Why does the first day of spring change?

March 20, 2018 officially marked the beginning of spring this year, but that date will change when we hit 2020. The March equinox, or vernal equinox, can occur on March 19th, 20th or 21st, depending on two factors: timing and location.

“The bottom line is our calendar system is out of sync with the sky…the sky doesn’t work on a 365 day basis,” Dr. Robert Cockcroft, assistant professor in the department of physics and astronomy at the University of Western Ontario said. “So we’re either off with the timing or we’re off with the location, one of those two things causes the slight change that we have from year to year.”

Time

As time passes, we continue to make small corrections for the inclusion or removal of a leap day in our calendar. If the year is a multiple of four then we have a leap year but on multiples of 100, we don’t have a leap year. If the year is a multiple of 400, it is in fact a leap year.

“With the timing, we have to correct that we go in and out of sync with leap years,” Cockcroft said. “What we’re doing when we make those corrections by either inserting or not inserting the leap day in February, is we’re kind of overcompensating for the shift.”

Side view of blooming tulips on a sunny spring day in Newfoundland/Getty Images

Location

In addition to timing, different locations on earth can change the date of this equinox, in addition to the position of the earth.

“Our orbit is not perfectly circular, it’s what we call elliptical orbit,” Cockcroft said. “When our elliptical orbit is taking us slightly closer to the sun, then we travel a little bit faster, and when we’re further from the sun… we’re traveling a little bit slower, and those faster and slower speeds are compared to the speeds we would be traveling if we were traveling in a perfect circle.”

The axis that the earth spins on is moving as well, causing the position to change over a period of 26,000 years. This spinning, which Cockcroft says is similar to a spinning top, allows us to define the exact moment of the vernal equinox.

“It’s when the sun’s path across the sky looks across the celestial equator, which is the projection of the earth’s equator into the sky,” Cockcroft said. “When the sun crosses that equator line, that’s the exact moment when the vernal equinox occurs.”

According to Scott Sutherland, meteorologist and science writer with The Weather Network, in the year 2020 the first day of spring will shift back to March 19th,  according to eastern standard time.

Meteorological seasons

Although the March equinox marks the beginning of spring from an astronomical perspective, meteorology has a different schedule for tracking seasons. Meteorological spring begins on March 1st, similarly summer, fall and winter start on the first day of June, September and December respectively.

These dates are more in line with climatological factors that influence weather patterns.

“Climatologically we find it easier to set that on a specific day,” Sutherland said. “If you took those dates and drew all those temperatures on a graph, it would vary here and there of course, but the general curve would bottom out right about the middle of that period of time.”

The consistent seasonal dates also makes it easier for meteorologists to keep track of forecasts on a year-over-year basis, but don’t think that the seasonal forecasts released by weather experts necessarily align with the meteorological beginning of each season.

“It would generally start around the astronomical season, even though our climatological records are recorded that way for the meteorological season,” Sutherland said. “When we talk about the forecast, we tend to try to align it to what people expect of the date…so we present that earlier but we don’t necessarily start it on the 1st.”