Astronomy dictionary – T

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.



A thin boundary that separates the Sun’s outermost layer, called the convective zone, from the denser internal regions. These layers rotate at different speeds to create the Sun’s magnetic activity.

Termination shock

This is where the solar wind confronts the interstellar wind and is slowed down considerably. The sudden deceleration of the gas causes it to become denser and to heat up.


The line on a planet or moon between the sunlit side, and the dark side.

Transient Lunar Phenomena

These are colour changes and brightenings on the Moon’s surface, reported by some lunar observers. Today TLPs also include the flashes produced when meteoroids impact the Moon, which have been recorded many times by CCD cameras.


A transit is when a celestial body passes in front of another body that is of a greater apparent diameter than itself. This could be, for example, one of Jupiter’s moons passing in front of Jupiter’s disc.

Transit method

A way of searching for exoplanets by looking for the telltale dip in brightness of a star as a planet passes in front of it.

Tube rings

Rings that attach to a telescope’s mounting plate (see dovetail plate) and hold the tube assembly in place.

Type Ia supernova

A supernova explosion that occurs when a white dwarf accretes enough matter from a companion star to exceed the Chandrasekhar limit.