SECT. I.
An Explanation of the Circles of the Sphere, and of some Astronomical Terms arising therefrom.
The Circles of the Sphere.
In order to determine the relative situations of places upon the Earth, as well as the positions of the fixed Stars, and other Celestial phænomena, the Globe of the Earth is supposed to be environed by several imaginary circles, and these are called the Circles of the Sphere. These imaginary circles are either fixed, and always obtain the same position in the Heavens, or moveable, according to the position of the observer.
Those circles that are fixed, owe their origin to the two-fold motion of the Earth, and are the Equator, and the Ecliptic, with their Secundaries and Parallels. These fixed circles are usually delineated upon the surface of the Globes.
The moveable circles are only the Horizon, its Secundaries and Parallels: These are represented by the wooden frame, and the brass ring, wherein the Globe is hung, and a thin plate of brass to be screwed in a proper place, upon the said ring, as occasion requires.
I. Of the Equinoctial.
The Equator, or Equinoctial.
1. The Equator, or the Equinoctial, is that great circle in the Heavens, in whose plane the Earth performs her diurnal motion round her axis; or it is that great circle, parallel to which the whole Heavens seem to turn round the Earth from East to West in 24 Hours.