The winter solstice (In Latin, sol, “Sun,” and stice, “come to a stop”) is the day of the year with the fewest hours of daylight. In 2015, this occurs for the Northern Hemisphere on Dec. 21 at 10:48 p.m. CST.
As Earth orbits the sun, its axis of rotation is tilted at an angle of 23.5 degrees from its orbital plane. Because Earth’s axis of spin always points in the same direction — toward the North Star — the orientation of Earth’s axis to the Sun is always changing as Earth orbits around the Sun.
As this orientation changes throughout the year, so does the distribution of sunlight on Earth’s surface at any given latitude. This links the amount of solar energy reaching a location to the time of year and causes some months of the year to always be warmer than others — in other words, the seasons.
On the Northern Hemisphere’s winter solstice, the northern spin axis is pointed away from the Sun and latitudes north of the Arctic Circle (66.5°N) have 24 hours of darkness.
The winter solstice is often referred to as the first day of winter. But there are other definitions of winter.
For example, the beginning of winter might be defined on the calendar day, on average, when precipitation has an equal chance of falling as rain or snow. For Madison, that calendar day is Nov. 15.
Meteorologists often define the three months of winter as December, January and February — the coldest months of the year.