What is the winter solstice?

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What is the winter solstice?

The winter solstice is the day of the year with the fewest hours of daylight. In 2009, the Northern Hemisphere winter solstice occurred on December 21.

2004 winter solstice viewed at the Lawrence Hall of Science in Berkeley, California, USA. Photo by:Tim Ereneta

As Earth orbits the Sun, its axis of rotation is tilted at an angle of 23.5 degrees from its orbital plane. Because the Earth’s axis always points in the same direction — toward the North Star — the orientation of Earth’s axis to the Sun is always changing during the orbit. That change, in turn, alters the distribution of sunlight on Earth’s surface at any given latitude.

This variation in the sun’s angle also causes some months to be warmer than others — manifesting our seasons. On winter solstice in the Northern Hemisphere, the northern portion of Earth’s axis points away from the Sun, and latitudes north of the Arctic Circle (66.5 degrees N) have 24 hours of darkness.

The winter solstice is often referred to as the first day of winter, but winter has other definitions. For example, the beginning of winter might be defined on the calendar day, when precipitation on average has an equal chance of falling as rain or snow. For Madison, that is November 15. Meteorologists often define the three months of winter as the coldest months: December, January and February.

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.