Tails of comets;Dissipating gases of a comet when it is close to the Sun. The solar wind excites the particles, which begin to emit light.
Terminator;Boundary line between the illuminated and the dark side of a planet or Moon.
Theory of relativity;Theory promoted by Einstein, stating that there is no absolute frame of reference for motion in space.
Tides;Periodic changes in the shape of a planet, moon or star caused by gravitational forces of a nearby body.
Transit;Passage of a small celestial body across the disc of a larger one.
Trojans;Group of planetoids located at two of the Lagrange points of the Saturn orbit.
Ultraviolet light;Section of the spectrum adjacent to the blue visible end of the scale.
Universe;Term describing everything that exists in space, including all matter.