update: I consulted my copy of 'The Sleepwalkers' again, and I see that Martianus Capella is listed twice in the index.
Koestler claims that Martianus Capella's system, wherein Venus and Mercury orbit around the Sun, while the rest of the planets orbit around the Earth, is originally due to Herakleides (https://en.wikipedia.org/wiki/Heraclides_Ponticus, not to be confused with Heraclitus), a 4th century BC Greek philosopher. But I see that wikipedia links to an article [1] by Eastwood from 1992 that claims this to be a complete misattribution lacking any evidence. Link for the interested: https://adsabs.harvard.edu/pdf/1992JHA....23..233E.
[1] Eastwood, B.S., 1992. Heraclides and heliocentrism: Texts, diagrams, and interpretations. Journal for the History of Astronomy, 23(4), pp.233-260.
Koestler claims that Martianus Capella's system, wherein Venus and Mercury orbit around the Sun, while the rest of the planets orbit around the Earth, is originally due to Herakleides (https://en.wikipedia.org/wiki/Heraclides_Ponticus, not to be confused with Heraclitus), a 4th century BC Greek philosopher. But I see that wikipedia links to an article [1] by Eastwood from 1992 that claims this to be a complete misattribution lacking any evidence. Link for the interested: https://adsabs.harvard.edu/pdf/1992JHA....23..233E.
[1] Eastwood, B.S., 1992. Heraclides and heliocentrism: Texts, diagrams, and interpretations. Journal for the History of Astronomy, 23(4), pp.233-260.