night hours are longer.
It may also refer to the time it takes a satellite orbiting a planet or moon to complete one orbit. For celestial objects in general, the orbital period is determined by a 360° revolution of one body around its primary, e.g. Earth around the Sun. Periods in astronomy are expressed in units of time, usually hours, days, or years.
From the 17th century to the late 19th century, planetary ephemerides were calculated using time scales based on the Earth's rotation: usually the mean solar time of one of the principal observatories, such as Paris or Greenwich. After 1884, mean solar time at Greenwich became a standard, later named Universal Time (UT). But in the later 19th ...
The geologic time scale is a way of representing deep time based on events that have occurred throughout Earth's history, a time span of about 4.54 ± 0.05 Ga (4.54 billion years). [3] It chronologically organises strata, and subsequently time, by observing fundamental changes in stratigraphy that correspond to major geological or ...
Date Time UTC Planet Angle distance Planet Elongation to Sun January 18, 2007 18:10:50 Venus 1°25' south of Neptune 20.1° East January 26, 2007 06:46:07 Mercury 1°28' south of Neptune 12.7° East February 7, 2007 13:13:57 Venus 44' south of Uranus 24.6° East March 25, 2007 07:23:59 Mars 1°00' south of Neptune 43.2° West April 1, 2007
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Sidereal time was defined such that the March equinox would transit the meridian of the observatory at 0 hours local sidereal time. [7] Beginning during the 1970s, the radio astronomy methods very-long-baseline interferometry (VLBI) and pulsar timing overtook optical instruments for the most precise astrometry.