Friday, September 23, 2011

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.


  1. One of the best things about watching the planets on a semi-regular basis, and especially watching Jupiter's moons with binoculars, is that you get a real sense for where the ecliptic lies in the sky. When you visualize it correctly, you suddenly understand that those of us here in Michigan are standing at a 45-degree angle -- more or less, depending on the time of year -- to the entire solar system. And then if you do it regularly enough in a dark place, you also get a much better sense for where the ecliptic fits in the Milky Way as a whole.

    It's one of the things I've always liked about watching the skies. It makes me feel more connected to the solar system and the greater universe by better understanding my place in it.

  2. So right. I got a sense of that during our years in Minnesota on evening, night-walks with a big-sky view and not much urban light, much as I assume our ancestors saw the world. Here it is much more difficult. Nice to be on the edge of the metro and the shore of a lake...

  3. FINALLY! i have found a correct and clear explanation of the ecliptic! thank you!