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I was trying to sleep but instead I'm wondering if the definition of a day means 24h between the sun being at its highest point in the sky, or 24h for a full rotation of the earth.
Because every day the earth has moved on its orbit so it has a slightly different angle around the sun and I think that makes a 4min difference 
I thoughts human timezones and stuff were making time complicated, but actually it's already super messy by itself.
Good morning
I'm looking up how the day duration works. So it seems I was right!
The day based on the rotation of the earth is called sidereal time (https://en.wikipedia.org/wiki/Sidereal_time) and is about 23 h 56 min long (I learned in passing that the rotation duration fluctuates every day, on the scale of milliseconds).
And the one based on the sun is the apparent solar time (https://en.wikipedia.org/wiki/Solar_time). It lasts up to 15min more or less depending on the season, but in average it's 24h.