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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.
A definition of a terrestrial time standard was adopted by the International Astronomical Union (IAU) in 1976 at its XVI General Assembly and later named Terrestrial Dynamical Time (TDT). It was the counterpart to Barycentric Dynamical Time (TDB), which was a time standard for Solar system ephemerides, to be based on a dynamical time scale ...
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.
The galactic year, also known as a cosmic year, is the duration of time required for the Sun to orbit once around the center of the Milky Way Galaxy. [1] One galactic year is approximately 225 million Earth years. [2]
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
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 ...
A graphical view of the Cosmic Calendar, featuring the months of the year, days of December, the final minute, and the final second. The Cosmic Calendar is a method to visualize the chronology of the universe, scaling its currently understood age of 13.8 billion years to a single year in order to help intuit it for pedagogical purposes in science education or popular science.