sidereal time

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sidereal time

(ST), timetime,
sequential arrangement of all events, or the interval between two events in such a sequence. The concept of time may be discussed on several different levels: physical, psychological, philosophical and scientific, and biological.
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 measured relative to the fixed stars; thus, the sidereal day is the period during which the earth completes one rotation on its axis so that some chosen star appears twice on the observer's celestial meridiancelestial meridian,
vertical circle passing through the north celestial pole and an observer's zenith. It is an axis in the altazimuth coordinate system.
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. Because the earth moves in its orbit about the sun, the sidereal day is about 4 min shorter than the solar day (see solar timesolar time,
time defined by the position of the sun. The solar day is the time it takes for the sun to return to the same meridian in the sky. Local solar time is measured by a sundial.
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). Thus, a given star will appear to rise 4 min earlier each night, so that different stars are visible at different times of the year. The local sidereal time of an observer is equal to the hour anglehour angle,
in astronomy, a coordinate in the equatorial coordinate system. The hour angle of a celestial body is the angular distance, expressed in hours, minutes, and seconds (one hour equals 15 degrees), measured westward along the celestial equator from the observer's
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 of the vernal equinox.

sidereal time

Time based on the rotation of the Earth with respect to the stars. It is measured in sidereal days and in sidereal hours, minutes, and seconds. The sidereal time at any instant is given by the sidereal hour angle of a catalog equinox and ranges from 0 to 24 hours during one day. The day starts at sidereal noon, which is the instant at which the equinox crosses the local meridian. The hour angle at a particular location gives the local sidereal time, the hour angle at Greenwich being the Greenwich sidereal time. A celestial object will be on the meridian of a particular place when the local sidereal time becomes equal to the object's right ascension.

Apparent sidereal time is measured by the hour angle of the true equinox and thus suffers from periodic inequalities, the position of the true equinox being affected by the precession and nutation of the Earth's axis. Mean sidereal time relates to the motion of the mean equinox, which is only affected by long-term inequalities arising from precession. Apparent minus mean sidereal time equals the equation of the equinoxes. Sidereal time is directly related to universal time and mean solar time and is used in their determination.

Sidereal Time

(religion, spiritualism, and occult)

Sidereal (from the Greek sidus, meaning “star”) time, like most ordinary measurements of time, is based on the rotational and orbital motion of Earth. However, unlike other ways of measuring the passing of time, sidereal time uses a fixed point in space (usually one of the fixed stars; hence the name sidereal) as a point of reference for the beginning and ending of a day, month, or year. By way of contrast, ordinary days and years, as well as lunar months (from one new moon to the next), use the constantly changing, relative positions of the Sun, the Moon, and Earth. As a result, there are slight differences in length between sidereal days, months, and years and ordinary days, months, and years. Sidereal time, which is also employed by astronomers, is used in tables of planetary positions (ephemerides) as well as tables of houses. The first step in casting a natal chart is to convert birth time to sidereal time.

sidereal time

[sī′dir·ē·əl ′tīm]
(astronomy)
Time based on diurnal motion of stars; it is used by astronomers but is not convenient for ordinary purposes.

sidereal time

The sidereal time at any place is the hour angle of the vernal equinox at that place.