View cloud cover over the UK in the latest satellite image from the Met Office

Astronomy Dictionary

Browse through Sky at Night Magazine’s comprehensive astronomical dictionary, with over 300 entries.

Lagrange point

An area of space where the gravity between two masses is balanced. There are five Lagrangian points for the Earth and Sun where a satellite would be locked in its orbit around the Sun, similar to a geosynchronous orbit.

Laminar air flow

This is where air flows smoothly in layers, without any interference to disrupt the flow. Such conditions enable better quality seeing. Turbulent air flow reduces the observing quality.

Lenticular galaxy

A lenticular galaxy (also known as a S0 type galaxy) is a galaxy that is disc shaped with no real evidence of spiral arms. Their name derives from how they look when they are edge on – like the shape of a lens.


This is the apparent wobble of the Moon as it goes around the Earth. Libration allows us to see a little more than half of the Moon’s disc.

Libration zone

Regions of the Moon occasionally visible due to the Moon’s apparent periodic wobble.

Light curve

A light curve is a graph of the brightness of an astronomical object. It’s usually applied to variable stars, novae or stars that are transited by extrasolar planets.

Light echo

This is the reflection of light usually seen around the gaseous ejections from massive stars or sometimes supernova. Perhaps the most prominent of all recently observed light echoes was the one seen around the exploding star V838 Monocerotis.

Light pollution

This is the pervasive orange glow which appears throughout much of the night sky in urban and indeed some rural areas. It is caused by light reflected of bright surfaces and poorly positioned (or badly designed) lighting.


A unit of measurement equal to the distance travelled by light in a year, that’s 9.46x10^12km. (5.88x10^12 miles).

Local group

A gathering of 36 nearby galaxies, including the Milky Way, make up the local group. The Andromeda Galaxy and M33 are both in our local group.


This is the name often given to the ‘light frames’ or exposures gathered from a monochrome CCD i.e. without colour.


This is the total amount of radiation emitted from a celestial object like a star or galaxy.

Lunar limb

The extreme edge of the disc of the Moon.


The time it takes for the Moon to make a complete cycle through its phases.

Lyman alpha blob

These are vast glowing gas clouds that emit a characteristic wavelength of light known as the Lyman-alpha spectral line in Hydrogen’s spectral signature.