Thus, the meteorological seasons were born. These variations in season length and start date would make it very difficult to consistently compare climatological statistics for a particular season from one year to the next. Additionally, the elliptical shape of Earth’s orbit around the sun causes the lengths of the astronomical seasons to vary between 89 and 93 days. This also causes the exact date of the solstices and equinoxes to vary. These seasons are reversed but begin on the same dates in the Southern Hemisphere.īecause Earth actually travels around the sun in 365.24 days, an extra day is needed every fourth year, creating what we know as Leap Year. In the Northern Hemisphere, the summer solstice falls on or around June 21, the winter solstice on or around December 22, the vernal or spring equinox on or around March 21, and the autumnal equinox on or around September 22. The equinoxes mark the times when the sun passes directly above the equator. Earth’s tilt and the sun’s alignment over the equator determine both the solstices and equinoxes. The natural rotation of Earth around the sun forms the basis for the astronomical calendar, in which we define seasons with two solstices and two equinoxes. People have used observable periodic natural phenomena to mark time for thousands of years.
So, why do meteorological and astronomical seasons begin and end on different dates? In short, it’s because the astronomical seasons are based on the position of Earth in relation to the sun, whereas the meteorological seasons are based on the annual temperature cycle.
You may have noticed that meteorologists and climatologists define seasons differently from “regular” or astronomical spring, summer, fall, and winter.
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