Astronomy dictionary – E

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.



When a planet, moon or other body enters the shadow of another. Therefore a solar eclipse is technically not an eclipse at all but an occultation of the Sun (see Occultation), or an eclipse of the Earth.

Eclipse mapping

A technique used to indirectly map the surface of an eclipsing binary star, to look for starspots and make other studies by using Doppler imaging.

Eclipsing binary

A star with a companion that periodically passes in front of it, and eclipses it as seen from Earth.


The ecliptic is the apparent path of the Sun across the sky, which is mapped onto the celestial sphere.


When a celestial body emerges from a shadow or from a transit.

Einstein Ring

This is a rare form of gravitational lens that occurs when a distant galaxy’s light is magnified by another galaxy directly in front of it, as seen from Earth. The result is a distorted image of a background galaxy in the shape of a perfect ring.

Ellerman Bombs

Ellerman bombs are bright explosions seen in the Sun’s chromosphere around newly formed active regions. They are named after the solar astronomer Ferdinand Ellerman who discovered them in September 1915.

Elliptical galaxy

These are galaxies that have predominantly old stars with a shape that ranges from nearly spherical to highly elliptical, and that contain little or no gas and dust.


Elongation is the angle in the sky between the Sun and a planet. For instance, the best time to view Mercury is when it’s at greatest elongation away from the Sun. Mercury is usually then close to its highest position in the sky.

Emission nebula

A cloud of interstellar gas that is being made to glow (emit). The emission is usually due to the strong radiation from hot young stars within the cloud or in its vicinity.

Equatorial mount

A telescope mount whose axis is aligned parallel with the Earth’s axis of rotation. This enables stars to be tracked as they drift across the sky.


The two times in the year when the Sun crosses the celestial equator.


An etalon is an optical device used to filter specific wavelengths of radiation by reflecting it many times between two parallel glass panels.


Better known as astrobiology, it combines the study of geology, biology, chemistry and astronomy to study the possibility of life in space and the origins of life on Earth.


An abbreviation of ‘extrasolar planet’, meaning a planet not in the Solar System but orbiting another star.


The upper part of an atmosphere where gas is very tenuous and rarefied. Earth’s exosphere lies about 500km above its surface. Mercury has no substantial atmosphere, but does have a very thin layer of gas extending around it that is essentially an exosphere

Eye relief

The distance from the eyepiece at which you can see the full field of view. People who wear glasses observe further away from the eyepiece and so need a longer eye relief.