# Do we use a fixed clock interval or the Sun to define the concept of "day"?

Follow up on the Earth stopping rotation question.

What if instead of stopping, the Earth simply slowed down or sped up its rotation rate (as it has, historically)?

Do we halachically count a single calendar day as passing using A) a fixed length of time, e.g. 24 hours or B) using the Sun's apparent position in a certain location [e.g. Jerusalem] to make a full cycle. or C) some other possibility.

To illustrate, let's say the Earth starts rotating 2x as fast. If A is true, then we'd still be counting seven 24-hour periods between one Shabbos and the next, but we'd have 14 sunsets during that time. If B is true, then from one Shabbos to the next would be seven 12-hour periods.

• I'm voting to close this question as off-topic because this type of question is a type of "riddle" that would involve fantasy analysis and is beyond our scope.
– DanF
Aug 8, 2019 at 1:07
• Not sure what you're asking, since we see that every person on every day in every location slices his halachic day into 2 sets of 12 hours. In Israel this hour can be as short as 50 minutes on your watch, in winter or as long as 70 minutes on your watch in summer. Aug 8, 2019 at 9:58
• @DanF Since when are fantastical questions off-topic? I’m voting to close as unclear, since none of these are actually how Day is determined. Aug 8, 2019 at 15:16
• @DanF@DonielF I've modified the question to make it more clear Aug 8, 2019 at 17:10
• @DanF No fantasy analysis required. The earth's rotation is indeed continuously slowing down. Aug 8, 2019 at 18:37