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Monday, August 21, 2017

Dark Moon Magic in the Day

 Eclipse of the sun! If you were one of the lucky ones to be in, or to have traveled to, the path of totality, I hope you thoroughly enjoyed the celestial magic. To those of you who, like me, watched the phenomenon from the much larger partial eclipse path, do not let the amazingness of the fact that it even happens at all, gets lost. I think it might be easy to do, in the wake of the excitement of its path of totality crossing a wide swath of the country.

Eclipses -- both solar and lunar, are phenomena based in the imperfections (not sure what word would work better, but that doesn't feel right) of the orbits of the earth around the sun and the moon around the earth. 

The path the earth takes around the sun and the one the moon takes around the earth are not both "flat" -- that is that they are not on the same plane. The Earth's orbit, known as the ecliptic, is by definition our reference point at 0 degrees inclination. The Moon's orbital inclination with respect to the ecliptic varies, but it is, on average 5.1 degrees. 

Think about this for a minute. The Moon's orbital inclination varies and most of the times that it goes 'round the earth, we just get the standard "new moon" (what I call dark moon) when it spends its days positioned more or less between us and the sun. As it moves around its orbit, we begin to see a small sliver of lit moon in the sky, the waxing crescent that appears in the western sky at sunset. As the moon continues around its orbit, it moves farther from being more or less between us and the sun and shows up later in the night until we see it fully illuminated at full moon. And then it continues around the orbit, rising later and later (or earlier and earlier by reference to morning, until we can only see that small sliver of waning moon, in the east just before sunrise.

We think of a lunar cycle as being 28 days, but while the moon completes its orbit around the earth in 27.3 days, due to the Earth's motion around the sun it has not finished a full cycle until it reaches the point in its orbit where it is in the same position with reference to the sun.

With the offset in orbital planes, not to mention the variation in the moon's orbit, the fact that they EVER line up is amazing... astronomical, in fact! 

Add to all of this, that for a solar eclipse to be total, the relative positions of sun and moon need to be just right for the moon to appear the same size as the sun. It's only by chance that the Moon and the Sun each take up approximately half-a-degree on the sky as seen from Earth's surface. Because both the Earth's orbit around the Sun and the Moon's orbit around the Earth are ellipses rather than circles, sometimes the Moon appears larger than the Sun, casting its shadow all the way down to Earth's surface, (a total eclipse when viewed from those locations on earth where the alignment is precise, but as a partial if viewed from other places nearby) while at other times the Sun appears bigger, with the Moon unable to completely cover the solar disk. This latter phenomenon is called an annular eclipse, and while nearly all of the suns's disk is obscured, we see a ring of sun around the moon, rather than the apparent flaming tendrils of the coronasphere that makes total solar eclipses so dramatic.

I am looking forward to the potential opportunity to see a total solar eclipse on April 8, 2024, right here in Maine. According to this image from Accuweather if the weather gods smile on us (April... Maine... start praying now!) we will only have to travel to northern Piscataquis, Penobscot or southeastern Aroostook
projecting the eclipse
counties. At the same time, I thoroughly enjoyed this opportunity to attempt to project this partial eclipse with a pair of binoculars, out in front of the house. Somewhere, I have an image of myself, as a teen, doing the same thing with my 4" refractor telescope, which I had recently completed. Believe me, I was longing for the good, solid mount for it that my dad built to go on a heavy, wooden surveyor's tripod that we found in one of our raids on the local surplus stores. By the time 2024 rolls around, I will have a more stable mount for whatever optics I use!