THE SOUTHWORTH PLANETARIUM

207-780-4249   www.usm.maine.edu/planet

70 Falmouth Street   Portland, Maine 04103

43.6667° N                   70.2667° W 

Altitude:  10 feet below sea level

Founded January 1970
Julian Date: 2458837.16

2019-2020:  LXXV

             "In the depth of winter I finally learned that there was in me an invincible summer."

                             -Albert Camus

THE DAILY ASTRONOMER
Thursday, December 19, 2019

Solstice Q and A

Astronomical autumn is drawing to a close.   In fact, tomorrow is the last full day of autumn.  Winter will start the next day, albeit at a late hour.    While winter's onset can engender dread in even the heartiest souls, it is not without its beauty.  If you don't believe me, go out and observe a fresh snowscape below a rising ember moon.     Today we welcome astronomical winter's arrival with a brief Q and A: solstice questions we've collected over the years.

When will the winter solstice occur this year?

Saturday, December 21st at 11:19 p.m.

At this precise moment, the North Pole will be aligned away from the Sun as far as it can be.  Astronomical winter will officially begin.

Why is the term "December solstice" preferable to "winter solstice?"

Simply because it is more accurate.       The December solstice will mark the beginning of the Northern Hemisphere's astronomical winter and the Southern Hemisphere's astronomical summer.   The summer solstice is more accurately known as the "June solstice."

Is winter really our shortest season?

Did you ever hear the old adage:   New England has only two seasons:  winter and Independence Day.   Well, New Englanders can be forgiven for not believing that winter is our shortest season, for it certainly seems interminable, especially during the April blizzards that we all adore.   However, astronomically, winter is truly the shortest season.  It lasts about 88 - 89 days, whereas summer is about 93 days in duration.  

 Why?    

Winter occurs when the northern hemisphere is directed away from the Sun.  Refer to the image below.  

The amount of time Earth requires to move from the winter (December) solstice point to the spring (March) equinox is shorter than the time it requires to move between any of the other seasonal points because in early January Earth reaches perihelion, the closest point in its orbit around the Sun.   A planet revolves more rapidly when it is closer to the Sun.     Around July 4th, Earth reaches aphelion, its greatest distance from the Sun.    Therefore, Earth is moving faster in January and February than it is traveling in July and August.    If Earth reached perihelion in July, our summer would be the shortest season.  

Since you can balance an egg on its end on the equinox, I assume you can lay an egg on its side on the solstice?

Are you serious?

Actually, a person can balance an egg on its end on any date of the year, provided they don't have any other commitments and are endowed with saintly patience.  This popular misconception is based on the mistaken idea that Earth is actually "upright" on the equinox.  It isn't.   Earth's tilt remains the same throughout the year.

Is the Sun directly overhead at the equator on the winter solstice?

No.  The Sun passes directly overhead at the Tropic of Capricorn on the winter solstice.   It passes directly overhead along the Tropic of Cancer on the summer solstice.   The Sun passes directly overhead along the equator on the spring and autumn equinox.

What season is it on the Equator?

The Equator is a line of no thickness. If you're standing on the Equator, the part of you in the southern hemisphere will be in one season and the part on the northern side will be in another season.

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