| by Olivia B. Waxman

As the winter solstice approached, TIME turned to scientists, including Charles A. Dana Professor of Science, Technology, and Society James R. Fleming, to point out that “the idea of the first day of winter is more complicated than you may think.” Meteorological winter differs from astronomical winter because “the alignment of the Earth’s axis doesn’t line up with traditional weather, or the common-sense idea of weather,” Fleming told TIME Dec. 20. While no one knows exactly when humans began marking the solstice, meteorological winter’s roots began “in the early-to-mid 20th century when the need for the statisticians to generate seasonal averages” grew, dating back to “prominent Austrian meteorologist Julius von Hann, who, in 1897, stated that climate is sum of the average weather conditions,” Fleming said.