Astronomical winter arrived at 5:44 Wednesday morning with temperatures in the teens, basically seasonable for the date.  Today was a typical winter day with plenty of sunshine and highs in the mid- to upper 30s.

The term astronomical winter is used because meteorological winter started three weeks ago.  How many of you think of Dec. 20 as fall anyway?

The significance of the word astronomical refers to the position of the sun in relation to Earth. The fact that the Earth rotates on its tilted axis around the sun is what gives us our seasons.

Although today marks the start of astronomical winter, the coldest week of winter usually occurs the third or fourth week of January. This is because the planet is so big, it takes several weeks to fully react to the loss of daylight.

As the Earth rotates around the Sun it gives us our seasons

As the Earth rotates around the sun, it gives us our seasons.

Today marks the point where the northern hemisphere is receiving the least amount of sunlight possible. The winter solstice is the day with least amount of solar energy reaching the northern half of the planet, as well.

Further, it’s the day where the sun is at its lowest point in the sky at noon.  If you look at the distance of the sun above the horizon at solar noon (11:42 a.m. this morning), it will only be 24.2 degrees above the horizon, that’s as high as it gets all day!  This is about the same position would be just after 7:30 a.m. on the first day of summer.

The sun is very low in the sky all day long this time of year.

The sun is very low in the sky all day long this time of year.

The change in light is one of the constant pieces of nature we can predict.  We know how much light is going to be available at any given day and how much we gain and lose each day as well. This information is part of the computer models we use to forecast, it’s the other information we can’t predict so well that makes the forecast a challenge.

A few interesting facts about the solstice you can share.  Today is not the day with the latest sunrise or earliest sunset.  Because of the tilt of the Earth and the way we measure time the latest sunrise and sunset are offset from the solstices.  You can see on the map below when the earliest sunset occurs and how it differs from the latest sunrise.  This doesn’t take into account daylight saving time.  In practice our latest sunrise of the year is actually the Saturday before we turn the clocks back in November.

The earliest sunset and latest sunrise doesn't occur on the winter solstice

The earliest sunset and latest sunrise doesn’t occur on the winter solstice

We’ve lost a lot of daylight since the first day of astronomical summer back in June.  As a matter of fact over 6 hours of it.   The speed at which we gain and lose daylight increases towards the equinoxes and then slows down at the solstices.

Over 6 hours of daylight has been lost since the third week of June

Over 6 hours of daylight has been lost since the third week of June

There is only around 9 hours of possible daylight this time of year.  We will gain a few seconds of it each day the rest of this week, but we will be gaining a minute a day by the end of the first week of January and 2 minutes a day by the 4th week of that month.

There isn't much daylight this time of year.

There isn’t much daylight this time of year.

So although we have lots of winter weather ahead of us, the amount of daylight is now increasing and eventually nature will respond kindly.
You can follow Dave Epstein’s forecast on Twitter @growingwisdom.

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