Astronomy Dictionary

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

Danjon scale

A method of measuring lunar eclipse brightness, ranging from 0-4, darkest to brightest respectively. The Danjon scale is represented by the letter L.

Dark frame

Exposure taken with a digital camera with the lens cap on, or aperture covered. All that is captured is noise and unwanted effects, such as hot pixels and amp glow. The dark frame is then used to remove the noise and unwanted effects when processing the image.

Dark matter halo

A giant halo of dark matter found around a galaxy. These have been detected by the effect their gravity has on the rotation of spiral galaxies.

Dark nebula

This is an interstellar cloud of gas and dust which is silhouetted against a brighter background such as a field of stars or a glowing nebula.

Declination (dec.)

The angular distance of a body north or south of the celestial equator. A positive value indicates a body north of the celestial equator, whereas a negative value indicates one to its south.

Deep-sky objects (DSOs)

This is the common name of a range of faint objects including galaxies, nebulae and star cluster. These objects are also often fondly referred to as ‘faint fuzzies’ by amateur astronomers.


When a planet or moon appears at 50 per cent phase (half lit). As viewed from Earth, Mercury, Venus and the Moon can reach dichotomy.

Diffraction-limited optics

The theoretical limit of the resolving power of a telescope’s optics. This limitation arises because the diffraction, or bending, of light as it passes through the aperture of a telescope causes a star to be observed as a bright disc (the Airy disc) with bright rings around it, rather than a point of light.


A measurement of the optical power of lenses and curved mirrors in telescopes and binoculars. The dioptre is the reciprocal of the lens or mirror’s focal length in metres.

Dioptre adjustment

This is the ability to adjust the power of the eyepieces on a pair of binoculars to suit your eyes.


The Dobsonian telescope is a design of telescope that was lauded by the famous American astronomer Jon Dobson. It is based on a simple design – usually a large Newtonian style reflecting telescope mounted on a basic alt-azimuth mount. This type of telescope excels if you are interested in observing the night sky; as the simple, relatively cheap design means that, for the cost, a larger primary mirror can be incorporated.

Doppler effect

The relative motion between a source and a receiver that causes a change in a wave’s observed frequency. In the case of light, it is more commonly referred to as redshift since a reduced frequency corresponds to a longer wavelength, shifting the light to the red end of the visible spectrum.

Doppler Imaging

A technique that uses high-resolution spectrographic data of a star to produce a map of the star’s surface.

Dovetail plate

A type of mounting plate, which attaches a telescope tube to a mount. Its name comes from the fact that the join is wedge-shaped, like a dove’s tail

Dwarf planet

This classification was introduced in 2006 for celestial objects that orbit the Sun, are nearly spherical in shape, but are not satellites and have not cleared their neighbourhood of other celestial bodies. This new class includes Pluto (formerly a planet), Ceres (the largest object in the Main Belt) and Eris (a trans-Neptunian object that was found to be larger than Pluto), the planet that started the debate.

Dwarf spheroidal

A dwarf galaxy that contains few stars and hardly any gas. Most of the Milky Way’s satellites are dwarf spheroidals.

Dwarf star

Most of the stars in the Universe are dwarf stars. Their masses are between a tenth and one hundred times the mass of the Sun.

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