Oblique Sphere.
17. An Oblique Sphere is when the equinoctial and the horizon make oblique angles with each other, which every where happens but under the equator and the poles.
Diurnal and Nocturnal Arch.
The arch of any parallel or declination, which stands above the horizon is called the Diurnal Arch; and the remaining part of it, which is below the horizon, is called the Nocturnal Arch.
That point of the equinoctial which comes to the (Eastern/Western) part of the horizon with any point of the Heavens, is called the (Ascension/Descension) of that point, counted from the beginning of ♈; and if it be in a right sphere, the ascension or descension is called right; but if it be an oblique sphere it is called an oblique ascension or descension. So that,
Right Ascension.
18. The Right Ascension of the Sun, Moon, or any Star, &c. is an arch of the equator contained betwixt the beginning of ♈, and that point of the equinoctial which rises with them in a Right Sphere, or which comes to the meridian with them in an oblique sphere.
Oblique Ascension.
19. Oblique Ascension, or Descension, is an arch of the equinoctial intercepted between the beginning of ♈, and that Point of the Equator which rises or sets with any point in the Heavens in an oblique sphere.
Ascensional Difference.