208. If we imagine twelve great Circles, one of which is the Meridian of
any given place, to intersect each other in the two Poles of the Earth,
and to cut the Equator _Æ_ at every 15th degree, they will be divided by
the Poles into 24 Semicircles which divide the Equator into 24 equal
parts; and as the Earth turns on it’s Axis, the planes of these
Semicircles come successively after one another every hour to the Sun.
As in an hour of time there is a revolution of 15 degrees of the
Equator, in a minute of time there will be a revolution of 15 minutes of
the Equator, and in a second of time a revolution of 15 seconds. There
are two tables annexed to this Chapter, for reducing mean solar time
into degrees and minutes of the terrestrial Equator; and also for
converting degrees and parts of the Equator into mean solar time.
209. Because the Sun enlightens only one half of the Earth at once, as
it turns round it’s Axis he rises to some places at the same moments of
absolute Time that he sets to others; and when it is mid-day to some
places, it is mid-night to others. The XII on the middle of the Earth’s
enlightened side, next the Sun, stands for mid-day; and the opposite XII
on the middle of the dark side, for mid-night. If we suppose this Circle
of hours to be fixed in the plane of the Equinoctial, and the Earth to
turn round within it, any particular Meridian will come to the different
hours so, as to shew the true time of the day or night at all places on
that Meridian. Therefore,
[Sidenote: And consequently to 15 degrees of Longitude.