concave which is the apparent annual path of the sun. (3.) Horizon: The great circle in which the tangent plane to the earth at the station of the spectator meets the celestial concave. The Secondary Great Circles. (1.) Circles of Declination: Great circles of the celestial concave perpendicular to the equinoctial. (2.) Circles of Latitude: Great circles of the celestial concave perpendicular to the ecliptic. (3.) Circles of Altitude: Great circles of the celestial concave perpendicular to the horizon. (1.) Points of Origin. }First Point of Aries: The intersection of the ecliptic and the equinoctial where the sun crosses from the south to the north of the equinoctial. (3. North and South Points: The points in which the meridian cuts the horizon; the north point being the one adjacent to the north pole of the heavens. Co-Ordinates. (1.) Right Ascension of a Heavenly Body: The arc of the equinoctial intercepted between the first point of Aries and the circle of declination passing through the body, reckoning eastward. (2.) Longitude of a Heavenly Body: The arc of the ecliptic intercepted between the first point of Aries and the circle of latitude passing through the body, reckoning eastward. (3.) Azimuth of a Heavenly Body: The arc of the horizon intercepted between the south and north point (according as it is the north or south pole which is elevated), and the circle of altitude passing through the body, the arc being that which is not greater than 180°. (1.) Declination of a Heavenly Body: The arc of the circle of declination passing through the body intercepted between the equinoctial and the body. (2.) Latitude of a Heavenly Body: The arc of the circle of latitude passing through the body intercepted between the ecliptic and the body. (3.) Altitude of a Heavenly Body: The arc of the circle of altitude passing through the body intercepted between the horizon and the body. Relations, Poles, &c. Obliquity of the Ecliptic: The angle at which the ecliptic is inclined to the equinoctial. Its value is about 23° 27' 30". Zenith: The superior pole of the horizon. II. POLAR CO-ORDINATES FOR THE SURFACE OF THE CELESTIAL SPHERE. Celestial Meridian: The great circle of the celestial concave passing through the pole and the zenith. The term is generally restricted to the superior half of the circle. Hour Circles: Great circles of the celestial concave passing through the poles. Hour Angle of a Heavenly Body: The angle at the pole between the celestial meridian and the hour circle passing through the body, reckoning westward. Polar Distance of a Heavenly Body: The arc of the hour circle passing through the body intercepted between the elevated pole and the body. Poles, Particular Cases, &c. Prime Vertical: Circles of altitude, from their being perpendicular to the horizon, are also called Vertical Circles; the prime vertical is the vertical circle perpendicular to the meridian. East and West Points: The points in which the prime vertical cuts the horizon, the east point being towards where the sun rises. They are the poles of the meridian. Amplitude of a Heavenly Body: The distance from the east point at which the body rises, or the distance from the west point at which the body sets. DIURNAL TIME. Day: The interval between two successive transits, over the same meridian, of some point in the celestial concave fixed upon as the point of definition. (1.) Sidereal Day: The interval between two successive transits of the first point of Aries over the same meridian. (2.) Apparent Solar Day: The interval between two successive transits of the actual sun's centre over the same meridian. (3.) Mean Solar Day: The interval between two successive transits of the mean sun over the same meridian; the Mean Sun being a fictitious body which is conceived to move uniformly in the equinoctial with the mean velocity which the real sun has in the ecliptic. Noon: The instant when some point in the celestial concave, fixed upon as the point of definition, is on the meridian. (1.) Sidereal Noon: The instant when the first point of Aries is on the meridian. (2.) Apparent Noon: The instant when the actual sun's centre is on the meridian. (3.) Mean Noon: The instant when the mean sun is on the meridian. Time: The angle at the pole of the heavens between the celestial meridian and the hour circle passing through some point in the celestial concave, fixed upon as the point of definition, reckoning westward. (1.) Sidereal Time: The angle at the pole of the heavens between the celestial meridian and the hour circle passing through the first point of Aries, reckoning westward. (2.) Apparent Time: The angle at the pole of the heavens between the celestial meridian and the hour circle passing through the actual sun's centre, reckoning westward. (3.) Mean Time: The angle at the pole of the heavens between the celestial meridian and the hour circle passing through the mean sun, reckoning westward. Equation of Time: The difference between apparent and mean time. With reference to the arrangement of its parts, the day is distinguished as the Astronomical and the Civil Day. The Astronomical Day is the day used by astronomers to which to refer their observations; the Civil Day is that which regulates the ordinary business of life. They differ in the following points: the |