Astronomy dictionary – R

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.


Radial velocity method

A way of searching for exoplanets by looking at the movement of spectral lines in a star’s light to infer the presence of a planet.


The radiant is the point in the sky where meteors (associated with a specific meteor shower) appear to come from. The constellation where the radiant is located determines the name of the meteor shower. So for example, the Orionids have their radiant in Orion.

Ray (moon)

When a large meteoroid or asteroid impacts the Moon it forms a crater. In doing so it blasts vast amounts of material over the surface. This material then falls back on to the lunar surface, showing up as a bright ‘ray’ extending out from the centre of the crater.

Red dwarf

Red dwarfs make up most of the stars in the Universe. They are small cool stars that usually have less than 10 per cent the mass of the Sun with surface temperatures of between 2,200 and 3,700°C.

Red giant star

Massive or Sun-like stars that have ended the steady ‘main sequence’ phase. They then swell and in the process become extremely luminous.


The increase in the wavelength of visible light from a distant celestial body towards the red end of the electromagnetic spectrum. This is due to the Universe’s expansion, which causes the wavelength of light to increase as it travels through space.

Refractory grain

A grain that has been formed at high temperatures. This type of grain was found in samples of cometary dust by the Stardust spacecraft, and hints at a process that might have deposited these grains in the far reaches of the Solar System.


The fine, powdery layer of soil on the surface of the Moon.


This is the greatest amount of detail that a telescope is capable of delivering. A telescope’s resolution can be roughly estimated by observing binary stars. If a binary pair can just be made out to show two separate stars, the telescope’s resolution (or ‘resolving power’) is roughly equivalent to their angular separation in the sky.

Retrograde motion

The movement of a celestial body in the opposite direction to the majority of other bodies. For example, most objects in the Solar System, including all of the planets, orbit the Sun in an anti-clockwise direction as seen from above the Solar System. Bodies that orbit clockwise are said to have retrograde orbits.


A long, narrow depression on the surface of the Moon – and other planetary bodies – that resembles a channel. They are thought to be caused by collapsed lava flows just under the surface.

Roche lobe

This is a theoretical spherical boundary around (for example) each star in a binary system. Matter is bound by the star’s gravitational attraction within each lobe. When two Roche lobes mix they may begin to transfer material.

Ronchi grating

A Ronchi grating is a special type of diffraction grating used in optical tests. It’s used to find the quality of a scope’s mirror by studying the diffraction patterns produced by light bounced off the mirror.