Sky

A sky was the visible part of a planet's atmosphere.

Earth
In 2009, Earth was one of 27 planets which were transported to the Medusa Cascade by the New Dalek Empire before being returned by the Children of Time. During this brief period, the other 26 planets were visible in the sky. (TV: The Stolen Earth/Journey's End) The next year, however, Amy Pond failed to remember this sight, (TV: Victory of the Daleks) which the Eleventh Doctor later attributed to cracks in time which erased the memory of events. (TV: Flesh and Stone)

Others
The sky of Gallifrey was burnt orange at night. (TV: "A Desperate Venture")

The sky of Quinnis was copper-coloured. (AUDIO: Quinnis)

The sky of an Earth-like planet was green, (PROSE: The Sour Note) as was the sky of Arcopolis. (PROSE: The Eyeless)

The sky of an Earth-like planet was purple. (PROSE: Follow the Phantoms)

Other references
By the year 5,000,000,000, mankind was said to have "touched every star in the sky." (TV: The End of the World)