Winter officially begins, at least astronomically, Thursday at 11:28 in the morning eastern standard time.

It’s always struck me as quite odd that we say winter is beginning in the final 10 days of December. By this time we’ve already had over 20 percent of the coldest 90 days of the year.   The sunsets have been getting later for just under two weeks.

One thing that is true: Starting Wednesday the amount of  time between sunrise and sunset increases.

There are two elements of weather and astronomy that haven’t bottomed out quite yet: Our sunrises will continue to get later until early January and the coldest average temperatures won’t occur for about another month.

The coldest average temperatures of the year occur around January 19th.

Interestingly, the Earth is also nearing its closest approach to the sun of the entire year – a phenomenon that will take place on Jan. 3.

You may wonder why winter begins on Dec. 21 in the first place. Meteorologists actually started winter three weeks ago and will end it on March 1. But for astronomers winter begins on the Dec. 21. This is because every day somewhere on the planet the sun is exactly overhead in one spot at noon. That position changes each day and on the first day of winter that spot is as far south as it will ever go for the year. On June 21 – the first day of summer – that overhead position of the sun will be as far north as it goes all year.


Winter arrives just before 11:30 Thursday.

The solstices don’t represent the extreme of sunrise or sunset period in the summer. The earliest sunrises have occurred by June 14 in this part of the world. And the sunsets don’t start getting later until early July. The main reason for this is because the Earth is tilted on its axis – known as obliquity – and there’s the fact that the Earth’s orbit is elliptical around the Sun. That’s called eccentricity.

These factors influence where we see the position of the sun every day, but you’d have to take a photograph at the same time each day minus daylight savings to really see it.  If you did this the sun’s path in the sky would make a pattern called the analemma.

If you own a globe, it might have an analemma pattern  on it.  The pattern is created by the sun’s changing north-south position throughout the year, as well as its east-west position.  If you looked at where the sun is at noon (minus daylight saving) each day for an entire year, you would notice a difference in time between what your watch reads and the position of the sun (clock time vs. sun time) is called the Equation-of-Time. Here in Portland and much of eastern New England, the sun is already lower at noon, so the equation-of-time is negative.

Examine the sunrise and sunset chart for December for Boston (the concepts are the same for your area, but the times would be different by a few mintues).  I have placed the letters A through E on the interesting things to note.  A shows the earliest sunset of the year. Although you don’t see a flip from 4:11 to 4:12 until Dec. 14, there’s a two-second change from Dec. 8 to 9.  The sunset Dec. 8 is 4:11:38 and the sunset the following day is 4:11:40.

December sunrise and sunset for Boston, MA can tell us a lot of interesting facts.

On Dec. 22, the gap between sunrise and sunset will grow by 3 seconds. This is why we say the “days are growing longer.”  The increase in the gap will become larger and larger until the first day of spring when we gain nearly 3 minutes each day.

Letter C is showing the sunrise still getting later, this will continue through Jan. 3. After that the daylight starts really increasing quite quickly.  We gain nearly a full hour in January.

Where I have noted D is noteworthy for two reasons. First, this is the day the sun will start getting higher in the sky each afternoon.  Also note that the sun reaches its highest point before noon, which is always the case when we are in standard time (and would be always the case throughout the year if we didn’t shift the clocks forward in March.  This because we are in the eastern part of our time zone.  In Detroit, farther west, the sun reaches its highest point after noon on the clock all year.  The final letter E  shows the Earth gets closer to the sun until Jan. 3 – that’s called perihelion – and from there it gets farther away until early July when it reaches aphelion.

All of this gets a bit heady for sure, and it’s always good to remember that all of this season, clock and calendar stuff is just made up by us silly humans to explain our cosmic condition anyway.

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