October 17, 2017

AstroEvent of the Week: 6.7.09: Can you spot the Penumbral?



This week, I give you what is surely the meekest of all eclipses; a very shallow penumbral eclipse. At about 5:38 Eastern Standard time on the morning of Tuesday, July 7th, the northern edge of the Moon will find itself not quite a quarter submerged in the Earth’s penumbra, the light outer part of the planet’s shadow. The geometry for most of the continental United States is not good, as the Moon will be setting in the brightening dawn twilight. So why should you care to wake up early for an almost non-eclipse? Two reasons; first, the challenge put forth here at Astroguyz is whether such a subtle eclipse can be noticed at all… “before” and “during” photographs can help in this aspect. The farther west of the US east coast you are located, the better your prospects of success. Secondly, this pass at the ascending node of the ecliptic sets the Moon , Earth, and Sun up for this months’ big event; a very long solar eclipse in two weeks time. More to come!

The astro-word of the week is saros. This cycle is all important to predicting and understanding the geometry of lunar and solar eclipses. One saros cycle is equal to 18 years, 11 days and 8 hours. After which time, an eclipse will occur shifted 120 degrees westward. That’s where the eight hours comes into play. Don’t fear the math= 8/24=1/3. 1/3 x 360=120! For example, an eclipse (an even worse penumbral!) related to this Tuesdays will occur on July 18th, 2027 that will “favor” the Indian Ocean. Of course, the specifics of the related eclipses can vary. This week’s partial-penumbral is part of saros 110. The life time of a typical saros cycle lasts around 1550 years, with a typical saros producing about 72 lunar eclipses. At member 71, you can see that cycle 110 is about played out!

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