Thursday, December 12, 2013
Around the autumn equinox, which occurred this past Sunday, sunlight in our part of the world, especially around dusk, takes on a clarity that seems practically supernatural. A certain light slants in through birches, oaks and pines in ways unseen at any other time of year. In late afternoon the goldenrod skeletons and scarlet sumac and even stands of horseweed radiate energy that practically sets the day on fire.
Part of what’s going on physically to create these effects on your bewildered, bedazzled autumn soul, I think, is the quick angles of the sunlight.
Toward the end of September (and of March) the sun sets more quickly than it does at other times.
Around the solstices (around June 21 and Dec. 21) at our latitude (which in Troy is 44.6 degrees north), it takes roughly 3 1/2 minutes from the moment the bottom of the sun’s disk first touches the western horizon to the moment the top of the disk disappears. At the equinoxes (around Sept. 21 and March 21), it takes less than three minutes. This doesn’t seem like much when you’re just reading these numbers, but imagine standing there waiting. Like watching the pot come to a boil, time is contextual in your mind.
Anyway, why does the sun set so quickly in September? The answer is that in September it’s making an essentially vertical descent to the horizon. In June, it approaches the horizon at an angle.
As noted here at this summer’s solstice, the Earth is tilted on its axis with respect to the sun. In other words, the North and South poles are tipped at an angle (of about 23.5 degrees) to the sun. This tilt gives rise to the seasons. In the part of the orbit when the North Pole is tipped toward the sun, the sun climbs higher and stays up longer, and we have summer. In the part of the orbit when the North Pole is tipped away, the sun is lower, the days are shorter, and we have winter. (This cycle is swapped in the Southern Hemisphere.)
In its course between its highest point in June and its lowest point in December, the sun naturally crosses a midpoint. This happens in September and March and is called the equinox. At that midpoint, the sun sets due west, neither high toward the north (as in June) nor low toward the south (as in December). So when it’s setting, it goes for all practical purposes straight down on the western horizon. The straight path, of course, is shorter and quicker than the path that angles down from the north or south.
The word “equinox” is from Latin words meaning “equal” and “night,” and the gist is that at the equinox, the hours of night are equal to the hours of daylight. But just as the longest and shortest daylight hours do not occur on the solstices, the days when light and dark hours are exactly equal (called the “equilux”) do not occur on the equinoxes either. At our latitude, the equilux is a few days after the equinox. This year our equilux was Wednesday, Sept. 25. (We are talking about differences of less than a minute here. See above: watched pot.)
Two factors influence this. One involves the way sunrise and sunset are calculated. Sunrise occurs, technically, at the moment the top of the sun’s disk appears on the eastern horizon. Sunset, technically, occurs the moment the top of the sun’s disk disappears on the western horizon. Now, the atmosphere is bending the sun’s rays. So in the morning, the rays are bending in over the horizon and the sun is visible there for several minutes before its actual, physical disk breaks over the plane of the horizon. Similarly, at sunset, the sun remains visible, with its rays bending back up over the horizon, for several minutes after the actual, physical disk has gone under the horizon. So the calculated time of sunset differs from the experienced time of sunset.
The second factor is that it does not become dark suddenly at the moment of sunset. Sunlight is still bending in through the atmosphere even after you’ve seen the sun wink under the horizon; our experience of this air-bending light is twilight. Technically there are three kinds of twilight, according to Dark Sky diarist Steve Owens, and “dark” hours are calculated according to which of the three kinds of twilight you take to be still within a range you want to call daylight.
The three kinds of twilight are: civil twilight, when the sun is up to 6 degrees below the horizon and enough light is still bending in to allow, say, a soccer game to continue; nautical twilight, when the sun is 6 to 12 degrees below the horizon and most of us would say it’s dark (like the referees who once ended a soccer game with the score 2–1, my son’s team pressing late — and 40 seconds to go), but sailors can still distinguish the horizon and take measurements from bright stars; and astronomical twilight, when the sun is 12 to 18 degrees below the horizon, dark to us, but enough light still bends in to interfere with stargazing.
The exact moment of equinox is calculated at the moment the center of the sun’s disk crosses the midpoint in its north-south track. So the calculation of sunrise and sunset by the top of the sun’s disk and the twilight factor throw seconds and minutes of discrepancies between equinox calculations and equilux calculations.
In late September, the sun is moving most quickly and directly toward the horizon. Its angles of light are transforming the landscape into a subtly turning kaleidoscope, to my eye at least. Light bending in through clear autumn air, firing red and yellow leaves and dry brown stalks and reed grass seems to pry something invisible loose. The secret of time appears to be underneath these autumn angles, and how they seem to move and play, and unfurl things there in the field.
Dana Wilde lives in Troy. His writings on the stars are collected in “Nebulae: A Backyard Cosmography,” available from Booklocker.com and online book sellers. Backyard Naturalist appears the second and fourth Thursdays each month. You can contact him at email@example.com.