Altitude
The angular distance of a celestical object above the local horizon. With azimuth, altitude defines the horizontal coordinate system. Altitudes of 90 degrees correspond to the zenith, while altitudes of zero degrees indicate an object on the horizon.

Analemma
The apparent path of the sun when plotted on the celestial sphere at a fixed local time for a location on earth. The analemma, often included on globes, shows a "figure 8" pattern, which has a width of 47 degrees, with the endpoints showing the height of the sun at the summer solstice and the depth of the sun during the winter solstice. The equinox is the midpoint of the figure, and does not coincide with the central intersection of the '8' due to the change in speed of the earth as it orbits the sun.

Angular Diameter
The apparent size of an object on the sky. The angular diameter increases with the linear size of the object, and decreases with the inverse of the object's distance. By coincidence the angular diameters of both the sun and moon are approximately 30 arc minutes, which makes solar eclipses possible.

Annual Motion
The apparent motion of the celestial objects seen from a single location at the same time. The shift of the apparent positions of the celestial objects occurs due to the motion of the earth along its orbit with the sun. To a geocentric astronomer, these motions were a source of mystery and they formed the mainstay of the development of astronomical cosmology. The annual motion of the sun is a path through the constellations of the zodiac, along a line which is the ecliptic plane. The annual motions of planets consist of similar motions through the zodiac, with additional retrograde loops due to the fact that inner (or inferior) planets move faster and pass the outer (or superior) planets. The annual motions of the stars cause them to appear to shift to the west relative to the sun each day, resulting in the stars rising 4 minutes earlier each day.

Azimuth
The angular heading of a celestial object on the local horizon, as measured eastward from north. With altitude, azimuth defines the horizontal coordinate system.

Celestial Sphere
The celestial sphere is a construct which allows astronomers to map the positions of stars and planets by projecting their positions in the sky above the terrestial globe. The result is a sphere which surrounds the earth, upon which one can define 'fixed' positions of the stars. The north pole of the celestial sphere correponds to the north pole of the earth, and defines the celestial pole. Similarly the equator of the sphere, known as the celestial equator, corresponds to the equatorial plane of the earth extended into the sky.

Conjunction
The alignment of an two astronomical objects along the same line toward the sun. A superior conjunction occurs when a planet is on the opposite side of sun from us in line with the sun. An inferior conjunction occurs when the planet is on the same side of the sun with us in line with the sun. The inner planets only can be in these conjunctions with the earth, while outer planets may be in either superior conjunction, or in opposition to the earth.

Constellation
A grouping of stars in the sky, generally representing a region from 1-15 degrees in the sky associated with a figure drawn using the brightest stars as vertices. Each culture has defined their constellations differently, with particular notice being paid to some of the same groupings, such as the Big Dipper, Orion, and Scorpio.

Culmination
The passage of a star or planet across the observer's meridian. This crossing of the meridian results in the maximum altitude for the star at the observer's location.

Declination
The angular location of an object on the celestial sphere, relative to the celestial equator. The declination of an object on the celestial sphere is analogous to the latitude of a point on the globe.

Deferent
In the Ptolemaic System, the deferent is defined as the largest circle of a planet's orbit centered at or near the earth through which the center of the epicycle progresses in its orbit around the earth.

Diurnal Motion
The apparent daily motion of stars and planets resulting from the earth's daily spin. The diurnal motion of stars are large circles across the sky which intercept the observer's horizon at different points depending on the latitude of the observer's location. For stars near the celestial poles, the diurnal motion consists of complete circles, which are either completely above or below the horizon. Objects closer to the equator desribe large arcs which rise on the eastern horizon and set in the west.

Ecliptic Plane
The ecliptic plane is the plane defined by the earth's orbit, which is inclined by 23.5 degrees from the equatorial plane due to the tilt of the earth's spin axis. The ecliptic plane defines the center of the zodiac, and most planets appear near the ecliptic plane due to the general flatness of the solar system.

Elongation
The elongation of a planet is the angular distance between the position of the planet and the sun. For interior planets such as Venus and Mercury, the maximum elongations are fixed by the sizes of the planet's orbit, and the relative positions of the planet and earth. For Venus the maximum elongations are approximately 47 degrees, while for Mercury, the maximum elongations range from 18 to 28 degrees. A planet which has an elongation of 90 degrees is said to be at quadrature, a planet with an elongation of 180 degrees is said to be at opposition, and a planet with an elongation of 0 degrees is in conjunction.

Epicycle
In the Ptolemaic system, an epicycle is a smaller circular motion added to the larger orbital motion of the planet. Using the correct combination of epicycle and deferent, most of the retrograde loops of the planets can be described quite accurately. The additional innovation of placing the largest circular motion slightly off-center from the earth allowed the Ptolemaic system to completely describe planetary motions consistent with the observations of the day, albeit with several unweildy assumptions.

Equant
In the Ptolemaic system, the equant is the name for the spheres of the different planets, and the equants divided the universe into different shells centered on the earth, within which each of the planets and the sun would move.

Equinox
The equinox is both a location and a time of year. The most common use of the term refers to the midpoint of the earth's motion in which the length of the day and night are equal. The vernal (or Spring) equinox occurs on or about March 21, while the autumnal equinox occurs on or about September 23.

Geocentric Theory
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Heliocentric Theory
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Hour Angle
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Line of Nodes
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Meridian
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Metonic Period
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Opposition
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Ptolemaic System
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Proper Motion
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Retrograde Loop
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Right Ascension
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Saros Cycle
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Sidereal Period
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Solstice
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Standstill
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Synodic Period
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Transit
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Zenith
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Zodiac
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