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Time, universal (UT)
Generally, mean solar time as measured on the Greenwich meridian. Formally, it is defined as a function of sidereal time by formula. Direct application of the formula supplies UT0, and correction for polar motion produces UT1, which is normally implied whenever UT is read. |
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Time zone
Originally, an approximately longitudinal strip on the Earth's surface in which the zone time, as whole number hours before or after UT, is adopted as the local civil time by agreement. |
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Tropical year
The period of revolution of the Earth about the Sun with respect to the vernal equinox. |
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UTC
See Coordinated Universal Time. |
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Vespers
A watch in the medieval monastery system, beginning an hour before sunset. |
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Waning
An adjective referring to the phase of the Moon when the phase is decreasing. Opposite of waxing. |
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Watch
Early cultures were forced to use manual methods to measure time by watches. These were necessarily variable in length and started at different times, depending upon the time of year. The watch system used by medieval monasteries eventually determined the structure of the day in Western usage. |
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Waxing
An adjective referring to the phase of the Moon when the phase is increasing. Opposite of waning. |
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Week
As commonly used today, the term refers to a period of seven days, which is continuously repeated across month and year boundaries. Traditional usage defines Sunday as the first day of the week. ISO usage defines Monday as the first day, while other calendars define still other days. |
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Western calendar
The Julian and Gregorian calendars, considered as one calendar. |
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Year, anomalistic
The interval of time between two successive passages of the Sun, using as a reference point the perigee. Equivalent to 365.2596 MSD. |
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