Astronomy Terms

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Altitude

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A measure of the angular distance of a celestial object above or below the observer's horizon.

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Zenith

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The imaginary point on the celestial sphere directly "above" a particular location.

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Perihelion

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The point along the elliptical orbit where an orbiting body (like a planet or comet) is closest to the Sun.

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Azimuth

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The angular distance of a celestial object measured east from north and parallel to the observer's horizon.

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Ellipse

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The shape of the path a planet takes when orbiting the Sun.

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Tropic of Cancer

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A parallel of latitude that lies at about 23 degrees north of the Equator.

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Tropic of Capricorn

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A parallel of latitude that lies at about 23 degrees south of the Equator.

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Aphelion

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The point along the elliptical orbit where an orbiting body (like a planet or comet) is most distant from the Sun.

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Astronomical Unit (AU or au)

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A unit of length effectively equal to the average, or mean, distance between Earth and the Sun.

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Light-year (ly)

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The distance that light travels in a vacuum in one Julian year.