We are in the second week of September and the heatwave in the US is still ongoing. How can we still have a summer heatwave, you might wonder, if it’s fall? Labor Day in the US marks for many the end of summer, some call September 1 the start of fall, while others will point out that September 22 marks the autumnal equinox, therefore marking the change of seasons.
Which begs the question: What really marks the start of the seasons? The answer is really not that complicated but slightly nerdy, so just bare with me.
Seasons happen because the earth is tilted on its axis at currently 23.4°. As the earth rotates around the sun in 365 days, this tilt affects the amount of sunlight each hemisphere receives. The equinoxes mark the times when both hemispheres receive the same amount: 12 hours each. Likewise, the June summer solstice marks when the Northern Hemisphere tilts the most towards the sun and the December winter solstice marks when it tilts the most away from the sun. The opposite seasons apply of course for the Southern Hemisphere.
Fun fact: The earth’s axis is not fixed, over the last million years it has oscillated between 22.1° and 24.5° on a 41,000 year cycle. The greater the tilt is, the more extreme our seasons are. Currently, the earth’s tilt is decreasing and will reach its minimum in 9,800 years.
The elliptical shape of Earth’s orbit around the sun causes the lengths of the astronomical seasons to vary between 89 and 93 days. In addition, the earth travels around the sun in exactly 365.24 days, which is why every four years we add a day to February to adjust for this. All this however causes the dates of equinoxes and solstices to vary from between the 19th to the 23rd of the month. This makes it difficult for meteorological statisticians to compare across the years, therefore meteorologists decided to use the first of the month to simplify comparisons.
So really, blame the statisticians for the confusion.
It’s Equinox in 1 hour! 22 Sep 2020, 13:30 UTC. Today everywhere has an almost** equal day and night length of 12 hours, while sunlight is at max intensity on the equator. Earth has seasons because of its axial tilt, which leads to day/night length changes throughout the year pic.twitter.com/L3pxQNFYnK
— Dr James O’Donoghue (@physicsJ) September 22, 2020