Dictionary
of astronomical
words and terms
Note - Words shown in Bold Italics are references to other entries.
A
Absolute Magnitude - The brightness of an object at a standard distance
of 10 parsecs (32.6 light years).
Aphelion - The point at which a planet, comet etc traveling around the Sun
in an elliptical orbit is at its furthest from the Sun. -See also perihelion.
Apogee - The point in an orbit round the Earth that is furthest from the
centre of the Earth.
Apparent Magnitude - The brightness of an object as seen from Earth.
Astronomical Units - Equal to the mean distance of the Earth from the
Sun, 149,597,870km / 92,955,800 miles. Abbreviation AU.
Autumnal Equinox (Autumn Equinox) - See equinox.
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B
Barlow Lens - A diverging lens used in conjunction with a telescope
eyepiece increasing the effective focal length of the telescope, which causes
the eyepiece to yield a greater magnification.
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C
Celestial Equator - The Earth's Equatorial Plane
projected onto the celestial
sphere, marking the boundary between
northern and southern hemispheres.
Celestial Sphere - An imaginary sphere surrounding the Earth on which
the celestial objects appear to be placed.
Conjunction - An alignment of two celestial bodies, so that they
appear to be at the same place (or nearly so) in the sky as seen from the Earth.
A planet is said to be at conjunction when it is at the same ecliptic longitude
as (and so is approximately in line with) the Sun. The planets Mercury and Venus
can form such a line by being either between the Earth and the Sun, when they
are said to be at inferior conjunction, or behind the Sun as seen
from Earth, an alignment called superior conjunction. The planets
further from the Sun than the Earth can only come to superior conjunction.
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E
Eclipse - A phenomenon where the light from a celestial body is temporarily cut off by the presence of
another.
Ecliptic - The mean plane of the Earth's orbit around the Sun.
Elongation - The angular distance
between the Sun and a planet or the Moon as viewed from the Earth.
Equinox - The point in Spring and Autumn where the Sun's path (the
ecliptic) crosses the celestial equator, so that day
and night are of approximate length. The vernal equinox (Spring)
occurs about 21 March and the autumnal equinox (Autumn) 23rd
September.
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G
Geocentric - Measured or Observed from the
Earth's centre. See also topocentric.
I
Inclination - Angle between the ecliptic and the plane
of the orbit of a planet, comet etc. In the case of the moon of a planet, it is
the angle between the plane of the moons orbit and the planets equator.
Inferior Conjunction - See conjunction.
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L
Light Year - The distance traveled by a beam of light in a vacuum in
one year. Approximately 9.46 trillion (million million) km / 5.88 trillion
miles.
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M
Magnitude - A measurement of the brightness of a star or other
celestial object. The larger the number the fainter the object. Zero or first
magnitude stars are some of the brightest. Even brighter stars are those of
negative magnitude, Sirius is such a star with a magnitude of -1.46. See
also apparent and absolute
magnitude.
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O
Occultation - The passing of one astronomical body in front of
another thereby obscuring it from view. For example, the moon passing in front
of a star or planet.
Opposition - A planet is said to be at opposition when it is opposite the
Sun in the sky. A line could then be drawn from the Sun through the Earth to the
planet. Only the planets outside the Earth's orbit can be at opposition.
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P
Parsec - Equal to 3.2616 light years, 2.063x10 astronomical
units, 3.086x1013km.
Penumbral - If the Moon only passes through the
outer part of the Earths shadow, the Lunar eclipse is an Penumbral type. See
also umbral.
Perigee - The point in an orbit round the Earth that is
nearest the centre of the Earth.
Perihelion - The point at which a planet, comet etc traveling around the
Sun in an elliptical orbit is at its closest to the Sun. -See also aphelion.
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Q
Quadrature - When the Moon or an outer planet forms a right angle,
when a line is drawn between it and the Earth, and the Sun and the Earth.
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S
Solstice - Solstices occur on about June
21st and December 21st. At the Summer Solstice, the Sun reaches its highest
altitude in the sky and the amount of daylight hours are at their greatest. At
the Winter Solstice, the altitude of the Sun is at its lowest and the amount of
daylight hours are at their minimum.
Spring Equinox - See equinox.
Summer Solstice - See solstice.
Superior Conjunction - See conjunction.
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T
Topocentric - In relation to a position on
the surface of a planet. See also geocentric.
U
Umbra (1) - An area of
total shadow, such as the zone on the surface of the Earth from which totality
is observed during a solar eclipse.
Umbra (2) - The dark central region of a sunspot.
Umbral - Type of lunar eclipse where the moon passes
through the center of the Earths shadow. See also penumbral.
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V
Vernal Equinox - See equinox.
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W
Winter Solstice - See solstice.