[Sidenote: Phenomena at the Equator. Fig. I.] 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. The Circles of perpetual Apparition and Occultation.]