It was a radical notion when Copernicus declared that the Earth rotates around the sun. Now, it might be just as unthinkable that December marks the first day of winter.

But it depends on whose information you’re referencing.

“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,” the National Oceanic and Atmospheric Administration says.

Examining the differences between astronomical and meteorological seasons, NOAA notes, “Meteorologists and climatologists break the seasons down into groupings of three months based on the annual temperature cycle, as well as our calendar.”

So meteorological winter includes December, January and February; spring includes March, April, and May; summer is in June, July and August; and meteorological fall includes September, October and November.

Astronomical observations of the seasons based on solstices and equinoxes are determined by the Earth’s tilt and the sun’s alignment when it passes directly above the equator.

Under those circumstances, astronomical winter and the solstice occur in the Northern Hemisphere on Tuesday, Dec. 21, at 10:59 a.m. in the D.C. area. Typically, it’s on or around Dec. 22.

The vernal, or spring, equinox usually happens on or around March 21; the summer solstice falls on or around June 21; and the autumnal equinox happens on or around Sept. 22. In the Southern Hemisphere, the seasons are reversed but begin on the same dates.

Now, if you’re wondering how different planetary hemispheres affect what direction water swirls down drains? The Library of Congress disputes the notion altogether that Mother Earth has anything to do with it, explaining: “It all depends upon how the water was introduced and the geometric structure of the drain.”