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Astro*Dictionary by Michael Erlewine





3 articles for "Equation of Time"

Equation of Time [Astro*Index]

Obsolete term. Replaced by the Equation of Universal Time, and distinguished from the Equation of Ephemeris Time.

See also: ♦ Time
Equation of Time [Munkasey M.]

The difference between mean and apparent time at any instance. That correction which must be added to to Universal Time to obtain Ephemeris time. See also: "Delta T".

See also: ♦ Time ♦ Delta T
Equation of Time [DeVore]

(1) Astronomy. The difference between mean solar time and apparent solar time. The moment the Sun is exactly on the Midheaven of any place is apparent noon at that place; hence an apparent solar day is the interval between two consecutive passages of the Sun across the Midheaven, or the elapsed time from one apparent noon to the next. However, since the Sun, or more correctly speaking, the Earth, does not move at a uniform speed throughout its orbit, the length of the apparent day varies at different times of the year. To make possible the use of time-keeping mechanisms, there was adopted a standard fixed day of 24 hours, known as a mean day — the length of which is the average of all the apparent days of the year. The result is that mean noon is sometimes earlier and sometimes later than apparent noon. The difference between mean and apparent noon on any particular day, the Equation of Time, may amount to as much as sixteen minutes.

(2) Astrology. It is generally considered that in a Figure erected for noon the Sun will be at the cusp of the Tenth House. This is approximately true, although at certain times of the year it will be two or three degrees removed, on one side or the other from the Midheaven. One sometimes hears the suggestion that the Figure should be erected by the Sun and not by the clock, which would involve the application of the Equation of Time as a correction of clock time. This is done when calculating the time of the rising of the Sun or other bodies. Its application to the erecting of the Figure, however, would be utterly unsound, for the time in which the birth is stated and the ephemerides which give the planets' places are both based on mean time. If the Figure were to be erected for apparent time, the birth moment would have to be corrected to apparent time, and the result would be the same.

(3) It is unfortunate that this term is incorrectly applied by some authorities to the difference between mean and sidereal time, more properly termed the correction employed in reducing to sidereal time the elapsed mean time of a given birth moment before or after noon or midnight.

(4) The term has frequently been incorrectly applied to the time equivalent of an Arc of Direction, in years, months and days — of which few points in Astrology have been more debated. The coordination of the 360° of the Equational circle and the 365¬-day year yields a mean value of 3m 56.33s per day, and a mean increment of either Right Ascension or Longitude of 59'8". Some authorities advocate an equation of 1° per year or 5' per month. Others advocate a method wherein the Arc of Direction is added to the R.A. of the Sun at birth — the number of days after birth at which the Sun attains this directional position, reduced to years at the rate of one day for a year or 2 hrs. for a month. Others divide the Arc of Direction by the Sun's mean motion per year (59'8"), the result converted into time at the rate of one degree for a year. (v. Directions.)

See also: ♦ Time ♦ Directions


Astro*Index Copyright © 1997 Michael Erlewine