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_.
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.
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.
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_.