[4] The square root of α + βi can be determined by the extraction of square roots of positive real numbers, without the trigonometrical tables.
EQUATION OF THE CENTRE, in astronomy, the angular distance, measured around the centre of motion, by which a planet moving in an ellipse deviates from the mean position which it would occupy if it moved uniformly. Its amount is the correction which must be applied positively or negatively to the mean anomaly in order to obtain the true anomaly. It arises from the ellipticity of the orbit, is zero at pericentre and apocentre, and reaches its greatest amount nearly midway between these points. (See [Anomaly] and [Orbit].)
EQUATION OF TIME, the difference between apparent time, determined by the meridian passage of the real sun, and mean time, determined by the passage of the mean sun. It goes through a double period in the course of a year. Its amount varies a fraction of a minute for the same date, from year to year and from one longitude to another, on the same day. The following table shows an average value for any date and for the Greenwich meridian for a number of years, from which the actual value will seldom deviate more than 20 seconds until after 1950. The + sign indicates that the real sun reaches the meridian after mean noon; the − sign before mean noon.
Table of the Equation of Time.
| m. | s. | m. | s. | m. | s. | ||||||
| Jan. | 1 | +3 | 26 | Mar. | 1 | +12 | 39 | May | 1 | −2 | 55 |
| 6 | 5 | 45 | 6 | 11 | 35 | 6 | −3 | 27 | |||
| 11 | 7 | 51 | 11 | 10 | 20 | 11 | −3 | 46 | |||
| 16 | 9 | 43 | 16 | 8 | 58 | 16 | −3 | 51 | |||
| 21 | 11 | 19 | 21 | 7 | 30 | 21 | −3 | 40 | |||
| 26 | 12 | 36 | 26 | 5 | 59 | 26 | −3 | 16 | |||
| Feb. | 1 | +13 | 42 | Apr. | 1 | +4 | 9 | June | 1 | −2 | 32 |
| 6 | 14 | 14 | 6 | 2 | 40 | 6 | −1 | 44 | |||
| 11 | 14 | 25 | 11 | +1 | 15 | 11 | −0 | 48 | |||
| 16 | 14 | 17 | 16 | −0 | 3 | 16 | +0 | 14 | |||
| 21 | 13 | 52 | 21 | −1 | 12 | 21 | 1 | 19 | |||
| 26 | 13 | 11 | 26 | −2 | 10 | 26 | 2 | 24 | |||
| July | 1 | +3 | 26 | Sept. | 1 | +0 | 9 | Nov. | 1 | −16 | 18 |
| 6 | 4 | 21 | 6 | −1 | 28 | 6 | −16 | 19 | |||
| 11 | 5 | 8 | 11 | −3 | 10 | 11 | −15 | 58 | |||
| 16 | 5 | 44 | 16 | −4 | 55 | 16 | −15 | 15 | |||
| 21 | 6 | 8 | 21 | −6 | 41 | 21 | −14 | 12 | |||
| 26 | 6 | 18 | 26 | −8 | 25 | 26 | −12 | 49 | |||
| Aug. | 1 | +6 | 10 | Oct. | 1 | −10 | 5 | Dec. | 1 | −11 | 7 |
| 6 | 5 | 47 | 6 | −11 | 38 | 6 | −9 | 9 | |||
| 11 | 5 | 9 | 11 | −13 | 2 | 11 | −6 | 57 | |||
| 16 | 4 | 17 | 16 | −14 | 14 | 16 | −4 | 35 | |||
| 21 | 3 | 12 | 21 | −15 | 11 | 21 | −2 | 7 | |||
| 26 | 1 | 55 | 26 | −15 | 52 | 26 | +0 | 23 | |||
EQUATOR (Late Lat. aequator, from aequare, to make equal), in geography, that great circle of the earth, equidistant from the two poles, which divides the northern from the southern hemisphere and lies in a plane perpendicular to the axis of the earth; this is termed the “geographical” or “terrestrial equator.” In astronomy, the “celestial equator” is the name given to the great circle in which the plane of the terrestrial equator intersects the celestial sphere; it is consequently equidistant from the celestial poles. The “magnetic equator” is an imaginary line encircling the earth, along which the vertical component of the earth’s magnetic force is zero; it nearly coincides with the terrestrial equator.