Astronomy dictionary – I

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.


Imbrian period

The period in the Moon’s history when the large basins Imbrium, Crisium, Tranquilitatis, Serenitatis, Fecunditatis and Procellarum were formed, and filled with basalts. The Imbrian period occurred between 3.8 and 3.2 billion years ago.

Inferior conjunction

This is the point when a either Mercury or Venus is exactly between the Sun and the Earth in its orbit.


The theorised rapid expansion (inflation) of the Universe right after the Big Bang.


The period during a transit or an eclipse when the smaller celestial body begins to cross the disc of the larger object. Egress marks the exit of the transiting or eclipsing body.

Integrated magnitude

The integrated magnitude is the magnitude of a diffuse object if it were gathered up into a point like source.


The process of combining light from two or more telescopes to achieve a greater resolving power.

Interstellar medium

The matter that lies between the stars in a galaxy. It is composed of 99 per cent gas, and 90 per cent of that is hydrogen. The remaining one per cent is dust mainly made up of carbon and silicates.

Interstellar Medium (ISM)

The matter that lies between the stars of a galaxy. It is composed of 99 per cent gas and 1 per cent dust. Hydrogen makes up 90 per cent of the gas and the remainder is helium and other heavier elements. The dust is mainly made up of carbon and silicates.


A camera accessory that measures time intervals, often used to create time-lapse sequences.


An atom that has gained or lost one or more electrons, giving it an overall negative or positive electrical charge respectively.


This is when an atom is stripped of or receives electrons. It often happens in astronomy where a gaseous nebula is ionized by stars within it.

Iridium flare

A bright flash in the night sky caused by sunlight glinting off solar panels of Iridium communications satellites. See for details of when an Iridium flare will be visible from your location.

Irregular galaxy

A galaxy that doesn’t show any obvious structure or form. They don’t fall into any of the normal classes of galaxy found in the ‘Hubble Tuning Fork Diagram’.

ISO number

This is the number that indicates the sensitivity (or ‘speed’) of a film or a given DSLR exposure.


This term describes anything relating to isotopes. Isotopes are atoms of the same element that have the same number of protons, but different numbers of neutrons.