EQUINOCTIAL
, a great circle in the heavens under which the equator moves in its diurnal motion. The poles of this circle are the poles of the world. It divides the sphere into two equal parts, the northern and southern. It cuts the horizon, of any place, in the east and west points; and at the meridian its elevation above the horizon is equal to the co-latitude of the place. The Equinoctial has also various other properties; as,
1. Whenever the sun comes to this circle, he makes equal days and nights all round the globe; because he then rises due east, and sets due west. Hence it has the name Equinoctial. All stars which are under this circle, or have no declination, do also rise due east, and set due west.
2. All people living under this circle, or upon the equator, or line, have their days and nights at all times equal to each other.
3. From this circle, on the globe, is counted, upon the meridian, the declination in the heavens, and the latitude on the earth.
4. Upon the Equinoctial, or equator, is counted the longitude, making in all 360° quite round, or else 180° east, and 180° west.
5. And as the time of one whole revolution is divided into 24 hours; therefore 1 hour anfwers to 15°, or the 24th part of 360°. Hence,
| 1° | of longitude answers | to | 4 | min. | of time, | |
| 15′ | " | to | 1 | min. | of time, | |
| 1′ | " | to | 4 | sec. | of time, | &c. |
6. The shadows of those people who live under this circle are cast to the southward of them for one half of the year, and to the northward of them during the other half; and twice in a year, viz, at the time of the equinoxes, the sun at noon casts no shadow, being exactly in their zenith.
Equinoctial Colure, is the great circle passing through the poles of the world and the Equinoctial points, or first points of Aries and Libra.
Equinoctial Dial, is one whose plane is parallel to the Equinoctial. The properties or principles of this dial are,
1. The hour lines are all equally distant from one another, quite round the circumference of a circle; and the style is a straight pin, or wire, set up in the centre of the circle, perpendicular to the plane of the dial.
2. The sun shines upon the upper part of this dialplane from the 21st of March to the 22d of September, and upon the under part of the plane the other half of the year.
Some of these dials are made of brass, &c; and set up in a frame, to be elevated to any given latitude.
Equinoctial Points, are the two opposite points where the Ecliptic and Equinoctial cross each other; | the one point being in the beginning of Aries, and called the Vernal point, or Vernal Equinox; and the other in the beginning of Libra, and called the Autumnal point, or Autumnal Equinox.
It is found by observation, that the Equinoctial points, and all the other points of the ecliptic, are continually moving backwards, or in autecedentia, i. e. westwards. This retrograde motion of the Equinoctial points, is that phenomenon called the Precession of the Equinoxes, and is made at the rate of 50″ every year nearly.
