Autumnal equinox kicks off today


By Tom Burns - Stargazing



Autumn will soon begin, and I must confess a certain trepidation. When I was director of Perkins Observatory, I always received some form of the following question several times this time of year: “When does fall begin? And I mean exactly, to the minute.” (I never could understand why people need to know so exactly. What are they going to do, sacrifice a chicken?)

The answer is not as easy as it sounds. (Pay attention. This is complicated.) We determine the seasons by the position of the sun in the sky. The sun’s position is complicated by two motions.

The yearly motion is determined by the Earth’s revolution around the sun. The daily motion is determined by the turning of Earth on its axis to make day and night.

It’s like trying to figure out exactly where you are in space while you’re on the teacups ride at the state fair. But let’s give it a go anyway.

As Earth moves around the sun, the sun seems to move slowly across the sky from day to day along a path called the ecliptic. Of course, the sun’s motion is purely an illusion. We’re the ones who are moving.

The sun’s path carries it once a year in front of the fixed background of stars along a great circle called the ecliptic. The ecliptic passes through certain constellations we have come to call the zodiac.

However, Earth, bless its heart, also spins once a day around a line from the north to south poles. The circle that the Earth moves around is its equator. Thus, the starry background that we are trying to measure the sun’s motion against moves parallel to a line above Earth’s equator. That imaginary line in the sky is called the celestial equator.

Sadly, Earth’s axis is tilted 23 1/2 degrees away from its path around the sun. That means that the ecliptic and the celestial equator are tilted with respect to each other and are bound to cross one another in two places. We officially mark the beginning of fall when the sun crosses the celestial equator.

Or to put it another way, for one magnificent moment, the sun is at that spot where the celestial equator and the ecliptic meet, a place we call the autumnal equinox.

Not coincidentally and for reasons too complicated to go into here, during the day of the equinox, day and night are nearly the same length, about 12 hours for each.

Day and night are nearly equal because on that day, Earth’s axis is perpendicular to its orbit. As a result, the shadow line on earth, called the terminator, that separates night from day, runs through both the north and south poles.

But notice that I said nearly equal. From where we sit on Earth, daylight is actually about eight minutes longer than nighttime. Our bounty of sunlight is the result of two factors. The sun is not a point of light, but a disk that takes a while to set completely.

Also, Earth’s atmosphere acts like a lens and bends the sunlight around the planet a little. The result is that when you see the sun touching the horizon, it is actually below the horizon. (Don’t look at the sun, BTW. It will harm your eyes. You’ll have to take my word for it.)

In case you were wondering, the day when daylight and nighttime are actually equal is Sept. 26 this year.

Unfortunately, the equinox moment doesn’t happen at the same time every year. The number of days in a year is not even. Earth takes 365 1/4 days to travel around the sun, and it’s tough to put 1/4 day on the calendar.

So, we add a leap day every four years to make the calendar come out right in the long run. In the meantime, our clocks and calendars are always wrong if time is measured precisely by the positions of the sun and Earth.

Taking all the above factors into consideration, I am pleased to report that the center of the sun is (or was, depending on when you are reading these words) at the autumnal equinox today, Sept. 22, at 9:30 a.m. (more or less, I think), and autumn will begin or has begun.

On March 21, when Earth is on the opposite side of its orbit around the sun, the sun will pass along the ecliptic across the celestial equator again, spring will spring forth, and day and night will be nearly equal again for one day.

You will please note that I have used the word “autumn” to refer to the season and not the more common “fall.”

Personally, I prefer “fall,” but I was recently chastised at for using it by one of my British Facebook friends. Apparently, some folks in England consider “fall” an American barbarism. “Autumn” is the preferred term in England.

For a long time before the terms “fall” and “autumn” ever existed, people in England referred to the season as “harvest,” which makes absolutely perfect sense.

During the 17th century, the English began to migrate from rural areas into the cities. “Harvest” didn’t make much sense in an urban context.

Two terms emerged, and both became popular. The more hoity-toity “autumn” comes from the Latin term “autumnus,” which comes (through Norman French) from the Latin verb “augere,” which means to “ripen” or “prosper,” more or less.

The Latin term really does maintain some of the sense of the old term “harvest,” but clearly the term is more likely to be used by the upper, Latin-educated classes.

The more plebeian word “fall” goes back centuries to the Old English word “fiaell,” which means to “fall from a great height.”

Yes, that’s right. “Fall” really does refer to the leaves falling from the trees. Or perhaps it refers to the great change that comes as the glorious heights of summer descend into the depths of winter.

Most of the folks who migrated from England to the New World were more likely to be from the lower classes, so “fall” took hold in America.

To be honest, I prefer “fall” over “autumn” because it gives me an excuse to tell an old Iroquois story that probably predates the language prejudices of the English and Americans.

As it turns out, more than just leaves are “falling from a great height.” At a great height from the ground is the collection of stars we call the Big Dipper. Some Native American cultures of North America see a bear in the bowl of the Dipper. The stars of the Dipper’s handle are often identified as three hunters stalking the bear.

The faint star called Alcor, above the bend in the handle, is the faithful hunting dog Ji-yeh.

In the old Iroquois story, the bear is a huge, monstrous beast that devours all the winter deer that the people depend upon for survival. Three brothers are given the honor of killing the bear. However, they soon discover that this is no ordinary beast. It can cover itself with an invisible net, which hides it from their sight.

Onward through the snow they plod, their moccasined feet slowly freezing, their hope slowly fading. At last, they approach the bear and make ready to loose their arrows.

All of a sudden, the bear disappears under its magic net, and the brothers are enveloped by a vaporous mist “like a hiding cloud that floats above the water,” as the old story goes. That night, as hope fades to frozen despair, the brothers all have the same dream — that they have finally captured the bear and roasted it on a warm fire.

They vow that they will not rest until they have vanquished the bear. Many days and nights pass without sleep. The cold winds blow and the snow piles up in great drifts, and still they stalk the bear. Higher and higher into the cold mountains they climb as the pitiless snowdrifts to the sky. And into the sky they go, upward and upward, until they at last see the bear pushing the clouds before it.

Seeing the hunters, the mighty beast casts the net of invisibility over itself. But this time the hunters are not fooled. They rush to the spot where the bear lay.

At that moment, the bear raises the net above its head. As a result, the bear, dog, and hunters are all cast into the sky under an eternal cloak of invisibility.

We see them now only as stars. The hunters and their dog are condemned to roam the skies, ignorant of their fate, and ceaselessly chase the bear. They never quite catch it because the circumpolar stars of the Dipper circle the pole and never quite set.

In the autumn, when the stars of the Dipper are lowest on the horizon in the early evening, the hunters manage finally to wound the bear with their arrows, and the beast’s blood drips down on the trees and smothers and colors their leaves. The glorious colors of autumn thus “fall from a great height” and remind us of the pain and sadness of the long, cold winter yet to come.

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By Tom Burns

Stargazing

Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.