altitude The angular distance between the direction to an object and the horizon. Altitude ranges from 0° for an object on the horizon to 90° for an object directly overhead. azimuth The angular distance between the north point on the horizon eastward around the horizon to the point on the horizon nearest to the direction to a celestial body. celestial horizon The circle on the celestial sphere that is 90° from the zenith. The celestial horizon is approximately the boundary between the Earth and sky. celestial sphere An imaginary sphere surrounding the Earth. The celestial bodies appear to carry out their motions on the celestial sphere. circumpolar stars A body is close enough to a celestial pole that its diurnal circle is always above the horizon. Circumpolar stars neither rise nor set. coordinate system A system in which numbers are used to give the location of a body or event. The longitude-latitude system is an example of a coordinate system used to locate things on the Earth’s surface. coordinates The numbers used in a coordinate system. Longitude and latitude are examples of coordinates. degree A unit used to measure angles. There are 360° in a circle. equator The line around the surface of a rotating body that is midway between the rotational poles. The equator divides the body into northern and southern hemispheres. great circle A circle that bisects a sphere. The celestial equator and ecliptic are examples of great circles. horizon system A coordinate system, using altitude and azimuth as coordinates, used to locate the positions of objects in the sky. latitude The angular distance of a point north or south of the equator of a body as measured by a hypothetical observer at the center of a body. longitude The angular distance around the equator of a body froma zero point to the place on the equator nearest a particular point as measured by a hypothetical observer at the center of a body. minute of arc A unit of angular measurement equal to one-sixtieth of a degree. prime meridian The circle on the Earth’s surface that runs from pole to pole through Greenwich, England. The zero point of longitude occurs where the prime meridian intersects the Earth’s equator. second of arc A unit of angular measurement equal to 1⁄60 of a minute of arc or 1⁄3600 of a degree. zenith The point on the celestial sphere directly above an observer. zero point The point from which the coordinates in a coordinate system are measured. For example, the vernal equinox is the zero point of right ascension and declination in the celestial coordinate system.

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