As with any field of scientific study, astronomy has a long list of important terminology. So we've put together this glossary where you can check you fully understand the meaning of the common terms, as well as some of the more unusual words.

A -- B -- C -- D -- E -- F -- G -- H -- I -- J -- K -- L -- M -- N -- O -- P -- Q -- R -- S -- T -- U -- V -- W -- X -- Y -- Z

Words beginning with 'D' :

Dark adaptation : a reflex adaptation of the eye to reduced illumination. The first rapid adaptation to darkness takes 5 to 10 minutes, whereas full adaptation requires at least 30 minutes.

Dark matter : a hypothetical form of matte that doesn't emit any form of electromagnetic radiation. Its existence would explain some puzzling astronomical observation such as the gravitational effect on the velocity and motion of stars, galaxies and galaxy clusters.

Declination (Dec.) : denotes how far north (positive Dec) or south (negative Dec) an object in the sky lies relative to the celestial equator. It is measured in degrees, arcminutes and arcseconds.

Dissociation : the process of a molecule being split into its constituent parts by UV light. For example water vapor in Venus's atmosphere went through dissociation with the molecules being split into oxygen atoms and hydrogen atoms, with the hydrogen atoms escaping into space.

Dobsonian : a type of Newtonian telescope that was made popular by John Dobson (an amateur astronomer). He designed a portable telescope with a large aperture but was still affordable.

Doppler shift : the wavelength of a radiation emitted by a body will arrive lengthened or shortened to an observer depending on whether the object is moving towards or away from the observer. A good example of this phenomenon is the change of pitch of an ambulance siren as it approaches or moves away from an observer.

Dwarf planet : an astronomical object with enough mass and gravity to be spheroid or near spheroid orbiting the Sun. Unlike planets they have not cleared a path of other objects in their orbit around the Sun.

Resources :

To support the quizzes in each section, we've put together some extra resources to provide further information: