Pythagoras at the same Time uses the Terms of Music, by calling the Space between the Earth and the Moon a Tone; saying, that from her to Mercury is Half a Tone: and from him to Venus about the same Space. But from her to the Sun so much and a Half more: but from the Sun to Mars a Tone, that is to say, as much as from the Earth to the Moon.

Pliny's Natural History. In Thirty-seven Books, Book II, Chapter XXII (p. 59), Printed for the Club by G. Barclay. 1847-1849