Which determines how many hours in a day?

Answer 1

The time it takes the earth to rotate on its own axis so that the sun is again in the same position determined the length of a day. It is then up to us to divide that period up. In olden days a day was split into hours of different length! We now have 24 hours in a day but there is no reason why we could not have a day consisting of 48 "semihours" - each lasting half as long, or 2.4 "decahours" each of 600 minutes or anything else.

Apart from the fact that it has now become an international standard so that changing it would be rather complicated, 24 has the advantage that it is divisible by 2, 3, 4, 6, 8, 12.