| Tektite |
A small, glassy material formed by the impact of a large body, usually a meteor or asteroid. Tektites are commonly found at the sites of meteor craters. |
| Telescope |
An instrument that uses lenses and sometimes mirrors to collect large amounts of light from distant objects and enable direct observation and photography. A Telescope can also include any instrument designed to observe distant objects by their emissions of invisible radiation such as x-rays or radio waves. |
| Terminator |
The sunset or sunrise line on the Moon or a planet. |
| Terrestrial |
A term used to describe anything originating on the planet Earth. |
| Terrestrial Planet |
A name given to a planet composed mainly of rock and iron, similar to that of Earth. |
| Tidal Force |
The differential gravitational pull exerted on any extended body within the gravitational field of another body. |
| Tidal Heating |
Frictional heating of a satellite's interior due to flexure caused by the gravitational pull of its parent planet and/or other neighboring satellites. |
| Transit |
The passage of a celestial body across an observer's meridian; also the passage of a celestial body across the disk of a larger one. |
| Trans-Neptunian Object |
Any one of a number of celestial objects that orbit the Sun at a distance beyond the orbit of the planet Neptune. |
| Trojan |
An object orbiting in the Lagrange points of another (larger) object. This name derives from a generalization of the names of some of the largest asteroids in Jupiter's Lagrange points. Saturn's moons Helene, Calypso and Telesto are also sometimes called Trojans. |
