8. All great circles passing through the Poles of the ecliptic, and consequently intersecting it at right angles, are called _Circles of Longitude_: So that,
[Sidenote: _Longitude of any Point in the Heavens._]
9. The _Longitude_ of any _Point_ in the Heavens (as a _Star_ or _Planet_, _&c._) is an arch of the ecliptic contained between the circle of longitude passing thro’ that point, and the equinoctial point ♈. And that degree of any sign which lies under the circle of longitude, passing thro’ any Star or Planet, is called the _Place_ of that Star or Planet.
_Note_, The _Sun_ never goes out of the ecliptic, and it is not usual to say the Sun’s longitude, but we commonly express it the _Sun’s Place_, which is that sign, degree, minute, _&c._, of the ecliptic, which he at any time passes.
10. All circles conceived to be drawn parallel to the ecliptic, are called _Parallels of Latitude_: So that,
11. The _Latitude_ of any point in the Heavens, (as a fixed Star, _&c._) is an arch of the circle of longitude, in passing thro’ that point, and intercepted betwixt it, and the ecliptic; or, the latitude is the distance from the ecliptic; and if the said point be to the Northward of the ecliptic, it is called North Latitude; but if it be to the Southward, is called South Latitude.
Upon the _Terrestrial Globe_, none of the circles of longitude are described; and upon the _Celestial_, they are commonly drawn thro’ the beginning of every _Sign_; but they are all supplied upon both Globes, by fastening a thin plate of brass over one of the Poles of the ecliptic, and so as to be moved to any degree thereof at pleasure. The parallels of latitude are also supplied by the graduations upon the said plate, as shall be shewn in a proper place.
We have now done with all those circles that are fixed, and such as are drawn upon the Globes themselves; we next proceed to the moveable circles.
12. The _Horizon_ is that great circle which divides the upper, or visible Hemisphere of the world, from the lower, or invisible: This circle is distinguished into two sorts, the _Sensible_, and the _Rational_.
The _Sensible_, or _Apparent Horizon_, is that circle which limits or determinates our prospect, whether we are at land or sea, reaching as far as we can see, or it is that circle where the Sky and the Earth, or Water, seem to meet. When we are on _Terra Firma_, this circle commonly seems rugged and irregular, occasioned by the unevenness of the ground terminating our prospect; but at sea there are no such irregularities; the semidiameter of this circle varieth according to the height of the eye of the observer; if a man of six feet high stood upon a large plain, or the surface of the sea, he could not see above three miles round.