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last quarter
the phase of the moon three-quarters of the way around its orbit from new moon; the western half is illuminated
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latitude
the angular distance north or south from the equator to a point on Earth's surface, measured on the meridian of the point
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lens
a curved piece of glass that brings light to a focus
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lenticular galaxy
a galaxy possessing a large bulge and small disk
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libration
the small oscillations in the moon’s motion that allow Earth-based observers to see slightly more than half the moon’s surface
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light pollution
light, typically from artificial sources, that reaches the night sky, obscuring the view of faint astronomical objects
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light-gathering power
the ability of a telescope to collect light; the larger a telescope’s aperture, the greater its light-gathering power
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light-year
the distance light travels in one year, equivalent to approximately 5.9 trillion miles (9.5 trillion km)
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limb
the apparent edge of a celestial object
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limiting magnitude
the apparent magnitude of the faintest objects that can be seen given the local observing conditions and any telescope, film, or other detector you may be using
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local supercluster
the galaxy supercluster to which the Local Group belongs; it spreads over 100 million light-years and boasts the Virgo Cluster as its dominant member
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longitude
the angular distance of a particular place on Earth as measured east or west from the prime meridian running through Greenwich, England
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long-period comet
Comets that have orbital periods greater than 200 years.
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luminosity
the total amount of light that an object radiates
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lunar eclipse
a phenomenon caused by the Earth passing between the sun and moon
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lunar month
the period of one complete revolution of the moon around Earth, 29.5 days
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lunation
the time between two successive new moons; approximately 29.5 days
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