While many of us are looking forward to the official start of fall Saturday, meteorological fall has already begun. So what determines the difference between the two?
Astronomical fall is marked by the Autumnal Equinox - when the sun’s rays cross south of the equator. It’s referred to as “equinox” because of the nearly equal length of day to night. Most of us consider this to be the official start of fall since we’ve used the astronomical calendar to mark our seasons for thousands of years.
Earth’s spin and 23.5 tilt determine the amount of sunlight received as it orbits the sun each year. Our seasons are marked by two equinoxes and two solstices annually: the winter solstice around December 21, Spring Equinox around March 21 and Summer Solstice around June 21.
The exact date of the season starts varies due to the Leap Year and the elliptical shape of Earth’s orbit. This causes the season length to vary from 89 to 93 days....which explains why some years the season starts on the 20th, 21st, 22nd, 23rd or 24th.
Although Astronomical Fall starts September 22 this year, Meteorological Fall starts on the first of September and goes through the end of November. Meteorological seasons are divided into four 3-month periods based off the annual temperature cycles – basically when the temperatures begin to change. The advantage of meteorological seasons is they are more consistent on a 90 to 92-day cycle (with the exception of leap years). This makes it easier to calculate seasonal statistics for agriculture and gather climate data.
Meteorological winter starts December 1st and goes through the end of February. Spring starts in March and goes through May...Summer: June through August. Meteorological seasons have the same start months as astronomical seasons, just different start dates.