Here comes the dark. The winter solstice — marking the longest night and shortest day of the year — is Monday night.

The solstice occurs at the same instant everywhere on Earth, according to earthsky.org. It happens Monday evening at 11:48 p.m. ET, 10:48 p.m. CT, 9:48 p.m. MT and 8:48 p.m. PT.

(So keep in mind that for most of the rest of the world — in Europe, Africa, Asia, and Australia — the December solstice actually comes on Tuesday Dec. 22.)

The solstice marks the precise moment at which the Northern Hemisphere is tilted as far away from the sun as it will be all year, and usually occurs near the day when there is the least amount of daylight and the most darkness.

It’s the astronomical beginning of the winter season (though meteorologists define the beginning of winter as Dec. 1, the start of the coldest three months in the Northern Hemisphere).

It’s the opposite in the Southern Hemisphere, where Dec. 21 marks the longest day of the year and the beginning of astronomical summer.

However, the shortest day of the year is typically not the coldest day of the year. There is bit of a lag between the shortest day of the year and 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 across the country, the minimum daily temperature occurs around 2-3 weeks later, in early to mid-January.

For example, the coldest days in Boston, on average, are Jan. 17 – 26. In Chicago, it’s Jan. 17 – 20, and in Miami, it’s Jan. 2 – 22. At the end of January, more heat finally begins arriving than leaving, and days slowly start to warm up.

The Earth’s tilted axis is the reason we have seasons. During the Northern Hemisphere 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.

And why is the Earth tilted? It’s probably the result of collisions with various proto-planets and other massive objects during the formation of the solar system billions of years ago, according to NASA. Just a bit unsettling to realize that the reason the Earth has the perfect temperature for life to form is a few random collisions with other space rocks a few eons ago.

In the meantime, happy solstice!

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