The first day of spring is Monday, March 20, 2023, at 5:24 p.m. EDT. For those of us in the Northern Hemisphere, this was marked by the arrival of the Vernal Equinox (otherwise known as the First Point of Aries). Vernal translates to new and fresh and equinox derived from the Latin aequus (equal) and nox (night).
Every year, weather forecasters welcome the arrival of spring on the first day of March—while others contend that the spring really begins a few weeks later with the equinox which falls on or around March 21. So who is right about when the seasons begin and end?
It depends on why you’re asking. Seasons are defined in two ways: astronomical seasons which are based on Earth’s position as it rotates around the sun and meteorological seasons which are based on annual temperature cycles. Both divide the year into spring, summer, fall, and winter—yet with slightly different start and end dates for each. Here’s what they mean and how to tell them apart.
Astronomical seasons
People have used observable periodic natural phenomena to mark time for thousands of years. 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. Earth’s tilt and the sun’s alignment over the equator determine both the solstices and equinoxes.
Ancient Rome was the first to officially mark those seasons with the introduction of the Julian calendar. Back then, the seasons began on different days than the modern era because of discrepancies with the Gregorian calendar used primarily today. Now, the start of each astronomical season is marked by either an equinox or a solstice.
The equinoxes mark the times when the sun passes directly above the equator. 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. These seasons are reversed but begin on the same dates in the Southern Hemisphere.
Solstices mark the brightest and darkest days of the year. They are also driven by Earth’s tilt and mark the beginning of astronomical summer and winter. When the Northern Hemisphere is tilted toward the sun, it is brighter and feels like summer while at the same time the Southern Hemisphere is tilted away from the sun, plunging it into a dark winter.
Because 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. This also causes the exact date of the solstices and equinoxes to vary. 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. These variations in season length and season start would make it very difficult to consistently compare climatological statistics for a particular season from one year to the next. Thus, the meteorological seasons were born.
Meteorological seasons
Since at least the 18th century, scientists have sought better methods of predicting growing seasons and other weather phenomena. Over time, that gave rise to the concept of meteorological seasons which are more closely aligned with both annual temperatures and the civil calendar.
Meteorological seasons are far simpler than astronomical seasons. They divide the calendar year into four seasons that each last exactly three months and are based on the annual temperature cycle. Winter takes place during the coldest three months of the year, summer in the hottest three months, and spring and fall mark the remaining transition months.
In the Northern Hemisphere that means the start date for each season is March 1 (spring), June 1 (summer), September 1 (fall), and December 1 (winter). In the Southern Hemisphere the seasons are reversed; spring begins in September, summer in December, fall in March, and winter in June.
Meteorological observing and forecasting led to the creation of these seasons and they are more closely tied to our monthly civil calendar than the astronomical seasons are. The length of the meteorological seasons is also more consistent ranging from 90 days for winter of a non-leap year to 92 days for spring and summer. By following the civil calendar and having less variation in season length and season start, it becomes much easier to calculate seasonal statistics from the monthly statistics, both of which are very useful for agriculture, commerce, and a variety of other purposes.
The consistency of meteorological seasons allows meteorologists to make the complex statistical calculations necessary to make predictions and compare seasons to one another. “Dealing with whole-month chunks of data rather than fractions of months was more economical and made more sense,” climatologist Derek Arndt told the Washington Post in 2014. “We organize our lives more around months than astronomical seasons, so our information follows suit.”
So when is the first day of spring? It isn’t March 1 or the spring equinox—it’s both.
But, if you live in Canada or the northern states you may argue, as I do, that we need a third means of determining the first day of spring. I suggest that spring really begins with the appearance of the first tiny leaves on the trees or the first crocus plants peeping through the snow? The First Leaf and First Bloom Indices are synthetic measures of these early season events in plants, based on recent temperature conditions. These models allow us to track the progression of spring onset across the country.
Take a look outside. If the birds are chirping, tree leaves are budding, the grass is turning green again—then it is spring!
>> DIG DEEPER
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Worth Pondering…
She turned to the sunlight
And shook her yellow head,
And whispered to her neighbor: Winter is dead.
—A.A. Milne, When We Were Very Young