Crossword Genius

Period of global revolution? (3)

Ross

I believe the answer is:

day

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'period' is the definition.
(I know that day is a type of calendar day)

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for day that I've seen before include "Doris -, US actress" , "24 hours" , "THIRD PART OF 23" , "24 hour period" , "24 23 acrosses" .)

I've seen this clue in The Independent.
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