EVECTION

, is used by some astronomers for the Libration of the moon; being an inequality in her motion, by which, at or near the quadratures, she is not in a line drawn through the centre of the earth to the sun, as she is at the syzygies, or conjunction and opposition, but makes an angle with that line of about 2° 51′. The motion of the moon about her axis only is equable, which rotation is performed exactly in the same time as she revolves about the earth; for which reason it is that she turns always the same face towards the earth nearly, and would do so exactly were it not that her menstrual motion about the earth, in an elliptic orbit, is not equable; on which account the moon, seen from the earth, appears to librate a little upon her axis, sometimes from east to west, and sometimes from west to east; or some parts in the eastern limb of the moon go backwards and forwards a small space, and some that were conspicuous, are hid, and then appear again.

The term Evection is used by some astronomers to denote that equation of the moon's motion, which is proportional to the sine of double the distance of the moon from the sun, diminished by the moon's anomaly: this equation is not yet accurately determined; some state it at 1° 30′, others at 1° 16′, &c. It is the greatest of all the moon's equations, except the Equation of the Centre.

EVEN Number, is that which can be divided into two equal whole numbers; such as the series of alternate numbers 2, 4, 6, 8, 10, &c.

EVENLY Even Number, is that which an even num- ber measures by an even number; as 16, which the even number 8 measures by the even number 2.

Evenly Odd Number, is that which an even number measures by an odd one; as 30, which the even number 6 measures by the odd number 5.

EVERARD's Sliding Rule, a particular sort of one invented by Mr. Thomas Everard, for the purpose of gauging. See Sliding Rule.