Question: Why does a year have 365 days and a day have 24 hours?

Thomas Biggans answered on 20 Jun 2016:

We base a year on how long it takes for the Earth to go around the sun in a circle (more of an oval). So on the 1st January next year the Earth will be at the same point around the sun as it was on the 1st January this year. It takes 365.25 days to do this so it was decided that a year would be 365 days but every four years we would have a leap year with an extra day to make up for the 0.25 because 4 x 0.25 gives us 1 day.

As for 24 hours in a day we have the Egyptians to thank for that. They decide to split the day time into 10 hours based on the sun and the shadow it cast on their clocks. They added an hour to count for the time where the sun was rising so it wasn’t quite day light but it wasn’t night time anymore and an hour for the sun setting where again it wasn’t quite day but it wasn’t quite night either. They came up with 12 hours to describe night time based on the stars. So all together we have 24 hours although the length of their hours tended to vary from season to season. We decided to define a day as the time it took for the earth to rotate around once and divided it equally over the previously used 24 hours.

Nowadays we base time on atomic clocks, we define a second based on the decay of a caesium atom and work our way up. But the Earth’s rotation is slowing down so there comes a point where our clocks finish the day one second earlier than the Earth does because of this we add a leap second to our clock so the Earth can catch up again. Who would’ve thought keeping time would get so tricky?