Syzygy

Partridge says that this word comes from the Greek zeugnumi which means “yoke” or “join” and that gives us the modern sense of the Late Latin word that emerged of “linked” or “paired together.” In astronomy a syzygy is a conjunction of two bodies, actually three, because you have to include the body the observer is on. In the case of an eclipse we have the lining up of Sun, Moon, and Earth for the solar variety; and Sun, Earth, and Moon for the lunar variety.

This happens monthly. Perhaps you had one of those teachers who tried to show you moon phases using a flashlight and a tennis ball. When the moon is between the earth and the sun the near side is in shadow and hence can’t be seen. We call this a new moon. When the earth is between the moon and the sun the near side is fully sunlit. We call this a full moon. But the moon and earth are offset slightly, otherwise we would have a solar or lunar eclipse every two weeks. A solar eclipse can only occur during the new moon. A lunar eclipse can only occur during the full moon.

But we only get a handful of eclipses per year, not two per month. This is due to the inclination of the moon’s orbit about the earth. The earth and other planets lie roughly in the same plane if looked at from outside the solar system. That is why planets appear in the sky very close to the sun’s path. The apparent path of our sun through the sky is called the ecliptic. If we project this line infinitely into space in all directions we get the ecliptic plane. The orbits of our celestial neighbors lie along this imaginary plane:

Ecliptic_plane_side_view

The moon, though, has its own ideas. Its orbital plane is tilted about five degrees relative to the ecliptic. Here’s a way to visualize that:

Moon-inclination

As you can see, the moon only puts itself in a blocking position when it is at the so-called nodes where the two planes intersect. Thus eclipses can only happen then, when the moon is at those points. One is called the ascending node and the other the descending node reflecting our earth-bound sense of up and down. Here’s another look:

MoonEarthShadowsNodes

Most of the time the moon is “above” or “below” the ecliptic and cannot eclipse or block the sun. Thus most new moons don’t produce solar eclipses. The same with lunar eclipses—most of the time the moon is not in the same plane as the earth and so the earth cannot cast its shadow on the full moon’s face. In order to have eclipses the moon must be ascending or descending through one of its two orbital nodes.

The moon is at or near its nodes twice per year. So we ought to have two solar and two lunar eclipses per year. And we do, usually. But it’s not that simple. The nodes are not fixed. This “regression of the nodes” is due to precession, a phenomenon that all non-uniform spinning bodies exhibit. Think of the wobbling of a spinning top—that’s precession about the spin axis. If the nodes were fixed, eclipses would happen at the same two times per year, half a year (six lunations) apart. But they don’t, they can happen during any month. This is because the eclipse half-year is about nine days short of half a solar year and thus two such eclipse half-years are about 18 days short of a full solar year. Consequently conditions for an eclipse (moon at the nodes) move “backward” through the calendar. This was known to the ancients and formed the basis of some of the first eclipse predictions.

This month we will have a syzygy of epic grandeur, that of a total solar eclipse. The moon and the sun and the earth are all in the right places at the right time and observers in the United States will get a chance to experience the turning of day into night. Although the path of totality restricts the viewing of the total eclipse to a narrow band, all fifty states will experience a partial eclipse somewhere within their borders.

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