So….two Full Moon posts in one month? Well it’s actually appropriate as in my previous Full Moon post I touched on the lunar eclipse and the “rare” selenelion that occurred on October 8th. But as far as the month of October goes, that wasn’t the full extent of the moon and sun’s celestial dance.
I know, you’re probably tired of hearing about orbital geometries in my posts by now, but in truth they explain the continuous rhythms of the two biggest acts in our sky’s showcase because like everything in nature, eclipses follow a rule of cycles. Yes, while it may seem that the varied differences in the orbits of the earth around the sun and the moon around the Earth would generate an eclipse schedule of complete randomness, they actually occur in a set pattern, a pattern that is dictated by the length of their orbit and the angles in which they all occur. Let’s take a basic look.
First we have the saros, an almost two decade time span to us that is nothing more than the blink of an eye in the cosmic scale. A saros is approximately 6585.3 days and marks the time it takes for the sun, Earth and moon to return to almost the exact same point relative to each other, that they started from at any given moment. This means that if there is a lunar or solar eclipse on any given day, the exact eclipse will occur 18 years and 11 1/3 days later (although it will not be visible from the same location on Earth each time). These identical eclipses are referred to as an ‘eclipse cycle‘ of which there are many, each having its own number, and are used to predicate and monitor eclipses by scientists all over the world.
On a smaller scale we have the synodic month, or 29.53 days (average, nothing is absolutely perfect as far as these things are concerned), the amount of time it takes for the moon to circle the Earth in one orbit. This quick trip around our planet is significant in the scheme of eclipses because the apparent angle of the orbit of the moon differs from that of our sun by a mere 5 degrees at their widest, which isn’t a lot it grand scheme of the cosmos, but it’s enough to have them skirt above or below each other for most of their orbits.
But at certain times the moon’s orbital plane crosses that of the sun’s at points called nodes, and these are the times when eclipses occur. If the moon is moving down across the orbital plane of the sun it is called the descending node, the opposite node, when the moon travels back up through the sun’s plane is called the ascending. This is a rather oversimplification of the whole eclipse process, as there are other variables involved, but its enough to give a basic understanding as to why we don’t see eclipses at every full moon (lunar) or new moon (solar) occurrence.
So while the fact that it takes the moon a mere 29.52 days to circle the Earth is an important constant in the equation of the eclipse cycle, it is a very important component in another phenomenon – eclipses are linked.
The time it takes the moon to go from one spot in its orbit to the complete opposite point is one half of a synodic month, called a fortnight (14.77 solar days). Yeah, that’s right, a fortnight isn’t just for you English Literature majors, it’s an actual thing. This means that the time it takes for the moon to go from full to new is fast enough so that the moon and the sun do not have time to, planarly speaking – get out of each others way. This results in a solar eclipse one fortnight after almost all lunar eclipses, the solar eclipse always happening in the opposite node from the one the lunar eclipse occurred in.
Such is the case this month, when the moon travels half way around the Earth from our recent Oct 8th lunar eclipse in the descending node for another rendezvous in our sky with the sun on Oct 23rd in the ascending. On that day, the moon’s passage between the Earth and the sun will give a large portion of North America a nice glimpse at a partial eclipse. The eclipse is partial due to the fact that center of the moon’s shadow (where a total eclipse can be seen) glances over our planet’s northern most point, which means the eclipse is not visible in the Southern hemisphere or in Europe where it’s night time. Unfortunately not everyone in North America is equally as lucky when it comes to this eclipse.
Just like the lunar eclipse earlier this month, this solar eclipse will be diving quickly out of the sky for most of us on the east coast. In fact, for those of us around Wilmington maximum eclipse occurs directly at sunset at 6:10pm, a mere 18 minutes after the moon first touches the solar disk. Sadly, many on the extreme east coast (like Boston) will be out of luck as the Earth’s shadow won’t make it there before the sun slips below the horizon.
So is this the best trick the moon and the sun have to offer? Of course not. On rare occasions, an eclipse alignment happens so that a whole synodic month can occur before the nodal alignment is broken. When this happens, we get three eclipses in a synodic month, each a fortnight after the previous. The next time this happens is 2018 with a partial solar eclipse(A) on July 13, and total lunar(D) on July 27, and the cycle ending with a partial solar(A) on August 11th. After that you’ll have to wait until 2020, when two penumbral lunar eclipses (June 5th and July 5th, both descending) bracket an annular solar eclipse(A) on June 21.
A solar eclipse on the summer equinox? I can’t wait to see all that doomsday hysteria!
Time for another beer…