Question: How were leap years discovered ?

Zena Hadjivasiliou answered on 15 Mar 2015:

The idea of days, months and years as a measure of time did not always exist! Humans decided to measure time in that way so that the way in which we understand time matches seasons and astronomical events. For example, a day is the time it takes the earth to rotate around itself. Where in this rotation the earth is, will determine what time of the day it is. In a similar way, it takes the earth about a year to rotate around the sun. And where in its rotation around the sun the earth is, determines what season it is!

Only I lied to you a little bit just now :/. It really takes the earth 23 hours and 56 minutes to rotate around itself (not 24 hours!). And it takes the earth NOT 365 days to rotate around the sun (the number of days in a normal year) but 365 days , 6 hours , 45 minutes and 48 seconds! When people realised that there was a little bit of error in their calculation they had to do something to make sure the way in which we define days and years stay in sync with the astronomical phenomena.

An easy way to fix this was to add an extra day to the year every four years so that the earth would come back into sync with the sun and its own rotation. So this is how humans came up with leap years!