sidereal time

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.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

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.

The Astrology Book, Second Edition © 2003 Visible Ink Press®. All rights reserved.

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.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

sidereal time

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

An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved