Celestial Alignments

© 2002 by Robert C. Moler

The alignment of celestial bodies, real or imagined, have fascinated astronomers from the very earliest times to today. In fact the ancient pseudo-science for which astronomy was developed was all about alignments, called conjunctions and other imagined configurations.

The most spectacular alignment of all is that of the sun, moon and you in the total solar eclipse. I didn't mention the earth, because to see a total solar eclipse you personally have to be near the axis of the alignment to see it.

This month (February, 2002) we will be able to see, weather permitting, the last occultation in the current series of the planet Saturn, and the moon seen from here. That will occur on the morning of the 20th. Like a solar eclipse, what you see, or if you see the occultation at all depends on your location. You are part of the alignment.

Conjunctions, when two solar system bodies appear close together, occultations and eclipses are as frequent as they are because the solar system is essentially flat. The orbits of the planets, including the earth, lie close to the same plane. The big exceptions are Mercury, with a 7 degree inclination and Pluto, with a 17 degree inclination to the earth's orbit. The projection of the earth's orbit on the sky is called the ecliptic, the apparent path the sun seems to travel, as the earth, in reality moves around it. Since Mercury has a small orbit, it doesn't stray far from the ecliptic.

Even the earth's moon cooperates, because unlike most moons, which orbit over their planet's equator, its orbit of the earth is aligned close to the earth's orbit of the sun. The moon's orbit is inclined about 5 degrees from the ecliptic. Each month it passes each of the planets. And about every 5.7 months it causes two and occasionally three eclipses of various sorts.

Since the plane of the moon's orbit is tilted at an angle to the earth's orbit, the planes intersect at a line, called the line of nodes. The node where the two intersect as the moon is crossing the earth's orbit traveling northward is called the ascending node. The opposite node is the descending node. It is when the sun, moon and earth line up when the moon is near a node that eclipses happen. Any other time and the moon will be out of the earth's orbital plane too far north or south for its shadow to be cast on the earth or the earth's shadow to be cast on it. The symbol for the ascending node looks like the Greek letter omega. It's called the dragon's head. The descending node uses the same symbol, but upside down, called the dragon's tail.

The line of nodes is not always aligned in the same direction. The line of nodes of the moon's orbit slowly rotates westward at a rate of 0.053 degrees per day, making one rotation in 18.6 years. Since the direction of rotation of the moon's nodes is westward and opposite the direction of most motion in the solar system, the rotation of the moon's nodes is called regression of the nodes.

Early on in the history of astronomy Babylonian astronomers discovered an important interval, the saros, the interval when eclipses of the sun and moon repeat. That interval is approximately 6585 1/3 days. That's 18 years plus 10 or 11 and a third days, depending how many leap years are in the interval. That one third day makes me believe that the Babylonians actually calculated the saros interval, rather than be an empirical, observational value. It is rare that two successive saros eclipses would be visible from a single location, though we have one coming up. Now, after 3 saros intervals, those one third days would add up, and an eclipse would repeat for that location. Three saros intervals are 19,756 days, or 54 years and one month, approximately.

I saw my first total solar eclipse on July 20, 1963 from Quebec. The next saros found the eclipse crossing Asia and the Pacific on July 31, 1981. The next saros brought the eclipse crossing Europe and the middle east on August 11, 1999. The third saros brings it back to North America, with the path of totality running from Oregon to South Carolina, on August 21, 2017.

Obviously there are many saros eclipse series running at the same time. We have about 2 solar and 2 lunar eclipses each year, give or take.

The saros is the near coincidence of three lunar month definitions. It is 223 synodic months. The synodic month is the lunation, or the interval between new moons. It is 242 draconic months. The draconic month is the interval between successive passages by the moon of its ascending node, the dragon's head. Remember the moon must be near a node to cause an eclipse. The saros is also 239 anomalistic months, the interval between successive passages of the moon through perihelion, its closest point to the earth.

This last interval makes the saros period always the same. The moon's orbit is elliptical, so doesn't travel at the same speed through out its orbit. If this period wasn't coincident with the other two, each saros interval could vary by several days. Also, since the moon's anomalistic period matches, it would be at nearly the same distance from the earth each time. So some saros series have only annular eclipses, and some have long total eclipses. The rest are in between.

Now all these different months don't all multiply out to the exact same value. So there's a bit of a creep north and south of each eclipse. Eclipses at the descending node creep southward each saros, those of the ascending node creep northward. The eclipse of July 20, 1963 occurred at the descending node, so it's heading southward. That's why the 1963 eclipse crossed Canada, and the 2017 eclipse crosses the United States.

Because of the north or south shift there is a limit to the number of eclipses in a saros series. A series of solar eclipses starts with partial eclipses in the earth's polar regions, then become annular or total eclipses. The series ends with partial eclipses at the other polar region. Eclipse saros series of the moon start with penumbral eclipses, then partial, then total. They then become partial and penumbral again to end the series.

Occasionally we get to see two consecutive saros eclipses. On May 30, 1984 our family saw an annular solar eclipse from Piedmont Park in Atlanta Georgia. On June 10th this year (2002) we will see the next eclipse in this series, just barely. It starts an hour before sunset.

Questions? Send Email to me at bob@bjmoler.org

Updated: 01/30/02