126. If the observer be any where on the Terrestrial Equator _eCq_, as
suppose at _e_, he is in the Plane of the Celestial Equator; or under
the Equinoctial _ECQ_; and the Axis of the Earth _nCs_ is coincident
with the Plane of his Horizon, extended out to _N_ and _S_, the North
and South Poles of the Heavens. As the Earth turns round the line _NCS_,
the whole Heavens _MOLl_ seem to turn round the same line, but the
contrary way. It is plain that this observer has the Poles constantly in
his Horizon, and that his Horizon cuts the Diurnal paths of all the
Celestial bodies perpendicularly and in halves. Therefore the Sun,
Planets, and Stars rise every day, and ascend perpendicularly above the
Horizon for six hours, and passing over the Meridian, descend in the
same manner for the six following hours; then set in the Horizon, and
continue twelve hours below it. Consequently at the Equator the days and
nights are equally long throughout the year. When the observer is in the
situation _e_, he sees the Hemisphere _SEN_; but in twelve hours after,
he is carried half round the Earth’s Axis to _q_, and then the
Hemisphere _SQN_ becomes visible to him; and _SEN_ disappears, being hid
by the Convexity of the Earth. Thus we find that to an observer at
either of the Poles one half of the Sky is always visible, and the other
half never seen; but to an observer on the Equator the whole Sky is seen
every 24 hours.
The Figure here referred to, represents a Celestial globe of glass,
having a Terrestrial globe within it; after the manner of the Glass
Sphere invented by my generous friend Dr. LONG, _Lowndes_’s Professor of
Astronomy in _Cambridge_.
[Sidenote: Remark.]
127. If a Globe be held sidewise to the eye, at some distance, and so
that neither of it’s Poles can be seen, the Equator _ECQ_ and all
Circles parallel to it, as _DL_, _yzx_, _abX_, _MO_, &c. will appear to
be straight lines, as projected in this Figure; which is requisite to be
mentioned here, because we shall have occasion to call them Circles in
the following Article[26].
[Sidenote: Phenomena between the Equator and Poles.