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Solstice
From the Latin sol stetit, meaning the "Sun stands (still)." The plural is solstices. The two solstices are the points at which the apparent longitude of the Sun is 90 degrees and 270 degrees, or by extension, the moments in time at which the Sun is found at one of these two points, which are the maximum distances north and south of the celestial equator. There is a summer solstice around 21 June and a winter solstice around 21 December of each year. |
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Sothic cycle
A period of 1,460 years, in which the heliacal rising of the star Sirius (Sothis to the Greeks), which occurs almost exactly on a 365.25-day cycle, reappears on the same day of the year in the vague Egyptian year of 365 days. The two-year measures differ by one-quarter day per year, or one day in four years, so they differ by a complete year of 365 days in (4365 =) 1,460 years. This cycle figures prominently in early calculations concerning the Egyptian calendar. |
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Standard time
Civil time according to the time zone in which the oberver is located. In particular, the term is distinguished from Advanced Time. |
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Synodic period
The time between two successive conjunctions in longitude. |
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Summer time
See Advanced Time. |
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TAI
See International Atomic Time. |
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TDT
See Terrestrial Dynamic Time. |
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Temporal hours
From the Latin horae temporales, hours measured by dividing periods of light and darkness into equal periods, usually 12 in number. These hours vary in length with the season of the year. Distinguished from equinoctial hours. |
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Terrestrial Dynamic Time (TDT)
Since 1984, TDT has replaced ET as the argument in theories of celestial dynamics. TDT is equal to TAI plus 32.184 seconds. |
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Tertia
A watch in the medieval monastery system beginning with the third hour, and therefore between sunrise and noon. The period for saying morning Mass. Also called terce. |
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Time, sidereal
Time measured with respect to the apparent motion of the fixed stars. |
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Time, mean solar
Time measured with respect to the mean Sun. |
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Time, solar
Time measured with respect to the real Sun. |
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