Page:Description and Use of a New Celestial Planisphere.pdf/18

13 beginning of the four Quarters of the Ecliptic are called Cardinal Signs, viz. Aries, Cancer, Libra, and Capricorn, becaue the Sun enters them at the beginning of the four Quarters of the year; the four Points of the Compas, called Eat, Wet, North, and South, are called Cardinal Points alo; and if you bring Cancer or Capricorn to the Meridian, the Cardinal Signs will coincide with, and lay over the Cardinal Points of the Horizon. Aries and Libra are alo called Equinoctial Points and Signs, becaue the days and nights are then equal, as you will ee by the Planisphere; if you take the Sun's diurnal Arc on thoe two days that the Sun enters thee Points, the two Meridians paing through thee Points, are called Colures, and that which paeth through the Equinoctial Points is called the Equinoctial Colure; and that which is at right Angles to it, paing through the Soltitial Points, is called the Soltitial Colure, as you will ee them named in the Planisphere. The use of the Prime Vertical in the Planisphere is very obvious; for when, by the diurnal rotation of the Earth, any celetial object comes to this circle in the eatern Hemiphere, it appears due Eat: and if it be in the wetern Hemisphere, it is due Wet, and its Altitude, in both Caes, is een by inpection; alo, when any celetial object comes to the outhern part of the Meridian, its meridian Altitude is een on the cale of Altitude; and when any object is riing or etting, its Azimuth and Amplitude are een on the Horizon, with the Point of the Compas on which it ries or ets.