Winter solstice: 7 things you should know about the shortest day of the year

There are sunnier days ahead! Say goodbye to shorter days, and ever so slowly start to welcome back more sun as the winter solstice arrives on Thursday.

The winter solstice is the shortest day and longest night of the year in the Northern Hemisphere, while the Southern Hemisphere experiences just the opposite.

CBS News asked University of Massachusetts astronomer Stephen Schneider to explain the astronomical cause of the solstice, and he answered seven basic questions about the phenomenon that everyone should know.

What is the winter solstice, and why do we have it?

The winter solstice occurs at the moment the Earth's tilt away from the sun is at a maximum.

"Earth's axis is tilted 23.5 degrees relative to its orbit, and on December 21, Earth will be at the point in its orbit when the North Pole is tilted at its maximum away from the sun," Schneider told CBS News. "The effect of this in the Arctic (within 23.5 degrees of the North Pole) is completely in darkness as Earth spins that day." 

For those of us in the Northern Hemisphere, it's the day with the shortest amount of daylight — less than 12 hours — and our longest night of the year. However, that's not the case for everyone. While it's winter for those of us in the Northern Hemisphere, people in the Southern Hemisphere experience it as their summer solstice with the longest stretch of daylight.

When does the winter solstice occur? 

The Earth's North Pole will be tilted farthest from the sun at 11:27 a.m. ET on Thursday, Dec. 21, 2017, according to the National Weather Service. For a complete listing of the dates of the winter and summer solstices and the spring and fall equinoxes through 2025, check out this calendar from the U.S. Naval Observatory.

Why isn't the winter solstice on the same day each year?

The date of the winter solstice varies from year to year for people in different time zones. It typically occurs around Dec. 21 or 22, though on rare occasions it can be as early as Dec. 20 or as late as Dec. 23, according to the Weather Channel.

That's because our calendars aren't a precise match to the solar year. 

Schneider writes: "Earth takes about 365 1/4 days to orbit the Sun. Next year, the moment of the solstice will be about 6 hours later at 5:23 p.m. ET. In 2019, it will be at 11:19 p.m. ET, so for people living in Puerto Rico and other time zones to the east, the date of the solstice will be on the 22nd."

Is the winter solstice the coldest day of the year?

The winter solstice is not normally the coldest day of the year. There is actually a lag between the shortest day of the year and the coldest average temperatures, the National Weather Service reports.

"The coldest time usually comes a month or two later because even though the amount of solar heating is beginning to increase, it isn't yet enough to reverse the cooling," Schneider says. "This effect, called 'the lag of the seasons,' is similar to how when you turn down the heat while you're cooking — the pot won't immediately reach its coolest temperature."

Will shadows on the day of the solstice be longer?

Noontime shadows on Thursday will be longest for those of us in the Northern Hemisphere because the sun doesn't rise as high in the sky. The closer you are to the Arctic, the longer the shadows (and the shorter your day) will be. 

"If you live 23.5 degree south of the equator (the Tropic of Capricorn), the sun will pass straight overhead at noon on December 21, and people south of that will have their shortest noontime shadows of the year," Schneider says.

Why doesn't the earliest sunset come on the shortest day?

The winter solstice may be the shortest day of the year, but that doesn't mean you'll have an early sunset. The exact date of the earliest sunset depends on your latitude, so it's not the same for everyone.

If you live in the southernmost U.S., your earliest sunsets are in late November, according to EarthSky.org. If you're farther north, your earliest sunsets are around December 7.

"We're actually closer to the sun this time of year (also a surprise to many people) and moving faster in our orbit, so the Earth has to turn a little extra to face the sun each day compared to other times of year," Schneider says. "The extra 30 seconds of a solar day makes true solar noon — and sunrise and sunset — about 30 seconds later each day."

What does all of this have to do with Stonehenge?

For more than 4,000 years, Stonehenge has stood like a Neolithic sundial, marking the longest day of the year and the shortest — taking aim at the sun like a giant stone gunsight.

Every year, people flock to the ancient monument in Wiltshire, England, to watch the sunrise during the winter solstice, when the structure is directly aligned toward the sun.

"For thousands of years people built structures to track the changing position of the sun, and of course the solstices were some of the most notable dates because the sun would reach its extreme north or south positions before reversing course for the next six months," Schneider says. "Ancient structures that allow you to track the shifting position of the sun between its extremes are found all over the world. Stonehenge is one of the larger of these structures begun perhaps 5,000 years ago with its main axis aligned toward sunrise on the June solstice."