One of the enduring terrors for primitive people was the possibility of the sun's disappearance.

Each year in the first three weeks of December, daylight continues to shorten. The weak sun crosses the southern horizon in a low arc to set earlier and earlier in the southwest. We notice, and so did ancient people.

To more primitive minds, there was worry that the sun would continue in its course and disappear. So, many of these early people thought the intervention of divinely appointed priests and specialized rituals ensured the sun's return. Because the sun didn't vanish forever and the days did get longer, the weeks after the winter solstice were traditionally a time for celebration.

Astronomical winter begins this year at 4:23 p.m. Friday in the Northern Hemisphere. Yes, astronomers can tell us the exact time when Earth is pitched 23 ½ degrees away from the sun at the North Pole — it's the winter solstice, when we have our longest night and shortest day. On Dec. 21, this first day of winter, the sun is 21 ½ degrees above the horizon at noon in the Twin Cities and only 18 degrees at International Falls. The Twin Cities receives 8 hours and 46 minutes of daylight, and people in International Falls must get by with a half-hour less.

So, we get more daylight — and, historically, lower temperatures. It should continue to get colder for five more weeks, with our coldest days often arriving in late January. It takes that long before radiation from the slowly lengthening sunlight can warm the frozen snow-covered ground and chilly air. Statistically, Jan. 25 is our coldest day of the year.

For meteorologists in the Upper Midwest, winter runs from Dec. 1 to the end of February, statistically the coldest 90 days of the year.

Jim Gilbert taught and worked as a naturalist for 50 years.