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apparent solar time
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time as measured by the position of the Sun in the sky (the time that would be indicated by a sundial)
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declination
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the angular distance north or south of the celestial equator
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great circle
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a circle on the surface of a sphere that is the curve of intersection of the sphere with a plane passing through its center
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International Date Line
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an arbitrary line on the surface of Earth near longitude 180° across which the date changes by one day
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lunar eclipse
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an eclipse of the Moon, in which the Moon moves into the shadow of Earth; lunar eclipses can occur only at the time of full moon
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mean solar time
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time based on the rotation of Earth; mean solar time passes at a constant rate, unlike apparent solar time
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meridian
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a great circle on the terrestrial or celestial sphere that passes through the poles
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phases of the Moon
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the different appearance of light and dark on the Moon as seen from Earth during its monthly cycle, from new moon to full moon and back to new moon
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right ascension
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the coordinate for measuring the east-west positions of celestial bodies; the angle measured eastward along the celestial equator from the vernal equinox to the hour circle passing through a body
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sidereal day
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Earth’s rotation period as defined by the positions of the stars in the sky; the time between successive passages of the same star through the meridian
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sidereal month
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the period of the Moon’s revolution about Earth measured with respect to the stars
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solar day
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Earth’s rotation period as defined by the position of the Sun in the sky; the time between successive passages of the Sun through the meridian
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solar eclipse
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an eclipse of the Sun by the Moon, caused by the passage of the Moon in front of the Sun; solar eclipses can occur only at the time of the new moon
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solar month
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the time interval in which the phases repeat—say, from full to full phase
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synchronous rotation
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when a body (for example, the Moon) rotates at the same rate that it revolves around another body
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tides
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alternate rising and falling of sea level caused by the difference in the strength of the Moon’s gravitational pull on different parts of Earth