What is "sidereal time"

Time \Time\, n.; pl. Times. [OE. time, AS. t[=i]ma, akin to t[=i]d time, and to Icel. t[=i]mi, Dan. time an hour, Sw. timme. [root]58. See Tide, n.]

1. Duration, considered independently of any system of measurement or any employment of terms which designate limited portions thereof.

   The time wasteth [i. e. passes away] night and day.
   --Chaucer.

   I know of no ideas . . . that have a better claim to be accounted simple and original than those of space and time.
   --Reid.

2. A particular period or part of duration, whether past, present, or future; a point or portion of duration; as, the time was, or has been; the time is, or will be.

   God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets.
   --Heb. i. 1.

3. The period at which any definite event occurred, or person lived; age; period; era; as, the Spanish Armada was destroyed in the time of Queen Elizabeth; -- often in the plural; as, ancient times; modern times.

4. The duration of one's life; the hours and days which a person has at his disposal.

   Believe me, your time is not your own; it belongs to God, to religion, to mankind.
   --Buckminster.

5. A proper time; a season; an opportunity.

   There is . . . a time to every purpose.
   --Eccl. iii. 1.

   The time of figs was not yet.
   --Mark xi. 13.

6. Hour of travail, delivery, or parturition.

   She was within one month of her time.
   --Clarendon.

7. Performance or occurrence of an action or event, considered with reference to repetition; addition of a number to itself; repetition; as, to double cloth four times; four times four, or sixteen.

   Summers three times eight save one.
   --Milton.

8. The present life; existence in this world as contrasted with immortal life; definite, as contrasted with infinite, duration.

   Till time and sin together cease.
   --Keble.

9. (Gram.) Tense.

10. (Mus.) The measured duration of sounds; measure; tempo; rate of movement; rhythmical division; as, common or triple time; the musician keeps good time. Some few lines set unto a solemn time. --Beau. & Fl. Note: Time is often used in the formation of compounds, mostly self-explaining; as, time-battered, time-beguiling, time-consecrated, time-consuming, time-enduring, time-killing, time-sanctioned, time-scorner, time-wasting, time-worn, etc. Absolute time, time irrespective of local standards or epochs; as, all spectators see a lunar eclipse at the same instant of absolute time. Apparent time, the time of day reckoned by the sun, or so that 12 o'clock at the place is the instant of the transit of the sun's center over the meridian. Astronomical time, mean solar time reckoned by counting the hours continuously up to twenty-four from one noon to the next. At times, at distinct intervals of duration; now and then; as, at times he reads, at other times he rides. Civil time, time as reckoned for the purposes of common life in distinct periods, as years, months, days, hours, etc., the latter, among most modern nations, being divided into two series of twelve each, and reckoned, the first series from midnight to noon, the second, from noon to midnight. Common time (Mil.), the ordinary time of marching, in which ninety steps, each twenty-eight inches in length, are taken in one minute. Equation of time. See under Equation, n. In time.

    1. In good season; sufficiently early; as, he arrived in time to see the exhibition.

    2. After a considerable space of duration; eventually; finally; as, you will in time recover your health and strength. Mean time. See under 4th Mean. Quick time (Mil.), time of marching, in which one hundred and twenty steps, each thirty inches in length, are taken in one minute. Sidereal time. See under Sidereal. Standard time, the civil time that has been established by law or by general usage over a region or country. In England the standard time is Greenwich mean solar time. In the United States and Canada four kinds of standard time have been adopted by the railroads and accepted by the people, viz., Eastern, Central, Mountain, and Pacific time, corresponding severally to the mean local times of the 75th, 90th, 105th, and 120th meridians west from Greenwich, and being therefore five, six, seven, and eight hours slower than Greenwich time. Time ball, a ball arranged to drop from the summit of a pole, to indicate true midday time, as at Greenwich Observatory, England. --Nichol. Time bargain (Com.), a contract made for the sale or purchase of merchandise, or of stock in the public funds, at a certain time in the future. Time bill. Same as Time-table. [Eng.] Time book, a book in which is kept a record of the time persons have worked. Time detector, a timepiece provided with a device for registering and indicating the exact time when a watchman visits certain stations in his beat. Time enough, in season; early enough. ``Stanly at Bosworth field, . . . came time enough to save his life.'' --Bacon. Time fuse, a fuse, as for an explosive projectile, which can be so arranged as to ignite the charge at a certain definite interval after being itself ignited. Time immemorial, or Time out of mind. (Eng. Law) See under Immemorial. Time lock, a lock having clockwork attached, which, when wound up, prevents the bolt from being withdrawn when locked, until a certain interval of time has elapsed. Time of day, salutation appropriate to the times of the day, as ``good morning,'' ``good evening,'' and the like; greeting. To kill time. See under Kill, v. t. To make time.

       1. To gain time.

       2. To occupy or use (a certain) time in doing something; as, the trotting horse made fast time. To move against time, To run against time, or To go against time, to move, run, or go a given distance without a competitor, in the quickest possible time; or, to accomplish the greatest distance which can be passed over in a given time; as, the horse is to run against time. True time.

          1. Mean time as kept by a clock going uniformly.

          2. (Astron.) Apparent time as reckoned from the transit of the sun's center over the meridian.

n. (context astronomy English) Time measured by the apparent diurnal motion of the vernal equinox; approximated by that based on the rotation of the Earth relative to the stars

n. measured by the diurnal motion of stars

Sidereal time is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky. Briefly, sidereal time is a "time scale that is based on Earth's rate of rotation measured relative to the fixed stars" rather than the sun.

From a given observation point, a star found at one location in the sky will be found at nearly the same location on another night at the same sidereal time. This is similar to how the time kept by a sundial can be used to find the location of the sun. Just as the sun and moon appear to rise in the east and set in the west due to the rotation of Earth, so do the stars. Both solar time and sidereal time make use of the regularity of Earth's rotation about its polar axis, solar time following the sun while sidereal time roughly follows the stars. More exactly, sidereal time is the angle, measured from the observer's meridian, along the celestial equator, to the great circle that passes through the March equinox and both poles, and is usually expressed in hours, minutes, and seconds. Common time on a typical clock measures a slightly longer cycle, accounting not only for Earth's axial rotation but also for Earth's annual revolution around the sun of slightly less than 1 degree per day (in fact to the nearest arcsecond, it takes 365.2422 days to revolve, therefore 360 degrees/365.2422 days = 0.9856° or 59′ 8″ per day, i.e., slightly less than 1 degree per day).

A mean sidereal day is 23 hours, 56 minutes, 4.0916 seconds ( hours or mean solar days), the time it takes Earth to make one rotation relative to the vernal equinox. (Due to nutation, an actual sidereal day is not quite so constant.) The vernal equinox itself precesses slowly westward relative to the fixed stars, completing one revolution in about 26,000 years, so the misnamed sidereal day ("sidereal" is derived from the Latin sidus meaning "star") is 0.0084 seconds shorter than Earth's period of rotation relative to the fixed stars.

The longer "true" sidereal period is called a stellar day by the International Earth Rotation and Reference Systems Service (IERS). It is also referred to as the sidereal period of rotation, or simply as the period of rotation or the rotational period.

Maps of the stars in the night sky use declination and right ascension as coordinates. These correspond to latitude and longitude respectively. While declination is measured in degrees, right ascension is measured in units of time, because it was most natural to name locations in the sky in connection with the time when they crossed the meridian.

In the sky, the meridian is the imaginary north to south line that goes through the point directly overhead (the zenith). The right ascension of any object crossing the meridian is equal to the current local (apparent) sidereal time, ignoring for present purposes that part of the circumpolar region north of the north celestial pole (for an observer in the northern hemisphere) or south of the south celestial pole (for an observer in the southern hemisphere) that is crossing the meridian the other way.

Because Earth orbits the sun once a year, the sidereal time at any given place and time will gain about four minutes against local civil time, every 24 hours, until, after a year has passed, one additional sidereal "day" has elapsed compared to the number of solar days that have gone by.