Astronomy dictionary – G

Browse through our astronomy dictionary to find definitions for some of the most common terms used in practical astronomy and space science.

Click on one of the letters below to search for a term.



A vast gathering of typically billions of stars. They come in many shapes and sizes throughout the Universe.

Gamma-ray burst

Flashes of high-energy electromagnetic radiation from random locations in space. They may be caused by the merger of two black holes or neutron stars, or the explosion of a supernova.

General relativity

This is Einstein’s theory of gravity, which states that mass and energy curve space-time – the fabric of the Universe. Gravity is the result of the curvature of space-time, which causes light to bend around an object and planets to orbit stars.

Geosynchronous orbit

An orbit in which the period of a satellite’s movement around the Earth is the same as the rotation period of the Earth on its axis. If the satellite is above the Equator this results in the satellite appearing stationary in space, as seen from the surface of the Earth.

Ghost crater

These are craters who have been infilled, buried or submerged by geological processes but are sometimes still just visible on the surface of a planet or moon.


The phase of a moon or planet when the illumination of the disc is greater than half the planet or moon’s visible disc but less than full.

Globular cluster

A dense spherical grouping of old stars bound together by their self gravity – these clusters are found in the halos of galaxies like the Milky Way and are truly ancient.


The raised area of a sundial that casts a shadow on a dial so that the time of day can be read.

Gravitational lens

A massive celestial object, like a galaxy, that curves space-time around it to such an extent that it bends the path of light from distant bright objects like quasars. This phenomenon was predicted by Albert Einstein in his general theory of relativity.

Gravitational waves

Predicted in Einstein’s general theory of relativity, these waves are believed to be ripples in the fabric of space-time caused by bodies such as binary systems (binary white dwarfs, neutron stars or black holes). The Laser Interferometer Space Antenna (LISA)
is one of several missions that will hopefully detect gravitational waves.

Gravity waves

These are waves produced by the gravitational effects of one body on another. The Moon causes rocks on Earth to physically bulge up – not as much as water, but measurably. Gravity waves shouldn’t be mistaken for gravitational waves predicted by general relativity.

Green flash

An optical phenomenon visible for a short time as a green spot in the sky just after sunset or before sunrise.


A (usually) smaller telescope operated in conjunction with a CCD camera or modified webcam, mounted on top of a main imaging scope. It’s used to guide the movement of the mount to iron out any imperfections in the tracking.


Scientists have found a more accurate way to calculate the ages of stars. Gyrochronology uses the rotation period and colour of a star to determine its age, with the age of the Sun being used to calibrate the measurements. Unlike another technique for calculating stellar ages, gyrochronology works well for main sequence stars (stars that are still burning hydrogen) and lone stars that don’t belong to a cluster.