Page:The New International Encyclopædia 1st ed. v. 07.djvu/191

* EQUATION OF EQUINOXES. 165 EQUESTRIAN STATUE. EQUATION OF EQUINOXES. The dif- ference between the actual position of the equi- noxes (q.v.) and the position calculated on the assumption that their motion is uniform. See Precession. EQUATION OF LIGHT. Tn astronomical observations, the ray of light by which we see any celestial body is not that which it emits at the moment we look at it, but which it did emit some time before, viz., the time occupied by light in traversing the space which separates us from the celestial body. The quantity of time so required for the passage of light from the sun to the earth is the so-called light-equation. It amounts to about eight minutes twenty seconds. EQUATION OF PAYMENTS. A method of finding the time when, if a sum of money be paid by a debtor, instead of several debts payable by him at different times, no loss will be sustained by either the debtor or creditor. The common rule is : Multiply each debt by its term of credit, and divide the sum of the products by the sum of the debts. The quotient will be the average term of credit. This added to the date from which the credits were reckoned will give the average time of payments; e.g. to find the average time of paving $200 due April 1st, $200 due May 11th, and $400 due June 30th: $200 + 40- $200 + 90- $400 = $44,000; $44,000 -r- $800 = 55. April 1st + 55 days = May 26th the equated time. This method is incorrect, except for equal debts, be- cause it takes no account of the balance of inter- est and discount. It is, however, sufficiently accurate for ordinary use. EQUATION OF THE CENTRE. A term used by astronomers in connection with the planets' orbital motions. The anomaly ( q.v. ) of a planet does not increase uniformly after peri- helion passage; because, according to Kepler's law, a line joining the planet with the sun sweeps over equal areas {not equal angles) in equal times. The difference between a planet's actual anomaly and that which its anomaly would be if equal angles were swept over is the equation of the centre. EQUATION OF TIME. The earth's motion in the ecliptic is not uniform. This want of uniformity would of itself be sufficient to cause an irregularity in the intervals of time between successive returns of the sun to the meridian, day after day; but besides this want of uniformity in the sun's apparent motion in the ecliptic, there is another cause of inequality in the time of his coming to the meridian, the obliquity of the ecliptic to the equinoctial. These two independent causes conjointly produce the inequality in the time of his appearance on the meridian, the cor- rection for which is the 'equation of time.' But from the causes above explained, mean and ap- parent noon differ, the latter taking place some- times as much as 16^4 minutes before the former, and at others as much as 14% minutes after. The difference for any. day. called the equation of time, is to be found inserted in ephemerides for every day of the year. ( See Ephemeris. ) It is nothing, or zero, at four different times in the year, at which the mean and unequal motions would exactly agree — viz.. about the loth of April, the 15th of June, the 31st of August, the 24th of December. At all other times the sun is either too fast or too slow for clock-time. EQUA'TOR, Celestial i MX. (equator, equal- izer, from Lat. aqua/re, to equalize). The great .uric which would be cut out on the sky by extending the plane of the earth's equator. EQUATOR, Terrestrial. The great circle on the earth's surface, half way between the poles, which divides the earth into the northern and bouI hern hemispheres. E'QUATO'RIAL (from ML. wqunlor, equal- izer). A term applied in astronomy to a method of mounting astronomical telescopes, by which a celestial body may be observed at any point of its diurnal course. It consists of a telescope fastened to a graduated circle, called the declina- tion circle, whosi axis is attached at right angles to that of another graduated circle called the hour-circle, and is wholly supported by it. The hour-circle axis, which is called the principal axis of the instrument, turns on fixed supports; it is pointed to the pole of the heavens, and the hour-circle is of course parallel to the equinoc- tial. This combination of axes gives us a uni- versal joint, thus enabling us to point the tube at any star in the sky; and with the pair of circles we can measure and record the exact posi- tion in the sky of the star under observation. ( In account of one axis being pointed at the pole, about which all the stars revolve in their diurnal course, it becomes possible to follow their motions by rotating the telescope about this one axis only, and this rotation can be effected easily and conveniently with clockwork. EQUESTRIAN ORDER (Lat. ordo eques ter), or EQ'UITES (Lat., knights, from equus, horse). Originally the cavalry of the Roman Army. It is said to have been instituted by Romulus, who selected from the three principal Roman tribes a body-guard of 300, en lied Celeres. The number was afterwards gradually increased to 3G00. who were partly of patrician and partly of plebeian rank, and required to possess a cer- tain amount of property ( 400.000 sestertii — about $17,000). Each of these equites received a horse from the Slate; but about B.C. 403 a new body of equites began to make their appearance, who were obliged to furnish a horse at their own expense. Until B.C. 123, the equites were exclu- sively a military body; but in that year Caius Gracchus carried a measure, by which all the judices (jurors) had to be selected from them. Now. for the first time, they became a distinct order or class in the State, and were called ordo equester. Sulla deprived them of this priv- ilege; but their power did not then decrease, as the farming of the public revenues appears to have fallen into their hands. After the con- spiracy of Catiline, the ordo equester began to be looked" upon as a third estate in the Republic; and to the title of senatus populusque Romanus was added et ordo equester. EQUESTRIAN STATUE. The representation in sculpture of a person on horseback. Equestrian statues were not commonly erected in Greece, but in Rome they were often awarded as a high honor to military commanders and persons of distinc- tion, and latterly were, for the most part, re- stricted to the emperors, the most famous in existence being that of the Emperor Marcus Aure- lius, which now stands in the piazza of the Capi- tol at Rome. It is the only ancient equestrian statue in bronze that has been preserved. The most famous equestrian statues of the Renais-