# mean solar time

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## mean solar time:

see solar time**solar 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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## mean solar time

Time measured with reference to the uniform motion of the mean Sun. The*mean solar day*is the interval of 24 hours between two successive passages of the mean Sun across the meridian. The mean solar second is 1/86 400 of the mean solar day. The difference between mean solar time and apparent solar time on any particular day is the equation of time. Since the mean Sun is an abstract and hence unobservable point, mean solar time is defined in terms of sidereal time. One mean solar day is equal to 24h 3m 56.555s of mean sidereal time.

Mean solar time was originally devised in order to provide a uniform measure of time based on the Earth's rate of rotation, which was assumed, incorrectly, to be constant. The Sun's mean daily motion does however conform closely to universal time.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

The following article is from

*The Great Soviet Encyclopedia*(1979). It might be outdated or ideologically biased.## Mean Solar Time

(or mean time), a system of time measurement based on the diurnal motion of the fictitious mean sun, whose instant of lower transit is called mean midnight. Mean solar time is counted from mean midnight and is numerically equal to the hour angle of the mean sun plus 12 hours.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

## mean solar time

[′mēn ′sō·lər ′tīm] (astronomy)

Time that has the mean solar second as its unit, and is based on the mean sun's motion.

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

## mean time

,**mean solar time**

the time, at a particular place, measured in terms of the passage of the mean sun; the timescale is not precisely constant

Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005