| apparent solar day | The amount of time that passes between successive appearances of the Sun on the meridian. The apparent solar day varies in length throughout the year.
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| apparent solar time | Time kept according to the actual position of the Sun in the sky. Apparent solar noon occurs when the Sun crosses an observer’s meridian.
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| autumnal equinox | The point in the sky where the Sun appears to cross the celestial equator moving from north to south. This happens on approximately September 22.
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| celestial equator | The circle where the Earth’s equator, if extended outward into space, would intersect the celestial sphere.
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| conjunction | The appearance of two celestial bodies, often a planet and the Sun, in approximately the same direction.
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| crescent phase | The phase of the Moon at which only a small, crescent-shaped portion of the near side of the Moon is illuminated by sunlight. Crescent phase occurs just before and after new Moon.
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| declination | The angular distance of a celestial body north or south of the celestial equator. Declination is analogous to latitude in the terrestrial coordinate system.
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| direct motion | The eastward apparent motion of a solar system body with respect to the stars. Direct motion is interrupted by regular episodes of retrograde (westward) motion.
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| diurnal | Daily.
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| diurnal circle | The circular path that a celestial body traces out as it appears to move across the sky during an entire day. Diurnal circles are centered on the north and south celestial poles.
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| ecliptic | The plane of the Earth’s orbit about the Sun. As a result ofthe Earth’s motion, the Sun appears to move among the stars, following a path that is also called the ecliptic.
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| equatorial system | A coordinate system, using right ascension and declination as coordinates, used to describe the angular location ofbodies in the sky.
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| full phase | The phase of the Moon at which the bright side of the Moon is the face turned toward the Earth.
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| gibbous phase | The phase of the Moon at which the near side of the Moon is more than half illuminated by sunlight. Gibbous phase occurs just before and after full Moon.
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| leap year | A year in which there are 366 days.
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| local hour angle | The angle, measured westward around the celestial equator, between the meridian and the point on the equator nearest a particular celestial object.
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| mean solar time | Time kept according to the average length of the solar day.
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| meridian | The great circle passing through an observer’s zenith and the north and south celestial poles.
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| new phase | The phase of the Moon in which none or almost none of the near side of the Moon is illuminated by sunlight, so the near side appears dark.
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| nodes | The points in the orbit of the Moon where the Moon crossesthe ecliptic plane.
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| north celestial pole | The point above the Earth’s North Pole where the Earth’s polar axis, if extended outward into space, would intersect the celestial sphere. The diurnal circles of stars in the northern hemisphere are centered on the north celestial pole.
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| north circumpolar region | The region of the northern sky within which the diurnal circles of stars do not dip below the horizon. The size of the north circumpolar region varies with the latitude of the observer.
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| opposition | The configuration of a planet or other body when it appears opposite the Sun in the sky.
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| prograde motion | The eastward (normal) revolution of a solar system body.
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| quarter phase | The phase of the Moon in which half of the near side of the Moon is illuminated by the Sun.
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| retrograde motion | The westward revolution of a solar system body around the Sun.
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| right ascension | Angular distance of a body along the celestial equator from the vernal equinox eastward to the point on the equator nearest the body. Right ascension is analogous to longitude in the terrestrial coordinate system.
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| sidereal clock | A clock that marks the local hour angle of the vernal equinox.
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| sidereal day | The length of time (23 hours, 56 minutes, 4.091 seconds) between successive appearances of a star on the meridian.
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| sidereal month | The length of time required for the Moon to return to the same apparent position among the stars.
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| south celestial pole | The point above the Earth’s South Pole where the Earth’s polar axis, if extended outward into space, would intersectthe celestial sphere. The diurnal circles of stars in the southern hemisphere are centered on the south celestial pole.
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| summer solstice | The point on the ecliptic where the Sun’s declination is most northerly. The time when the Sun is at the summer solstice, around June 21, marks the beginning of summer.
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| synodic month | The length of time (29.53 days) between successive occurrences of the same phase of the Moon.
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| synodic period | The length of time it takes a solar system body tore turn to the same configuration (opposition to opposition, for example) with respect to the Earth and the Sun.
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| tropical year | The interval of time, equal to 365.242 solar days, between successive appearances of the Sun at the vernal equinox.
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| vernal equinox | The point in the sky where the Sun appears to cross the celestial equator moving from south to north. This happens approximately on March 21.
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| waning crescent | The Moon’s crescent phase that occurs just beforenew Moon.
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| waxing crescent | The Moon’s crescent phase that occurs just afternew Moon.
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| winter solstice | The point on the ecliptic where the Sun has the most southerly declination. The time when the Sun is at the winter solstice, around December 22, marks the beginning of winter.
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| year | The length of time required for the Earth to orbit the Sun.
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| zodiacal constellations | The band of constellations along the ecliptic. The Sun appears to move through the 12 zodiacal constellations during a year.
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