This circle determines the rising and setting of the Heavenly bodies, and distinguishes Day and Night.
The _Rational_, or true _Horizon_, is a great circle passing thro’ the center of the Earth, parallel to the sensible Horizon, being distant from it by the Earth’s semidiameter, which is about 3980 miles: This distance is nothing in comparison of the immense distance of the Sun and the fixed Stars, therefore astronomers make no distinction between these two circles, but consider the apparent Horizon, or that wherein the Sun appears to rise and set, as passing thro’ the center of the Earth.
[Sidenote: _Cardinal Points of the Horizon._]
This circle is divided by astronomers into four quadrants, and each of the quadrants into 90 degrees, _&c._ The four points quartering this circle are called the _Cardinal Points_, and are termed the _East_, _West_, _North_, and _South_. The _East_ is that point of the Horizon where the Sun rises when he is in the equinoctial, or on that day when he ascends above the Horizon exactly at six o’clock; and the _West_ is that point of the Horizon which is directly opposite to the East, or where the Sun Sets when he is in the Equinoctial. The _South_ is 90 degrees distant from the East and West, and is toward that part of the Heavens wherein the Sun always appears to us in _Great-Britain_ at Noon; and the _North_ is that part of the Heavens which is directly opposite to the South: Or, the North and South points of the Heavens may be found by turning yourself either directly towards the East or the West: If you look towards the (East/West) the (South/North) will be to the right Hand, and the (North/South) to the left.
Besides the aforementioned divisions of the Horizon into degrees. _Mariners_ divide it into 32 equal parts, which they call the _Points of the Compass_; to each of which points they give a particular name, compounded of the four Cardinals, according to what quarter of the Compass is intended.
The center of the Horizon is the place of observation, and the Poles of it are one exactly over our heads, called the _Zenith_; and the other exactly under our feet, called the _Nadir_.
13. All circles conceived to pass thro’ the Zenith and Nadir, are called _Vertical Circles_, or _Azimuths_. Of these circles, that which passeth thro’ the North and South points of the Horizon, is called the _Meridian_; so that when any object is upon the Meridian, it then bears either due South, or due North from us; and the _Azimuth_ of any object is an arch of the Horizon intercepted between the vertical circle passing through it, and either the North or South part of the Meridian; which part is commonly specified.