What is "equation of 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.

Equation \E*qua"tion\, n. [L. aequatio an equalizing: cf. F.

1. A making equal; equal division; equality; equilibrium.

   Again the golden day resumed its right, And ruled in just equation with the night.
   --Rowe.

2. (Math.) An expression of the condition of equality between two algebraic quantities or sets of quantities, the sign = being placed between them; as, a binomial equation; a quadratic equation; an algebraic equation; a transcendental equation; an exponential equation; a logarithmic equation; a differential equation, etc.

3. (Astron.) A quantity to be applied in computing the mean place or other element of a celestial body; that is, any one of the several quantities to be added to, or taken from, its position as calculated on the hypothesis of a mean uniform motion, in order to find its true position as resulting from its actual and unequal motion.

   Absolute equation. See under Absolute.

   Equation box, or Equational box, a system of differential gearing used in spinning machines for regulating the twist of the yarn. It resembles gearing used in equation clocks for showing apparent time.

   Equation of the center (Astron.), the difference between the place of a planet as supposed to move uniformly in a circle, and its place as moving in an ellipse.

   Equations of condition (Math.), equations formed for deducing the true values of certain quantities from others on which they depend, when different sets of the latter, as given by observation, would yield different values of the quantities sought, and the number of equations that may be found is greater than the number of unknown quantities.

   Equation of a curve (Math.), an equation which expresses the relation between the co["o]rdinates of every point in the curve.

   Equation of equinoxes (Astron.), the difference between the mean and apparent places of the equinox.

   Equation of payments (Arith.), the process of finding the mean time of payment of several sums due at different times.

   Equation of time (Astron.), the difference between mean and apparent time, or between the time of day indicated by the sun, and that by a perfect clock going uniformly all the year round.

   Equation clock or Equation watch, a timepiece made to exhibit the differences between mean solar and apparent solar time.
   --Knight.

   Normal equation. See under Normal.

   Personal equation (Astron.), the difference between an observed result and the true qualities or peculiarities in the observer; particularly the difference, in an average of a large number of observation, between the instant when an observer notes a phenomenon, as the transit of a star, and the assumed instant of its actual occurrence; or, relatively, the difference between these instants as noted by two observers. It is usually only a fraction of a second; -- sometimes applied loosely to differences of judgment or method occasioned by temperamental qualities of individuals.

   Theory of equations (Math.), the branch of algebra that treats of the properties of a single algebraic equation of any degree containing one unknown quantity.

The equation of time describes the discrepancy between two kinds of solar time. The word equation is used in the medieval sense of reconcile a difference. The two times that differ are the apparent solar time, which directly tracks the motion of the sun, and mean solar time, which tracks a theoretical "mean" sun with noons 24 hours apart. Apparent (or true) solar time can be obtained by measurement of the current position ( hour angle) of the Sun, as indicated (with limited accuracy) by a sundial. Mean solar time, for the same place, would be the time indicated by a steady clock set so that over the year its differences from apparent solar time would resolve to zero.

The equation of time is the east or west component of the analemma, a curve representing the angular offset of the Sun from its mean position on the celestial sphere as viewed from Earth. The equation of time values for each day of the year, compiled by astronomical observatories, were widely listed in almanacs and ephemerides.