A calendar year is 365 days long – the number of days it takes the Earth to complete one orbit around the Sun. But 365 is actually a rounded number; it takes Earth 365.242190 days to orbit the Sun, according to the National Air and Space Museum. That extra five hours, 48 minutes and 56 seconds needs to be accounted for somehow. Otherwise after about 700 years our summers in the northern hemisphere would be in December.
Extra day recognized long ago
The sun is captured in a "starburst" mode over Earth's horizon by one of the Expedition 36 crew members aboard the International Space Station as the orbital outpost was above a point in southwestern Minnesota.





