The meridian passes thro’ the Poles of the world, as well as through the Zenith and Nadir, and therefore is a secundary both of the equinoctial and the horizon: This circle divides the globe into the Eastern and Western Hemispheres, and the Poles of it are the East and West points of the Horizon. All the heavenly objects are, during one half of their continuance above the horizon, in the Eastern Hemisphere, and for the other half in the Western; so that whenever the Sun arrives upon the upper part of the meridian, it is then Noon, or Mid-day, which is the reason why this circle is called the meridian; and when he comes to the lower part, it is then Midnight.
Prime Vertical.
The vertical circle passing thro’ the East and West points of the horizon, is called the Prime Vertical, or Circle of East and West, so that when any object is upon this circle in the Eastern hemisphere, it appears due East; and if it be in the Western hemisphere, it appears due West.
Amplitude.
That degree in the horizon wherein any object rises or sets from the East or West points, is called the Amplitude; which for rising is called Amplitude Ortive, and Occasive for setting; which must be also denominated whether it be Northerly or Southerly.
It may be observed, that the Amplitude and Azimuth are much the same; the amplitude shewing the bearing of any object when he rises or sets, from the East or West points of the horizon; and the azimuth, the bearing of any object when it is above the horizon, either from the North or South point thereof. As for example, if an object rises or sets within 10 degrees of the East or West, suppose towards the South, we accordingly say, its Amplitude is 10 degrees Southerly; but if an object, that is of any height above the horizon, should be in the vertical circle, passing thro’ the before-mentioned point, we then say, its Azimuth is 80 degrees from the South, or 100 degrees from the North, both which expressions signify the same.
Almacanthers.
Altitudes.
Meridian Altitude.
Zenith Distance.
14. All circles drawn parallel to the horizon, in the upper hemisphere, are called Almacanthers, or Parallels of Altitude: So that the Altitude of any point in the Heavens is an arch of the vertical circle passing thro’ that point, and intercepted betwixt it and the horizon; and if the object be upon the meridian, it is commonly called the Meridian Altitude. The complement of the altitude, or what it wants of 90 degrees, is called the Zenith Distance.
The horizon (by which we mean the rational) is represented by the upper surface of the Wooden frame, wherein the globes are placed; upon this horizon are described several concentric circles, the innermost of which is divided into degrees, which ought to be numbered both ways from the East and the West, until they end at 90 degrees in the North and South points. The use of these divisions is to shew the amplitudes of the Sun and Stars, at their rising and setting: Also in some convenient place upon this horizon, there is commonly noted the points of the Compass. Without the before-mentioned circle there is drawn the ecliptic with its divisions, into signs, and degrees, and a circle of months and days: The use of these two circles is to serve as a kalendar to shew the Sun’s place at any time of the year, and by that means to find his place in the Ecliptic, drawn upon the globe itself.
The Vertical Circles, and the Parallels of Altitude, are supplied by a thin plate of brass, having a nut and screw at one end to fasten it to the brass meridian in the Zenith point; which being done, the lower end of it may be put between the globe it self, and the inner edge of the horizon, and so turned round about to any point required.