An observer on Earth sees both real and apparent movement of objects in the sky, which results in their view of the sky changing constantly.
This is the verb to turn combined with the ideograph for sky. Therefore, the symbol is an ideograph for the verb to change.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-1909 A (NB. Image A is a mirror of the image that would appear on a sealing.): Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and
Inscriptions: Volume 3,1: Page 76: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.