L

Lacertid

A type of peculiar extragalactic radio source showing a starlike image but usually no emission or absorption lines in its spectra.

 

Latitude

The angular distance on the terrestrial sphere measured from the equator, north or south, along the meridian which passes through the place.  

 

Law of areas

Kepler's second law of planetary motion, which states that the radius vector (line joining the planet and sun) sweeps equal areas in its orbital plane in intervals of time.

 

Law of the red shift

The radial velocity of a distant galaxy, which is measured by the red shift, is proportional to its distance; therefore, the red shift is a measure of the galaxy's distance.

 

Leap year

A calendar year of 366 days, which occurs in the year divisible by four except in century years not divisible by 400. 1900 was not a leap year. 2400 will be a leap year.

 

Libration

A real of apparent oscillation of a body that permits the observer on the earth to see more than one hemisphere of the body during a given period of time.

 

Libration (latitudinal)

The libration due to the moon's equator being inclined about 6¹/₂^o to its orbital plane, which permits the observer on the earth to see 7¹/₂^o beyond the east and west limbs of the moon during one lunar month.

 

Light

An electromagnetic radiation visible to the eye.

 

Light curve

A plot of the variation of the magnitude of a variable star or an eclipsing binary against time.

 

Light-gathering power of a telescope

The amount of light a telescope collects, which is proportional to the area of its objective.

 

Light year

The distance light travels in one year in space (vacuum), which is approximately 6 x 10¹² miles, or 9.7 x 10¹² km. 

 

Limb

The apparent edge of the sun, moon, or planet.

 

Limb darkening

Term describing the fact that the sun's limb appears darker than the center of its disk, because at the disk's center the observer sees into deeper and hotter layers of the sun's photosphere.

 

Line broadening

The phenomenon that increases the width of spectral lines.

 

Line of apsides

The major axis of the elliptical orbit of a body.

 

Line of nodes

The line that connects the ascending and descending nodes of an orbit and that intersects a reference plane such as the ecliptic.

 

Local apparent time

The local hour angle of the apparent sun plus 12 hours.

 

Local group

The cluster of galaxies, including the Milky Way, that appears to form a group.

 

Local mean time

The local hour angle of the mean sun plus 12 hours.

 

Local standard of rest

The coordinate system in which the motions of the stars in the neighborhood of the sun average zero, that is, they appear to be at rest within the system.

 

Longitude

The angular distance on the terrestrial sphere measured from the greenwich meridian, east or west along the equator to the meridianthat passes through the place.

 

Luminosity

The rate at which a star emits electromagnetic radiation into space. It is usually expressed in terms of the sun's luminosity.

 

Luminosity function

The relative number of stars of various absolute magnitudes in a unit volume of space.