Penning Penumbral Predictions

Jupiter reaches opposition on August 14th, and Neptune on the 21st. They’re about four degrees apart in eastern Capricornus. Uranus runs a bit ahead, in Pisces, so it’s easy to pick up all three on any August evening.

Saturn is barely glimpseable at dusk – catch it early in the month before it fades away completely.

Venus and Mars compete for your attentions in the morning sky ... a battle that Venus, at magnitude -3.8, will surely win over the dim and currently distant Mars.

Pluto is up too, though unfortunately not very far up – it only transits at 35 degrees, and that happens before the sky gets fully dark. Still, it should be reachable by a dedicated Plutocrat: look for it in northwestern Sagittarius, about a third of the way from M23 to M17. Here in the heart of the Milky Way, you’ll definitely need a good finder chart to tell it apart from background stars.

This month the moon gives us something of an oddity – *another* penumbral lunar eclipse, on the morning of the 5th. I wrote last month about July’s penumbral eclipse and how unlikely you were to see anything. Well, this one’s pretty much the same, except worse: this time, it happens during our day when the moon isn’t even up. So why mention it at all? Only because it seems relatively unusual to have lunar eclipses two months in a row.

But is it really that unusual? In fact, having two of them close together almost guarantees that neither one will be impressive visually.

Eclipses, either solar or lunar, happen when the moon is on the ecliptic during the new or full moon. That’s the only way the shadows can line up. Of course, the moon doesn’t have to be centered on the ecliptic; it just has to be within a few degrees. The moon takes up about half a degree in our sky, while the earth’s umbra – the dark part of our shadow – spans about a degree and a half. So if the moon is within two degrees of the ecliptic, we’ll see an umbral eclipse. Our penumbra is bigger, so we can get a penumbral eclipse, like the ones last month or this month, if the moon is within about three degrees of the ecliptic.

Of course, the moon’s orbit isn’t entirely in the ecliptic: it differs by an inclination of about five degrees. As the earth/moon system orbits the sun, the moon’s height above or below the ecliptic at full moon will change from month to month.

Normally, it changes by enough that if the full moon is on the ecliptic one month – a lunar eclipse – then by the next month, it will have moved off. But in a case like last month, a penumbral eclipse where the moon was just barely skimming across one edge of the earth’s shadow, it’s not all that surprising that the following month might find the moon at the opposite edge of the penumbra.

So how common is this? I was curious, so I grabbed tables of lunar eclipses for the previous and current centuries from http://eclipse.gsfc.nasa.gov and wrote a Python script to analyze them.

First I looked at how many eclipses happened per year. I found that it really wasn’t terribly unusual to have three eclipses in a year, with two in successive months: the 20th and 21st century each have 22 years in which that happens.

Less common is having four lunar eclipses in a year. That happens seven times in the 20th century and six in the 21st. 2009 is the first 4-eclipse year of this century, with penumbral eclipses in February, July and August and an umbral (but not total) eclipse in December.

What about eclipses within a month of each other? There are19 of those in the 20th century and 17 in the 21st. None are total, and almost all are penumbral; the only exceptions are May 1958 and April 2013. 1904 is the only year with two eclipses in the same month (March), while 1915, 1933 and 2096 each have two pairs of adjacent-month eclipses.

So this year’s double penumbral eclipse is interesting, but not quite as unusual as I had first suspected.

Speaking of the moon, NASA’s LCROSS mission has successfully launched. It will smack into the moon in early October (alas, probably during the wee hours of the morning here in San Jose) but meanwhile it should be sending back some nice pictures. So go to NASA’s LCROSS site and check out the photos while you’re not watching our invisible and only slightly rare penumbral eclipse on August 5th.