Midwinter Minima

We’re in the valley of the year.

This is how the valley is depicted in the “Zodiac Wavy Chart” for 2018.  The chart now available is “Zodiac Wavy Chart 2019“; I made the 2018 one for my own use before I thought of publishing it.  These charts look like rolling landscapes.

The Sun is at its southernmost on December 21, the solstice – day light is shortest, night longest, sunlight reaches no farther north than the Arctic Circle.  The Sun is drawn at mid-month; in the month it travels from the right to the left edge of the glare.

Other wintry events cluster in the valley.  The day that seems shortest, because the Sun goes down earliest, is December 7 (for about latitude 40° north, where sunset reaches 4:35 in the afternoon).  Latest sunrise comes in early January.

December 13 is St. Lucy’s Day, the “yeares midnight” of John Donne’s poem; it used to be thought of as midwinter.  A story recently came to light about T.S. Eliot’s reaction to a St. Lucy’s Day custom in Sweden when he was there to receive the 1948 Nobel literature prize.

Near the Sun’s most southerly moment comes the Moon’s, though this year it was by no means as southerly as it can be when the orbit is differently tilted.  New on Dec. 7, the Moon passed just north of the Sun, so it was southernmost on Dec. 9, at -21.5°, before slanting southward across the ecliptic (though now recovering slightly northward in relation to the equator) on Dec. 10.  In years when the “valley” of the Moon’s orbit is centered ;like the valley of the Sun’s – at the solstice – it can dip to nearly -29°.

Your horizon could be drawn as a curve across our Zodiac chart.  Or, making the horizon itself the baseline, the ecliptic and celestial equator become curves cutting across it, as in this evening scene for December 13.

The Sun’s usual companions, Mercury and Venus, as well as Jupiter, are below this horizon because they are now on the Sun’s western or morning side.  In the other direction, not far along the ecliptic eastward, is Saturn.  The Moon has raced much farther out, to pass Mars and Neptune; in fact these three will cluster in a circle less than 5° wide on December 14.

At this time, the three bodies are only just past their closest-packed moment.  They lie 83° from the Sun, and the “antapex of Earth’s way” is 90° from the Sun), which means that the Moon is at First Quarter.  It may outdazzle Mars, and Neptune anyway is far below naked-eye visibility.

Another accompaniment to midwinter is the great Geminid meteor shower, expected in the nights of December 13 and 14.  The meteors are fragments of a small asteroid, rather than of a comet as with most meteor streams, and some of them explode as fireballs; an outlying Geminid fireball appeared a few days ago near Comet Wirtanen.

For Earth’s southern hemisphere, everything I’ve called a “valley” is hill.  if you live down (pardon me!) there, I’d like to know whether the sky maps you use have north or south at the top, and, whichever, how your brain reconciles this with using terrestrial north-upward maps.

I feel like saying something more about valleys and hills.

The profile of the year, of the ecliptic, is a perfect curve, symmetrical, a smooth valley and a smooth hill, able to be defined by a single equation.  (Declination equals 23.45 sin right ascension.)

The profile of real valleys and hills is entirely different.  Let’s use the more general terms “lowland” and “upland.”  The profile across an island or a continent is a mixture of flat lowlands and wrinkled uplands.

We may have been surprised by this, as by all else, at an age before we can remember, but experience and education have made us used to it.

In a contour map, you can readily see the upland, because it’s a mass of closely spaced wriggly contours.  It is visually distinct from the lowland with its few widely separated contours.  The mere mass of contours seems to say “upland.”

Yet why should this be so?  Why shouldn’t a continent typically consist of flat lowland and flat upland? – or wrinkled lowland and wrinkled upland?  Why is flatness the defining feature of lowlands and wrinkledness the defining feature of uplands?

(There are exceptions, such as plains that have been uplifted and into which canyons have been carved, but I’m making a general point.)

The explanation is that the real difference between the two types of terrain is aggradation versus degradation.

A young stream high up near its source has energy and tears at the ground, carrying it downstream.  From the surrounding slopes rain sends mud and rocks down into the stream.  In the hills, every pebble or tree that is loosened has nowhere to go but down.  An “old” slow-moving river in the lowland deposits more silt than it can carry.  It floods, spreading silt more widely, and so sometimes does the sea.

Contours represent height, but their closeness represents slope, and a map of hilly country could be drawn, instead, with shadings for slope.  Hill country consists of a lot of slope, and in high mountains everything is slopes, steep ones.

A river plain is a positive surface, growing slowly upward (and often outward to sea, in the form of a delta).  And very definitely a mountain landscape, which we think of as something sticking up, is a negative surface.  All its slopes result from down-cutting.  It’s obvious in a picture of, say, the Matterhorn.

You can almost see a giant backhoe that has chopped it down on four sides.  The mountain is all that is left of a layer of rock that was much thicker before a tectonic force thrust it up to where it could be best attacked by the weather.  The Matterhorn is high, but you can imagine the vanished surface much higher, and the quantity of rock that is now soil in a plain.  The noble mountain chains are  wrecks. residues.

Our planet would become flat, with a complete coat of water over a complete coat of silt, if it weren’t for plate tectonics.

We may find it hard to accept that geological processes apply to the human-fashioned landscape we daily see.  Streams aren’t allowed (usually) to get at the street, sidewalk, houses, lawns and erode them or silt them over.  So is a lowland covered by a city really aggrading?  It’s a matter of time-scale.  Buildings ultimately crumble, and their material, brought from quarries in the hills, aggrades the land.  The “tells” of Iraq and Syria, which fascinated me when archaeology was my passion, are mounds built by rather rapid human aggradation: houses wore out or were burned, new floors were laid over old rubbish.  The difference between a tell and Manhattan is that the tell has a small tight area.  My front door is two steps below the street because the old street has been many times resurfaced.

Though I had a young interest in geology, it was only later, when a book by a New Zealand geologist was given to me, that I learned of “aggrading” and “degrading”.  He applied them to watercourses, which are either degrading or aggrading their beds and banks, by erosion or deposition.  Soon afterwards, we took a walk along a Lakeland valley, I think it was Eskdale, and I noticed a rock reef across the narrow valley; above it, the river was delayed, aggrading; breaking through, it was vigorous, degrading; then it resumed peaceful aggradation.  And yesterday I kept contemplating these surfaces along the route of the railway through what I call the Crewkerne Rumple of hills between the counties of Somerset and Dorset.  There is one hillside where the degrading is being done by the hooves of sheep, pushing the turf downward, forming their many contour-like “sheep-walks”, emphasized by grazing sunlight.

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DIAGRAMS in these posts are made with precision but have to be inserted in another format.  You may be able to enlarge them on your monitor.  One way: right-click, and from the drop-down list choose “View image”  Or from that list choose “Copy image”, then put it on your desktop, then open it.  On an iPad, tap with three fingers to enlarge. I would be glad to know whether these work for you, and what the equivalent actions are on a phone.  I would welcome learning of any other methods.

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