Days are getting shorter, but not for long

Published 7:00 am Saturday, December 15, 2018

By Skip Rigney

We are headed toward the shortest day of the year, also known as the winter solstice. Not just us, but everyone north of the equator experiences their shortest period of daylight on Friday, December 21st. Of course, that also means we’re in for our longest night of the year.

The exact duration of daylight depends on how far north of the equator you are. Folks in Prudhoe Bay on the northern coast of Alaska haven’t seen the sun since it slipped below the horizon at 12:59 PM on November 24th. It won’t be back until January 19th. That’s not to say that it’s continuously pitch black dark there. The sun is lurking just below the horizon for a few hours each day providing the northern Alaskans and their Arctic neighbors a short dose of daily twilight.

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Northern residents of the lower 48 states don’t have to deal with that sort of daylight deprivation, but midwinter days are still noticeably shorter than ours. For example, people in Seattle, Washington, will have to make do with 8 hours and 25 minutes of daylight this Friday.

In contrast, Picayune will have 10 hours and 10 minutes of daylight on Friday, which is the same for all locations at Picayune’s latitude (distance north of the equator).

On Saturday we will have a few seconds more daylight. That daily increase will accelerate as we get closer to the spring equinox on March 20th. By February each new day will have a couple of minutes more daylight than the day before.

These changes in the amount of daylight we get each day are due to the earth being tilted on its axis at an angle of about 23 degrees. As the earth orbits the sun, those of us north of the equator are tilted the farthest away from the sun in December and the most toward the sun in June.

The resulting changes in the amount of daylight, in conjunction with the changing angle of the sun’s rays hitting the earth, are the reasons for the seasons.

You might expect the winter solstice, the shortest day of the year, to also be the day with the latest sunrise and the earliest sunset. Not true. Sunset throughout this week will actually be several minutes later than it was during the last week of November and the first part of December.

Strangely, even though it is by just a few minutes, our earliest sunset occurs in December a couple of weeks before the winter solstice. Our latest sunrise occurs in early January a couple of weeks after the winter solstice.

A full explanation would take an entire column, so suffice it to say that this offset is related to the interplay between two factors. The first is the earth’s tilt. The second is the fact that the earth’s orbit around the sun is an ellipse with the distance between the sun and the earth changing throughout the year.

Even though our days will be getting longer and the sun higher in the sky after this Friday, winter is just getting started. Poplarville’s historical records show that our temperatures can fall into the teens anytime from December through early March.