[Sidenote: _Twilight_, or _Crepusculum_.]
This small illumination of the atmosphere, and the state of the Heavens between day and night, is called the _Twilight_, or the _Crepusculum_.
The duration of twilight is different in different climates, and in the same place at different times of the year. The beginning or ending of twilight being accurately given, we may from thence easily find the height of the atmosphere, which is not always the same. The mean height of the atmosphere is computed to be about 40 miles; but it is probable, the air may extend itself a great deal further, there being properly no other limits to it, as we can conceive, but as it continually decreases in density the farther remote it is from the Earth, in a certain ratio; which at last, as to our conception, must in a manner terminate.
_Of the Situations of Places upon the Earth; of the different Situations of its Inhabitants; of Zones and Climates._
The situations of places upon the Earth, are determined by their Latitude and Longitude.
1. The _Latitude_ of any place (upon the Earth) is its nearest distance, either North or South from the Equator; and if the place be in the (Northern/Southern) hemisphere, it is accordingly called (_North_/_South_) _Latitude_; and is measured by an arch of the meridian intercepted betwixt the zenith of the said place, and the equator. And all places that lie on the same side, and at the same distance from the equator, are said to be in the same parallel of latitude: the parallels in _Geography_, being the same with the parallels of declination in _Astronomy_.
From this definition arise the following Corollaries.
(1.) _That no place can have above 90 degrees of latitude, either North or South._
(2.) _Those places that lie under the equinoctial (or thro’ which the equator passes) have no latitude, it being from thence that the calculation of latitudes is counted; and those places that lie under the Poles have the greatest latitude, those points being at the greatest distance from the equator._
(3.) _The latitude of any place is always equal to the elevation of the Pole in the same place above the horizon; and is therefore often expressed by the Pole’s height, or elevation of the Pole; the reason of which is, because from the equator to the Pole there is always the distance of 90 degrees, and from the zenith to the horizon the same number of degrees, each of these including the distance from the zenith to the Pole: That distance therefore being taken away from both, will leave the distance from the zenith to the equator, (which is the latitude) equal to the distance of the Pole to the horizon._