| altitude | The angular distance between the direction to an object and the horizon. Altitude ranges from 0º for an object on the horizon to 90º for an object directly overhead.
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| 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.
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| celestial horizon | The circle on the celestial sphere that is 90º from the zenith. The celestial horizon is approximately the boundary between the Earth and sky.
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| celestial sphere | An imaginary sphere surrounding the Earth. The celestial bodies appear to carry out their motions on the celestial sphere.
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| 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.
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| 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.
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| coordinates | The numbers used in a coordinate system. Longitude and latitude are examples of coordinates.
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| degree | A unit used to measure angles. There are 360º in a circle.
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| 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.
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| great circle | A circle that bisects a sphere. The celestial equator and ecliptic are examples of great circles.
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| horizon system | A coordinate system, using altitude and azimuth as coordinates, used to locate the positions of objects in the sky.
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| 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.
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| 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.
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| minute of arc | A unit of angular measurement equal to one-sixtieth of a degree.
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| 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.
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| second of arc | A unit of angular measurement equal to 1⁄60 of a minute of arc or 1⁄3600 of a degree.
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| zenith | The point on the celestial sphere directly above an observer.
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| 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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