- The winter solstice – which marks the beginning of astronomical winter in the Northern Hemisphere – is Tuesday.
- The solstice occurs at the same instant everywhere on Earth.
- The Earth's tilted axis causes the seasons.
Welcome back, winter.
The winter solstice – which marks the beginning of astronomical winter in the Northern Hemisphere – is Tuesday. It's the precise moment at which the Northern Hemisphere is tilted farthest from the sun.
The solstice occurs at the same instant everywhere on Earth: Here in the U.S., it happens at 10:59 a.m. EST on Tuesday, Dec. 21.
At that moment, the sun's rays will be directly over the Tropic of Capricorn, a line of latitude that circles the Earth in the Southern Hemisphere.
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Though the solstice marks the astronomical beginning of winter, meteorologists view the first day of winter as Dec. 1, which is the start of the coldest three months in the Northern Hemisphere.
Most locations don't have their earliest sunset or latest sunrise on the solstice. Those events occur either weeks before or after Dec. 21.
The date of the winter solstice varies from year to year and can fall anywhere from Dec. 20-23, but the 21st and 22nd are the most common dates.
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The solstice is typically not the coldest day of the year. There's a bit of a lag between the solstice and the day with the coldest average temperatures for most spots in the USA.
This lag in temperature occurs because even though the amount of daylight is increasing, the Earth's surface continues to lose more heat than it receives from the sun. In most locations, the minimum daily temperature occurs around two or three weeks later, in early to mid-January.
For example, the coldest days in Boston, on average, are Jan. 17-26. In Chicago, they're Jan. 17-20, and in Miami, Jan. 2-22, according to the National Weather Service. By the end of January, more heat finally begins arriving than leaving, and days slowly start to warm up.
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The Earth's tilted axis causes the seasons. During the Northern Hemisphere's winter, the land north of the equator is tilted away from the sun, which lowers the amount of the sun's energy warming the Northern Hemisphere.
Why is the Earth tilted on its axis? It's probably the result of collisions with various protoplanets and other massive objects during the formation of the solar system billions of years ago, according to NASA.
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