labes
A landslide.

labyrinthus
An intersecting valley complex.

lacus
A lake.

Lagrange, Joseph Louis 1736-1813
French mathematician and astronomer. Lagrange made a number of contributions to the study of celestial mechanics. He showed that three bodies can lie at the apexes of an equilateral triangle which rotates in its plane. If one of the bodies is sufficiently massive compared with the other two, then the triangular configuration is apparently stable. Several examples are known in the solar system: Such bodies are sometimes refered to as Trojans. The leading apex of the triangle is known as the leading Lagrange point or L4; the trailing apex is the trailing Lagrange point or L5.

Lagrangian point
One of the solutions to the three-body problem discovered by the eighteenth century French mathematician Lagrange; the two stable Lagrangian points, L-4 and L-5, lie in the orbit of the primary body, leading and trailing it by a 60-degree arc.

Lassell, William 1799-1880
British astronomer. Lassell discovered Neptune's largest satellite, Triton and (with William Cranch Bond) discovered Saturn's moon Hyperion. A successful brewer before turning to astronomy.

lava
A general term for molten rock that is extruded onto the surface.

lava tube
A tunnel formed underneath the surface of a solidfying lava flow.

leading hemisphere
The hemisphere that faces forward, into the direction of motion of a satellite that keeps the same face toward the planet.

lee
The side of an object that is sheltered from the wind.

levee
An embankment, continuous dike or ridge.

Le Verrier, Urbain Jean Joseph 1811-1877
French astronomer. La Verrier's prediction of the position of an undiscovered planet (Neptune) that caused perturbations in the orbit of Uranus was the first to be confirmed (by Johann Gottfried Galle). However, John Couch Adams had made a similar prediction some months earlier.

light
Electromagnetic radiation that is visible to the eye.

light year
The distance light travels in a year, at the rate of 300,000 kilometers per second (671 million miles per hour); 1 light-year is equivalent to 9.46053e12 km, 5,880,000,000,000 miles or 63,240 AU.

limb
The outer edge of the apparent disk of a celestial body.

linea
An elongate marking.

lineament
Linear topographic feature that may depict crustal structure.

lobate
Having lobes or resembling a lobe.

Lowell, Percival 1855-1916.
American astronomer. He founded the Lowell Observatory in Arizona (1894), where his studies of Mars led him to believe that the planet was inhabited. His successors later discovered Pluto.