Winter Solstice--and the Reason for the Seasons

Winter solstice this year was on December 21.  (Depending on when leap years fall, it is sometimes the 22nd.)  The solstice (means "sun stops") occurs when, from our point of view, the noonday sun reaches its lowest point in the sky in the northern hemisphere--it stops getting lower, and will begin rising again after this date.  How low it goes depends on the observer's latitude: the higher your latitude, the lower the sun, until you reach the Arctic Circle inside which the sun will not rise at all on this date. 

The winter solstice is the boundary between fall and winter--all the seasons begin and end as astronomical events, rather than changes in the weather. 

Recall that all globes come with a built-in 23 1/2 degree tilt.  That tilt represents the degree to which earth's axis of rotation is out of perpendicular to its orbit around the sun--as it would be if you put a (small) model of the sun on the same table with the globe.  Model our seasons in the following way.  Put a powerful lamp in the middle of a large table.  Put the globe on the edge of that table with the north pole tilted as far as it can go toward the sun: it is June 21st, the summer solstice.  As you spin the globe, notice how high the sun would get in the imagined sky of North America; notice also that much more than half the northern hemisphere is illuminated.  (It's hard to see this with the usual shiny-surfaced globe; I dust it with chalk dust to make clearer how much is lit.)  Because of this, the northern hemisphere day will be longer than its night, and the sun will shine more directly (so more intensely) on the surface at noon.  Both of these factors mean more heating of the northern hemisphere, bringing on warmer weather as the heat builds up. 

Now slide the globe counter-clockwise around the edge of the table to make the weeks and months pass.  While you do, be careful to keep the axis of the globe point in the same direction all the time (keep it aimed always at the same side of the room).  When you reach the opposite side of the table, you will find the north pole now tilted away from the sun.   It is now Decenber 21 and the winter solstice.  Notice that the situation is reversed from six months ago: the sun will not rise nearly as high for North America, and since most of the hemisphere is in darkness, the daytime will be short.  The earth's surface in the northern hemisphere is receiving much less heat than six months ago, so it is getting colder.

Several lines on the globe are defined by the solstices.  The Tropic of Capricorn is the line at 23 1/2 degrees south latitude where the sun will be directly overhead on the winter solstice, while on that date nothing inside the antarctic circle will get any sun at all, while inside the arctic circle on that date the sun won't even set!  Similarly for the tropic of cancer and the summer solstice.  The "tropics" is therefore that band from 23 1/2 degrees north to 23 1/2 degrees south where the sun will pass directly overhead at least once each year.

This might put a few misconceptions to rest.  First, notice that our distance from the sun is not a big factor.   In fact, the earth is actually a bit closer to the sun right now than it will be in June!  Notice also that the seasons will be opposite in the two hemispheres: summer has just begun for my friends in New Zealand, who are enjoying their longest days right now.  Notice that the tilt of the earth'a axis is not changing--just where the sun is in relation to it.  [You might remember that the north pole always points (approximately) at Polaris, the "pole star."]  Finally, don't confuse orientation with distance: plenty of students do this very activity, then get mixed up explaining that when the north pole is tilted toward the sun that hemisphere is actually closer to the sun--true, sort of, but only by a miniscule fraction of a percent! 

Remember: the changing seasons are about the length of days and the directness of rays!

One question might still occur to you: why isn't the winter sostice the coldest day, and the summer solstice the warmest?  It's strange to think that, as the days lengthen into January, the weather is still cooling off!  The secret is to think in terms of the balance between heat gain and loss: the summer solstice is when the northern hemisphere gains heat fastest, but it takes time for that temperature to rise; similarly, the decreased heating that reaches its lowest ebb on Dec 21 will take time to have its full effect.  So it really does make sense to begin winter with the winter solstice: though the fastest cooling is past, the chill will still deepen further.

 

Material Guy

I guess I am ultimately a materialist.

For a long time after we bought this house fifteen years ago I took a certain delight in fiddling with things--particularly outdoor things.  Since we had only rented before, owning property was a novel experience.  Almost before we moved in, I had begun drawing up a design for a native meadow garden to replace the "wildflower mix" garden in the strip between house and driveway, dreaming my way through catalogs from native plant nurseries.  The first spring I could not bear to cut the back lawn until I had traversed the whole on hands and knees, cataloging the striking diversity of grasses, sedges and other herbs that made up our "lawn."  (Today an artistic arrangement of dried plants from the yard adorns our wall.)  That year or the next I built the "grownup" tree platform in the tiny woods out back.  And a year or two after that I began a protracted war against the English ivy that had invaded and taken over a large part of the little woods.  Homeowner delight, naturalist-style.

 

My strip of meadow.  It will be more impressive in a month or so. 

Twice as old now, the grasses form a solid mass as tall as me in late summer.  (6/1/2006)

 

 

 

In some years I couldn't bear to mow down all these beautiful plants until July. (7/2/2005)

Then I got too busy to use my little tree platform, the meadow thrived until it no longer needed me much, other interests distracted me until the ivy had grown back, and plumbing and other issues made me sometimes yearn for the days when I could just call my landlord for a fix.

But through it all I've never lost my interest in our slightly mysterious property lines.  The border with the neighbors on each side hasn't much wiggle-room, but the woods of a number of neighbors runs together with ours, and property lines don't all run straight, so I've never been exactly sure where our land ended, so have never exactly known how much fiddling I could get away with out there.  There are (were) some majestic red oaks back there that I hoped were ours, but probably weren't.  And what about the little stand of spindly, old white oaks way back there?

New impetus for the settling of our property lines came when the accumulation of standing dead trees came to include a small elm near the property line that clearly threatened two houses.  Was it our problem? or would the bill go to the neighbors?  (It was too tiddly a situation to be safely taken down by me and my little electric chain saw--the diy savings would have been overwhelmed by the cost of repairing at least one home and also our relations with the neighbors.)

This at last sent me in search of plot plans for our neighborhood on the net.  Lo and behold, they were there--something I guess we wouldn't have found thirteen years ago.  The scanned image of the old drawing announced that we owned a bit more land than I'd thought, and it extended a bit farther back.  They also gently informed me I'd be paying a hefty bill to professional arborists.

 

My "new" scarlet oak. (12/11/2013) 

The upside is that I suddenly have more trees, including at least one fine, strapping scarlet oak and a multiple-trunked pignut hickory, as well as some of the spindly white oaks.  Pride of ownership in my majestic 4/10 of an acre--complete with ramshackle house and leaky roof--swells once more.

For a bit of contrast, here's Thoreau in Walden:

I have frequently seen a poet withdraw, having enjoyed the most valuable part of a farm, while the crusty farmer supposed that he had got a few wild apples only.  Why, the owner does not know it for many years when a poet has put his farm in rhyme, the most admirable kind of invisible fence, has fairly impounded it, milked it, skimmed it, and got all the cream, and left the farmer only the skimmed milk.

If the shortest day is Dec 21st, why is the earliest sunset weeks before?

A recent post at EarthSky explains a confusing fact: the earliest sunset of the year is NOT on the shortest day of the year--December 21st--but weeks earlier.

The days shorten until December 21st or so, due to the earth's revolving around the sun to the point at which the earth's tilt has the northern hemisphere tilted as far as possible away from the sun.  It would be reasonable to expect that the days would shorten equally at both "ends"--so later sunrises and earlier sunsets--but that turns out to be wrong.

On the right is the situation we're approaching: notice that

much of the northern hemisphere in darkness, so that our daylight hours are few.

I first discovered this years ago as a junior high school science teacher.  I liked to get out of the book sometimes, and do big outdoor things.  One favorite was to make the school yard into a giant sundial using the flagpole as a gnomon.  (My hope was that such a tall pointer would make a shadow you could watch move just standing there for a few minutes, but the shadow turned out to be too indistinct to work well.)  I wanted to use the shadow to establish the exact direction of south by looking at the shadow at "local noon"--the time when the sun is directly over your meridian, that is, your longitude line.  I figured to find that time as the half-way point between sunrise and sunset.  (That didn't work as planned, owing to the definitions of sunrise and sunset!)  It was in studying the newspaper almanac day after day that I became confused by the seeming lack of a pattern in the times. 

Note: I've discovered that the explanation below (as well as the EarthSky post referenced above) has errors. I will fix these in a post later in January. (Edited 1/3/14)

EarthSky describes the reason for the mismatch this way: "The time difference is due to the fact that the December solstice occurs when Earth is near its perihelion – or closest point to the sun* – around which time we’re moving fastest in orbit. Meanwhile, the June solstice occurs when Earth is near aphelion – our farthest point from the sun – around which time we’re moving at our slowest in orbit."

That explanation is good, but has gaps I think need filling.

First, why should the speed of the earth in its orbit around the sun affect sunrise and sunset times?  For convenience, let's count a day as being from one solar noon to the next.  (The Royal Navy used to do this.)  We think of the 24-hour day as the result of the earth's rotation on its axis, but in fact the sun is also moving about one degree of its 360 degree annual trip around the sun in that same day.  That means that the earth not only has to rotate one degree MORE than 360 degrees on its axis in order for our location to return to pointing at the sun--that is, back to solar noon.  As long as the earth kept a steady speed in it's orbit, all would be well.  But instead the earth speeds up a bit as winter approaches, and begins slowing again after early January.  Because it is going more than one degree around the sun in December, the earth must rotate farther to bring it around to the same solar noon, making everything (all things being equal) a bit later.  The opposite occurs when the earth is at its slowest, in June and July.  This effect combines with the shortening days of fall to create the odd timing of sunrises and sunsets.

Here's an animation that makes the below clearer.

Imagine you are standing on earth where the left-pointing arrow begins. From "Day 1" to "Day 2" your location has rotated 360 degrees PLUS an additional amount to point back toward the sun, since the earth has moved a bit further around the sun.

At least one more question occurs: why does the earth change speed in the first place?  That has to do with the elliptical shape of earth's orbit, and a law first discovered by Johannes Kepler centuries ago (soon after Copernicus and Kepler established that the earth revolved around the sun, instead of the reverse).  Earth's orbit is an ellipse (oval) that is not quite circular, with the sun a bit nearer to one end of the oval.  Kepler discovered that planets move slowest when they are farthest from their parent body, and fastest when closest.  To be precise, any orbiting body sweeps out equal areas of its orbit in equal times.  (You can think of these areas as pie-slices of the whole orbit: it will take a wider pie slice to cover the same area when the wedge is shorter.) 

Enough for now.  More about the "reasons for the seasons" soon!

*This surprises a lot of people, who assume that summer is warm because the earth is closer to the sun, while winter is colder because it is farther--but the opposite is true.  It turns out that the difference in earth-sun distance from summer to winter is not very great, and the reason for the seasons lies elsewhere, as I'll explain at the solstice in a couple of weeks.

The Generation Passing



hairy woodpecker & northern flicker photos from Cornell's
http://www.allaboutbirds.org/guide/search.aspx

We have been blessed to hear the percussion of woodpeckers in recent years, and occasional sightings of the striking black and white plumage with red accent of (most likely) the hairy woodpecker.  (We've also several times hosted northern flickers, a ground-feeding relative.)   The downside of their frequency is the reason for it: we have lost a number of our trees.

 A pair of big red oaks died on their feet about six or seven years ago, the second coming down in a blizzard only last winter. (Enjoying the blizzard in Thoreauvian fashion at the time, I dodged it by noticing how alarmingly it swayed in the wind.) A pretty little scarlet oak in the backyard died about the same time. I am embarrassed to say I didn't realize these had died until a year or more had passed. These three were apparently in the prime of life, and may have succumbed to the winter moth caterpillars that infested us around that time.

The big old three-stemmed black cherry that supported the kids' tree fort has been dying gradually. One stem came down in a rainstorm last spring: the break was twenty feet up, in an area weakened (unbeknownst to us) by insects.

And a small elm in the narrow space between us and a neighbor--a bit of a weed, really--has been leafless for several years, and without bark for the last year.

Most of these are reason to be a little sad, especially without their children waiting in an understory to spring up to take their place, as would be typical in a more natural setting. But the little oak in the backyard threatened our garage, while the elm could not be felled at all in usual way without taking off part of our roof and maybe the neighbor's as well. It was time to call in the professionals, and to figure out how to pay the bill.

    Once a strapping young adult scarlet oak.                  Worldly remains of a young elm.

Now then, how will I get the woodpeckers back?