Bless That Tilt

The odds that the tilt of the earth’s axis relative to the sun’s (23.4°) will change in anybody’s lifetime—including the babies who are being born as I write—is virtually zero. Never absolutely—not in this dimension where everything is ruled by flux. But given that zero, it is amusing to discover how many people have taken time to think about the consequences that would ensue if our axis and the sun’s were parallel—not least among them Isaac Asimov. We did so too, this morning, talking about seasons, and that because its “unseasonably” warm on this late January day. If our own axis matched that of the sun (see link on this blog), seasons as we know them would disappear—so what else would change?

Endless opinion, mostly negative. Brigitte took a, for her, innately more positive stance—always discovering, at least where humanity was concerned, that we would be, as always, active and creative. Thus she rejected the view that humanity would just be scratching out a meager living in an essentially undeveloped state just north and south of the tropical regions. No. Humanity would make the most of it; if the season did not change, humanity would do the moving. But the environment, at least, would be much less interesting. No seasons, neither Spring flowers nor the dreary view of leafless trees. No animal—and worse yet no butterfly—migrations. Some say no technology would ever have developed for lack of hardship that winter provides. But then, Brigitte says, there is human curiosity—also left out of the equation.

Asimov predicts an environment in which Ice Ages are Ice Permanencies—and keeps most of his article on another subject. His article was originally published in the August 1977 issue of The Magazine of Fantasy and Science Fiction†; in it he is poking fun at John Milton who, in Paradise Lost, thought that the tilting of the axis was a kind of punishment that went with the Fall. To the contrary, says Asimov, the tilt was a blessing. Brigitte and I certainly agree. We’d rather believe in Global Warming causing this “hot” January morning (34° F) than the beginning of a slow process of axial tilt movement to the vertical. Global Warming, turns out, will have at least some of the same effects as a tilt-adjustment.

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†Available on this site—if you are willing to page down, down, and down until the magazine’s cover comes into view.

The Ecliptic

Today’s the day of the autumnal equinox. I understand it better this year than ever before. In the course of writing a post on the astrolabe for LaMarotte, I came to understand what’s known as the ecliptic. It is one of those maddening words. It reflects simultaneously two different points of view and two theories of the solar system. In one the sun goes around the earth; in the other we go around the sun. Here for instance is Webster’s definition of the ecliptic:

The great circle of the celestial sphere that is the apparent path of the sun among the stars or of the earth as seen from the sun: the plane of the earth’s orbit extended to meet the celestial sphere.

If it is a circle, why call it the ecliptic? It comes from creating eclipses, not the shape, although the earth’s orbit is ever-so-slightly elliptical, but not so much that you would notice. That path of the sun among the stars is the legacy of the geocentric view. It prevailed from the end of the Hellenistic era to the acceptance of Copernicus and Galileo in the West. The earth as seen from the sun is the other, the heliocentric view. Now, confusingly, virtually all pictures of the ecliptic have the earth smack in the center—so that the sun seems to be doing all the moving. That, of course, shows that our real orientation is egocentric. The ecliptic only interests humans because it explains the seasons on earth. An honest picture is the following:

The sun is squarely in the center. We do the traveling. The line we describe is the ecliptic. The sun is “in Aquarius,” as people say, meaning from our perspective; therefore it’s late January or early February. The circle of the zodiac was first determined by people waiting for sunrise. As soon as the first light appeared, they looked above that spot at the still dark sky. The constellation they detected right above the sun was the “house” in which the sun was rising. In July-August, when the earth will be where Aquarius is shown on the sketch, the dawn-watchers will see the constellation of Leo instead. Brigitte was born in Aquarius; I was born in Leo. Astrologically we complement each other. But, of course, neither Aquarius nor Leo ever move. Nor does the sun except around its axis. Anyone who could survive on the sun and live in a fixed spot there could divide his sun-day into twelve sun-hours by looking at the sky and reading off a sign of the zodiac. “See you no later than Sagittarius…”

But let’s now turn to the more usual illustration. An attractive one is show below from Wikipedia’s page on the Equinox (link). As expected, it shows the ecliptic circling the earth, whereas the earth is circling the sun, but never mind. What Wikipedia is doing here is depicting the first part of Webster’s definition above, the sun’s apparent path.

Notice next that the earth, which circles the sun following the ecliptic (the horizontal light-green ellipse), does so at a tilt to the ecliptic. This also means that our equator—and the “celestial equator” that we project from it (tilted dark-green oval)—are also at a tilt. As this diagram clearly shows, the ecliptic is therefore located below the equator through half the year and above it through the other half. And where we see the ecliptic, there we see the sun. Therefore the sun appears below the equator and then above it. The exceptions are two days of the year when the ecliptic makes its two crossings. Those days mark the autumnal and the vernal equinoxes; the sun is precisely over the equator. During those two 24-hour periods, day and night are of equal length. Note that the point of crossing is at the intersection of the vertical line drawn at 90° to our actual orbital plane, thus to the ecliptic.