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laser An acronym for light amplification by stimulated emission of radiation; a device for amplifying a light signal at a particular wavelength into a coherent beam. 
latitude A north-south coordinate on the surface of the Earth; the angular distance north or south of the equator measured along a meridian passing through a place. 
law of areas  Kepler's second law: the radius vector from the Sun to any planet sweeps out equal areas in the planet's orbital plane in equal intervals of time. 
law of the redshifts  See Hubble law.
leap year  A calendar year with 366 days, inserted approximately every 4 years to make the average length of the calendar year as nearly equal as possible to the tropical year.
light year The distance light travels in a vacuum in one year; 1 LY = 9.46 x 10^12 km, or about 6 x 10^12 Mi. 
line broadening The phenomenon by which spectral lines are not precisely sharp but have finite widths.
line profile A plot of the intensity of light versus wavelength across a spectral line.
Local Group The cluster of galaxies to which our Galaxy belongs.
local standard of rest A coordinate system that shares the average motion of the Sun and its neighboring stars about the galactic center.
Local Supercluster The supercluster of galaxies to which the Local Group belongs.
longitude An east-west coordinate on the Eartb's surface; the angular distance, measured east or west along the equator, from the Greenwich meridian to the meridian passing through a place.
luminosity The rate at which a star or other object emits electromagnetic energy into space.
luminosity class A classification of a star according to its luminosity within a given spectral class. Our Sun, a G2V star, has luminosity class V
luminosity function The relative numbers of stars (or other objects) of various luminosities.
lunar eclipse An eclipse of the Moon.
Lyman lines A series of absorption or emission lines in the spectrum of hydrogen that arise from transitions to and from the lowest energy states of the hydrogen atoms.
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