Because celestial bodies such as the sun and stars were used to tell time, so if they were obstructed by clouds it would be hard to tell time thanks to

Well considering there aren't really any clouds at night it would be easier to navigate and tell time at night now during the day clouds would be in the way of the sun at SOME times which is probably one out of many struggles our human kind had back then.

If the clouds were covering the sun whilst someone was trying to tell the time you'd just have to either wing it OR try your hardest to find the lightest parts of the clouds and assume the sun is in that position.

Explanation:

not really sure sorry just trying to get points

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