Thursday is the shortest day of the year. Here’s what the winter solstice means for Chicago.

Nine hours, 7 minutes and 44 seconds.

The winter solstice, which occurs precisely at 9:27 p.m. Thursday, marks the shortest day of the year and the start of astronomical winter in the Northern Hemisphere. Chicagoans will only experience about nine hours of daylight.

“The winter solstice has a pretty firm connection to astronomy,” said Hunter Miller, public observing educator at the Adler Planetarium. “There’s a whole lot of stuff going on out in space to explain what’s happening here on Earth.”

What is the winter solstice?

The winter solstice is the moment the Northern Hemisphere is tilted farthest from the sun at about 23 degrees, Miller said. It’s easiest to understand, he said, if you imagine looking at Earth from far away and there’s a big stick that goes through the North and South poles.

“With the winter solstice, that stick that comes out of the North Pole is facing away from the sun, and the South Pole is facing more toward the sun,” Miller said. “The winter solstice in particular, it’s going to be the day that that big stick would point farthest away from the sun.”

The length of the day varies based on latitude, Miller said. The closer to the equator, the longer the day, because it’s pointed more toward the sun. Chicago, for example, will have less daylight Thursday compared to Miami. Far north places in Canada or Scandinavia might only experience a couple hours of daylight, Miller said.

In the days leading up to the winter solstice, days will slowly get shorter — usually changing by less than a minute per day. On Wednesday, there will be about 3 more seconds of daylight in Chicago, for instance. After the winter solstice, days will gradually lengthen until the summer solstice in June. On Friday, the city will gain about one second of daylight.

But the sun will set before 4:30 p.m. for the rest of December in Chicago, Miller added. The winter solstice doesn’t line up perfectly with the earliest sunset, which explains why earlier this month the sun set at 4:19 p.m. in Chicago, but it will set at 4:22 p.m. on Thursday.

The Southern Hemisphere will experience at this time its longest day of the year, called the summer solstice.

The coldest day of the year?

For meteorologists, winter begins on Dec. 1, lasting until the end of February. They split the seasons up into easy-to-remember chunks, grouping the three months that are usually the coldest together, according to the National Weather Service. This means that the meteorological winter doesn’t always line up with astronomical winter — the dates people are most familiar with.

“What we’re using with the solstice are much more focused specifically on our placement in space, whereas the meteorological calendar ends up being more focused on our experience here on Earth,” Miller said.

But just because it’s the shortest day of the year, doesn’t mean it’s the coldest day, Miller said. He said the “bone-chilling” cold days are usually about a month to six weeks after the solstice.

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Due to a strong El Nino, scientists predict that Chicago’s winter will be milder and drier than normal this year.

Culturally significant?

Many cultures around the world have held feasts and religious observances around the winter solstice — Miller added that it isn’t surprising that there’s so many popular holidays this time of year. One of the most famous celebrations is at Stonehenge in England. Humans may have started observing the solstice during the Neolithic period in 10,200 B.C., according to the History Channel.

Many Native American communities have also held solstice ceremonies. Miller said about 1,000 years ago people at the Cahokia Mounds State Historic Site in southwestern Illinois used “woodhenge” or large timber circles as a calendar to keep track of the sun.

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“They would mark all of these equinoxes and solstices and things,” he said. “That was really important back then — they used it to determine when their agricultural cycles would be.”

Miller said the winter solstice is a good opportunity to get outside and observe the night sky.

“You got lots of good nighttime to appreciate the stars, and this is a great season to check out some constellations that everybody knows like Orion,” he said. “It’s rising up over the lake pretty early in the evening around 8 o’clock and is always a beautiful sight.”

rjohnson@chicagotribune.com