ELi5: If seasons are due to the tilt of the earth and not the orbit around the sun, how do they occur on roughly the same days every year?

Question

Is it just coincidence that the 4 season begin and end at same point in the orbit around the sun?

In: Physics

Answer 1

We’ve specifically designed the calendar so that the seasons don’t drift.

This actually was a problem in early calendars – people tried to divide the year up into lunar cycles (month comes from moon-th) but this leaves you nearly a week off of the solar cycle at the end and the error rapidly builds.

The Romans first tried a 365-day calendar that corrected this, but even that isn’t quite perfect. Under Caesar they developed the 365.25 day “Julian” calendar that corrects the drift better, and this was further refined in the middle ages into the current 365.2425 day calendar.

It takes 365.2425 days to compete an orbit around the sun and reset the seasons, and the modern “Gregorian” leap year system corrects for those dangling fractions of days.