Gemini

Gemini was a constellation visible from Earth. It included the stars Castor and Pollux. Once Ian Chesterton showed it to Vicki Pallister. (PROSE: Byzantium!)

Astrology
In 1746, in Inverness, the Second Doctor mentioned it while pretending to use astrologic criteria as a physician. (TV: The Highlanders)

In 2009, the "Children of Gemini" were the first to be brought under the control of the Ancient Lights when Martin Trueman invoked the sign into his circle. Among the Children of Gemini were Clyde Langer and Gita Chandra. Gemini was followed by Cancer. (TV: Secrets of the Stars)