A landslide.

An intersecting valley complex.

A lake.

Lagrange 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. Bodies at such points are sometimes referred 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. Collinear with the two large bodies are the L1, L2 and L3 unstable equilibrium points which can sometimes be useful places for spacecraft, eg SOHO. (For a more detailed explanation, click on the diagram to the right).

The angular distance north or south from the equator.

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

An instrument similar to radar that operates at visible wavelengths.

The illumination of the night sky by waste light from cities and outdoor lighting, which prevents the observation of faint objects. This is why it is hard to see stars in big cities.

The distance light travels in one year. 1 ly = 9.46*10^15 meter = 5.9 billion miles.

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

Elongate marking.

A substance that behaves like both a liquid and a solid.

= 1000 cm3 = 1.06 US quarts.

The angular distance east or west from the prime meridian.

Relating to the moon.

The average time between successive new or full moons, equal to 29 days 12 hours 44 minutes. Also called synodic month.