Winter is coming ... but when? A look at the astronomical winter v meteorological winter.

Did you know that winter begins on two different dates? It depends who you ask.

The winter solstice is often classified as the season's first official day. But that isn't true, according to some scientists.

Meteorologists and climatologists define seasons differently from the "regular" cycle of equinoxes and solstices, the National Centers for Environmental Information reports. The system we know is called "astronomical seasons."

There is, however, another basis, resulting in "meteorological seasons." Here's what to know about the two:

Astronomical winter v. meteorological winter, explained

"Astronomical seasons" are defined by the Earth's position in relation to the sun, while "meteorological seasons" are dictated by temperature, the National Centers for Environmental Information reports.

Meteorological winter begins on Dec. 1, while astronomical winter starts on Dec. 21.

The Earth is tilted approximately 23.5 degrees on its axis. As a result, there are days when our planet faces toward the sun, and others when it faces away. The tilt and "the sun's alignment over the equator" are what shape the equinoxes and solstices, according to National Centers for Environmental Information.

The Earth's orbit is also elliptical, meaning it can be closer or farther to the sun, depending on where the planet is in its 365.24 day cycle. This shapes when the astronomical seasons fall. Each astronomical season varies in length from 89 to 93 days based on these factors.

Meteorological seasons were created as a result of the variation in season length.

Meteorological seasons are more consistent in comparison, since they always begin on the first of certain months. This season system is broken down into three-month groupings:

• Winter: December, January and February

• Spring: March, April and May

• Summer: June, July and August

• Fall: September, October and November

The annual temperature cycle is responsible for the classification of meteorological seasons. This allows for more accurate observing and forecasting. "Average temperatures are colder in December, January, and February rather than January, February, and March," the Mount Washington Observatory reports.

Temperatures in March could be warmer since the Earth is further along in its orbit, bringing the planet closer to the sun. This could skew seasonal statistics, making average temperatures higher.

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When is the first day of winter 2024?

In the Northern Hemisphere, the winter solstice will occur on Saturday, Dec. 21 at 4:21 a.m. EST.

What happens on the winter solstice? A guide to the shortest day of the year

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This article originally appeared on USA TODAY: Astronomical winter v meteorological winter: How each system works