In old days (primary school) all was simple. A year had 365 days in which the earth rounded the sun.
After some years we learned about leap years and later about the earth being titled over its axis. Now learn about an elliptical trajectory where one hemisphere will have a cold winter where the other will have a mild winter.
Just when we think that all is defined in this way other and again other variables are added. For instance what caused the ice ages or how come that the pole star referred to other stars in history?
See this video to get enlightened or more confused……