September is here! And according to the calendar and the Farmers’ Almanac, we still have a few weeks of summer left to enjoy, as fall doesn’t officially start until mid-September with the arrival of the autumnal equinox. But in meteorological circles, September 1st marks the first day of fall. So why the difference?
What’s the difference between “meteorological fall” and “astronomical fall”?
Meteorologists divide the year up into seasons a bit differently than our calendars do. According to the meteorological calendar, the 4 seasons occur as follows:
- Fall – Begins September 1 (September, October, November); ends November 30
- Winter – Begins December 1 (December, January, February); ends on the last day in February
- Spring – Begins March 1 (March, April, May); ends May 31
- Summer – Begins June 1 (June, July, August); ends August 31
The reason is that meteorological seasons are based on the annual temperature cycle. So, summer, for example, would run June through August, which makes perfect sense. Those are the months we most associate with “summer” and “summer temperatures.” Astronomers, on the other hand, determine the seasons by the Earth’s tilt. While it may seem that the seasons are caused by Earth’s changing distance from the Sun, it’s really due to the tilt of the Earth’s axis. This tilt—a 23-degree slant—enables the Sun to appear above the horizon for different lengths of time during the various seasons. The tilt determines whether the Sun’s rays strike at a low angle or more directly onto Earth.
- Fall begins with the autumnal equinox. At this point, the Earth’s tilt is moving away from its maximum lean toward the Sun; its rays are aiming directly at the equator.
- Winter begins with the winter solstice. The Earth tilts away from the Sun, and the Sun’s rays are aiming directly at southern latitudes.
- Spring begins with the vernal equinox. At this point, the Earth moves from its maximum lean away from the Sun to a point that’s equal distance from the Sun; the Sun’s rays are aiming directly at the equator.
- Summer begins with the summer solstice. The Earth is tilting its farthest toward the Sun, and its rays are aiming directly at northern latitudes.
In short, the fall you are familiar with, which starts on the date of the autumnal equinox as listed on your calendar, is “astronomical.” But for some, fall starts September 1.
What do you think: would you rather follow the meteorological seasons, with fall starting September 1? Share your thoughts in the comments below!
Caleb Weatherbee is the official forecaster for the Farmers' Almanac. His name is actually a pseudonym that has been passed down through generations of Almanac prognosticators and has been used to conceal the true identity of the men and women behind our predictions.