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