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Winter Solstice and Dark Days

© Keith C. Heidorn

As the calendar year winds down, so too does our Northern Hemisphere solar year, except that the sun reaches its cycle's end nine days before the last page of 2001 is ripped from the wall.

If each day we could mark off the altitude above the southern horizon of the noon sun, we would see it drop to its lowest angle in the sky on December 21, the Winter Solstice. To the astute eye, its lowering toward the southern horizon halts (thus the name solstice from "the sun standing still"). The following day the sun begins to again slowly rise in the sky, reaching its noon zenith next June, on the day of the Summer Solstice.

On the Winter Solstice, darkness reigns over all but the most equatorial latitudes north of the equator. Indeed, north of the Arctic Circle (latitude 66.5 deg N), the sun will not be seen today, and at latitudes just south of the Circle, it will tease us by rising and then quickly falling back below the horizon, shining weakly for only a few hours.

Note that sunrise is defined to occur when the upper edge of the Sun's disk appears on the horizon, and sunset is defined as the moment when the upper edge disappears below the horizon. These are the instants of first and last direct sunlight, but at these times the centre of the solar disk is still about a degree of arc (50 minutes) vertically below the horizon. Refraction of sunlight by the atmosphere will also make the sun appear to rise earlier and set later than would be the case if we had no atmosphere. As a result, some northern latitudes have a midnight sun but no 24-hour night, and the longest day is longer than the longest night, the shortest day longer than the shortest night.

The period from about November 1 to February 1 can be defined as solar winter, the period of the year with the least potential daylight. Thus, the total hours of potential daylight have been slowly decreasing for us for seven weeks or so, before bottoming out on the Winter Solstice. The Winter Solstice is the shortest day of the year, but ironically is not the date of either the earliest sunset or the latest sunrise. The former has passed as of this writing, having occurred around December 8, at latitude 40 deg N (the exact date depends on the latitude). The latest sunrise will come sometime in late December/early January (around December 26 at latitude 60 deg N and about January 4 at latitude 30 deg N).

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