Altitude. Angular distance above the horizon: the arc of a vertical circle between the horizon and a point on the celestial sphere, measured upward from the horizon.

Angle. The inclination to each other of two intersecting lines, measured by the arc of a circle intercepted by the two lines forming the angle, the center of the circle being the point of intersection.

Angular Distance. The angle between two directions as seen from a given point.

Apogee. That orbital point farthest from the earth when the earth is the center of attraction (as in the case of the moon).

Apparent sun. The actual sun as it appears in the sky.

Apparent time. Time based upon the rotation of the earth relative to the apparent (true) sun.

Azimuth. The horizontal angle between a reference direction and another direction of interest. It is measured from 0° at the reference direction clockwise or counter clockwise through 360.

Bearing. The horizontal angle between a reference direction and another direction of interest. It is usually measured from 0° at true north clockwise through 360°.

Celestial body. Any aggregation of matter in space constituting a unit, such as the sun, a planet, etc.

Celestial Sphere. A sphere concentric with the earth, on which all celestial bodies except the earth are imagined to be projected.

Coordinate. One of a set of magnitudes defining a point in space.

Declination. The angular distance north or south of the celestial equator: the arc of an hour circle between the celestial equator and a point on the celestial sphere, measured northward or southward from the celestial equator through 90°, and labeled N or S to indicate the direction of measurement.

Ecliptic. The apparent annual path of the sun among the stars.

Equator. The primary great circle of the earth, or a similar body, perpendicular to the polar axis.

Equinox. One of the two points of intersection of the ecliptic and the celestial equator, or the instant the sun occupies one of these points, when its declination is 0°.

Great circle. The intersection of a sphere and a plane through its center.

Hour circle. A great circle perpendicular to the celestial equator.

Latitude. The angular distance north or south of the equator and a point on the surface of the earth, measured northward or southward from the equator through 90°, and labeled N or S to indicate the direction of measurement.

Longitude. The angle between the prime or reference meridian and another meridian of interest, measured from the prime meridian E or W through 180°.

Mean sun. A fictitious point conceived to move eastward along the celestial equator at a uniform rate equal to the average rate of the apparent sun along the ecliptic.

Meridian. A great circle perpendicular to the equator.

Parallax. The difference in the apparent direction or position of an object when viewed from different points.

Perigee. That orbital point nearest the earth when the earth is the center of attraction (as in the case of the moon).

Polar distance. The angular distance from a celestial pole, usually the elevated pole.

Precession. The change in the direction of the axis of rotation of a spinning body, as a gyroscope, when acted upon by a torque.

Prime meridian. The meridian of longitude 0° used as the origin for the measurement of longitude.

Sidereal day. The duration of one rotation of the earth on its axis, with respect to the vernal equinox.

Sideral hour angle. The angular distance west of the vernal equinox; the arc of the celestial equator, or the angle at the celestial pole, between the hour circle of the vernal equinox and the hour circle of a point on the celestial sphere, measured westward from the hour circle of the vernal equinox through 360°

Sidereal time. Time based upon the rotation of the earth relative to the vernal equinox.

Solar day. The duration of one rotation of the earth on its axis, with respect to the sun.

Vernal equinox. That point of intersection of the ecliptic and the celestial equator, occupied by the sun as it changes from south to north declination on or about March 21, or the instant this occurs.

Zenith distance. The angular distance from the zenith.

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