While the first day of fall is marked on the calendar as Friday, Sept. 22, for meteorologists across the world it’s already been the fall season for the past three weeks. There are two types of seasonal cycles, astronomical and meteorological.

The astronomical cycle is what is marked on the calendar, where each season begins on the solstices and equinoxes, which all depends on the Earth’s orbit around the sun. The summer and winter solstices being when the sun is directly over the poles. The spring and fall equinoxes being when the sun is directly over the equator.

The meteorological cycle is based on climate and temperatures, where each season is broken into three-month periods. Winter is considered the coldest three-month period, summer the warmest, and fall and spring the transition periods. Each period starts at the top of the month, spring on March 1, summer on June 1, fall on Sept. 1 and winter on Dec. 1.

But why not just use one seasonal cycle?

As stated above, the astronomical cycle is based on the Earth’s orbit around the sun. Since its orbit is not exactly 365 days, but 365.24 days, the solstices and equinoxes are offset every few years. So to make it easier to keep consistent weather and climatological data, the meteorological seasonal cycle was born!