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Last Sunday was the vernal equinox, or, as we say in English, the first day of spring. It is an occasion that often slips by unnoticed in Chicago. For us, late March brings blizzards more often than balmy breezes and nodding daffodils. We have to wait a whole month before the first pale green leaves emerge on the trees.

Nonetheless, the equinox, one of the two moments in the year when the sun is directly over the equator, happened on schedule at 3:39 AM on March 20. March 21 is the usual date, but the insertion of February 29 into this leap year advanced the event one day.

Way back in the sixth grade, I learned that the spring and fall equinoxes were the two occasions when day and night were of equal length--12 hours each--all over the world. But recently I found out that my sixth grade certitudes were just a little off.

I was I looking at a 1988 calendar published by the Field Museum. It features some excellent photographs by Dave Walsten, and it also lists the time of sunrise and sunset each day. I was surprised to discover that our closest approaches to equality of dark and light were four and five days prior to the 20th. Specifically, on the 15th the sun rose at 6:01 AM and set at 5:59 PM, giving us 11 hours and 58 minutes of daylight. On the 16th, Sol peeked over the horizon at 5:59 and set at 6:00, providing our closest approach to absolute equality.

On the 20th, we enjoyed 12 hours and 11 minutes of daylight. Why, I asked, myself, are we four days off here?

The first possibility that came to mind was that our calendar is out of kilter. Keeping accurate track of the passing years has been a problem for humanity since the beginning of our history. Some anthropologists think that the earliest elites used their knowledge of the timing of the seasons to justify their privileged positions. The presumed trade-off is something like "you support me in lavish style, and I will tell you when to plant your crops."

The Mayas of southern Mexico and Central America seem to fit that theory. Their astronomers calculated the length of the year as 365.2422 days, a calculation so accurate that our own astronomers have been able to improve upon it only in this century.

The Mayan elites made good use of their knowledge. At the ruins of Chichen Itza, the north side of the giant pyramid called El Castillo is adorned with a carving of Kukulkan, the Mayan version of Quetzalcoatl, the plumed serpent. The snake's tail is at the top of the pyramid, and the body descends in loops to a head at ground level.

The pyramid is precisely oriented so that on the morning of the equinox, the rising sun illuminates first the tail and then each successive loop until it reaches the head. The god Kukulkan is descending to earth. The importance of show biz to religion was recognized long before John Paul II and Jimmy Swaggart were around.

Speaking of popes, the calendars used in the Western world since the Middle Ages were both named after bishops of Rome. The Julian calendar was widely used until the 18th century, by which time it was two weeks off. Most of Europe then adopted the Gregorian calendar, which we still use. The Russians stayed with Julian until 1917, which is why the October Revolution is now celebrated in November.

The problem for all calendars is the .2422 of a day that is left over at the end of the year. We cant have people gathering in Times Square to ring in the new year at 5:45 in the morning one year and at 11:30 AM the next year, so we use leap years and other less frequent adjustments to keep ourselves more or less in sync with the earth's revolutions. Those adjustments are why the explanation for the length of the day on March 20th does not lie with the calendar.

The cause, as I learned from Jim Seevers, an associate astronomer at the Adler Planetarium, lies in the atmosphere, but before we get into the details on that one, we ought to understand why the length of the day varies at all.

If Plato had organized the universe, doubtless the earth would rotate on a perfectly vertical axis, the sun would always be above the equator, and we would have perpetual springtime. However, the evidence suggests that God is not a Platonist, so we have an earth that rotates on an axis tipped 23.5 degrees from the vertical.

I asked Jim Seevers why we had this tilt, and he said it was a good question, especially since the moon's gravitational pull is constantly trying to tug us into a vertical position. However, the earth weighs six quadrillion tons and it started out spinning this way, and anything that weighs six quadrillion tons is hard to move. Which is another way of saying that the earth is tilted 23.5 degrees from the vertical because it is tilted 23.5 degrees from the vertical.

As the earth revolves around the sun, the tilt changes our relationship to that star. When it is summer here, the tilt leans the northern hemisphere toward the sun and the sun seems to move north until it is directly above the Tropic of Cancer, 23.5 degrees north of the equator. In December, when we are on the other side of the sun, the tilt favors the southern hemisphere and the sun reaches the Tropic of Capricorn, 23.5 degrees south of the equator.

The tilt causes the changes in our weather from season to season. When the sun is high in the sky, its rays hit us more directly and thus we receive more energy per square foot than we do in winter, when the sun is lower. You can demonstrate the effect with a flashlight and a piece of paper. Hold the paper at right angles to the light, and the beam will be concentrated in a very small area. All the energy will be hitting that one small space. Tilt that paper so that the light hits at 45 degrees, and the same amount of energy will be spread over a much larger area

In spring and fall, the earth is tilted sideways. It leans to the right or left relative to the sun, and not toward it or away from it. That is why the sun is directly over the equator and the days and nights are of equal length.

Or almost. As I said, the atmosphere upsets the perfection of this arrangement by distorting the light we receive from outer space. No doubt everybody has noticed that both the sun and moon look large and red as they rise and set and get smaller and paler as they climb into the sky. When they are near the horizon, their light has a longer path to travel through the atmosphere, and the trip through this lens enlarges them; it also filters out most of the blue light, making the red much more prominent.

The atmospheric lens also bends the light from space. This curvature causes us to see the sun while it is still actually below the horizon. When we look to the east and see the top of the sun at the official moment of sunrise, we are actually looking at a mirage. Thus our days are slightly longer than they would be if the earth did not have an atmosphere.

The precise moment of sunrise can also be thrown off by differences in atmospheric pressure. If there is high pressure to the east there is more air for the light to pass through and more refraction. Low pressure means less refraction. The moment of sunrise or sunset can vary as much as 30 seconds either way depending on differences in barometric pressure. Which is why the official times of sunrise and sunset published by the Naval Observatory are stated to the nearest minute, not to the nearest second.

The equinox is also the day when the sun rises and sets at due east and due west. In the winter, the sun comes up south of east, and by midsummer it will be rising and setting about 30 degrees north of east and west. If you have a north-facing window, you can see this phenomenon quite easily. Starting now, you will begin to get direct sunlight in the room each morning and evening.

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