How can there be a day in the universe. Days are measured on each planet differently depending on their rotati

Answer 1

We use ourselves as a reference, so whenever you here "Day", it's still referring to the length of a day on Earth, which is 24 hours.

Likewise, a lightyear would have no meaning to an alien culture because it's a constant rate (the speed of light) multiplied by an arbitrary amount of time, which is our year, which only has meaning for Earthlings. But what else are you going to use then? Everything is arbitrary in one form or another. 360 degrees in a circle, the length of a mile, even our measurement of a second is arbitrary.

Answer 2

Time is a man made concept, we had to invent it in order to measure speed and distance of motion. Even on Earth any two clocks that are spaced more than one thousand miles apart read different times. Synchronized clocks placed on four different planets will eventually read different times because they are affected by the different speeds of motion of the planets. Motion is the pulse of the universe.

Answer 3

You are correct. The length of one day depends entirely on the rotational characteristics of the planet you're on at the time.

However, since most people are not astronomers, the generally accepted meaning of "day" is 24 hours.