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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