‘Tis the season!

When I was a kid, summer was June, July and August – the months we were out of school – but we’re often told that the official start of summer is June 21st.  What’s the deal?

Simply, there are two different methods defining the seasons.

In temperate and subpolar regions, four seasons based on the Gregorian calendar are generally recognized. The dates for the seasons depend on whether one is talking about meteorological  seasons or astronomical seasons as well as where on Earth one is.

Meteorological seasons

In 1780, an early meteorological organization, the Societas Meteorologica Palatina, defined seasons as groupings of three whole months as identified by the Gregorian calendar. This definition has been used in meteorology ever since.

Astronomical seasons

In the astronomical timing of temperate seasons, summer and winter begin at the solstices,^{1, 2} while spring and fall begin at the equinoxes.³ 

Length of seasons

The meteorological seasons are well balanced in length.  In the northern hemisphere, spring and summer are 92 days each, autumn is 91 days, and winter is 90 days, except on leap years when it is 91.

Astronomical seasons, on the other hand, are not as exact because of the elliptical nature of the Earth’s orbit.  Again for the northern hemisphere, spring is currently 92.75 days, summer is 93.65 days, autumn is 89.85 days and winter is 88.99 days.

The timing of the equinoxes and solstices are not fixed to the calendar, shifting six hours later each year, accumulating one full day every four years, when they are reset with the addition of an extra day on leap years.

‘Tis the season! – stay cool!

Endnotes

1. either the shortest day of the year (winter solstice) or the longest day of the year (summer solstice) – Collins English Dictionary (accessed May 25, 2019)
2. Either of the two times in the year, the summer solstice and the winter solstice, when the sun reaches its highest or lowest point in the sky at noon, marked by the longest and shortest days. – Oxford Living Dictionaries (accessed May 25, 2019)
3. either of the two times each year (as about March 21 and September 23) when the sun crosses the equator and day and night are everywhere on earth of approximately equal length – Merriam-Webster (accessed May 25, 2019)