Thursday, November 15, 2012

Another meteor shower, another full moon

Well here it is the middle of a new month, the presidential election is over, there's yet another scandal involving high government officials with trouble keeping their ego and their libido (and the associated body parts) in check, and still I have not provided the astronomical highlights for November.  But not to worry, you haven't missed a thing unless you wanted to send a birthday card to Fred Whipple, the American astronomer who worked and taught at Harvard for more than 70 years - he was born on November 5, 1906 and, among his other accomplishments, invented the Whipple shield to protect spacecraft from small particles by vaporizing them. Other November birthdays of note are the eponymous Edmund Halley (of comet fame), born November 8, 1656, and my favorite astronomer Carl Sagan who was born on November 8, 1934.  And if you were in northern Australia on the 13th you might have witnessed the total solar eclipse that occurred there.  But other than that, you haven't missed a thing, so on to the second half of the month which is much more exciting (and not just because that's when my birthday falls).

First and foremost, the Leonid meteor shower peaks during the early morning hours of Saturday the 17th.  The moon will have set before the meteor shower gets going so we should be able to see about 20 meteors per hour as the Earth passes right through the debris field of the Comet Tempel.  So stay up late or get up early, either way it should be an experience that will make you glad you did.

The other highlight for me will be the full moon on the 28th (at 9:46 am so the viewing should be really good on the 27th, too), which is called the Frosty or Beaver moon (I'm going to let circumstances determine which one I settle on).  A penumbral lunar eclipse happens on the night of the 28th (OK, I didn't know what that means either, so here's what Google has to say about lunar
Lunar Eclipse Geometry
Geometry of the Sun, Earth and Moon During an Eclipse of the MoonEarth's two shadows are the penumbra and the umbra.
(Sizes and distances not to scale)

Types of Lunar Eclipses

An eclipse of the Moon (or lunar eclipse) can only occur at Full Moon, and only if the Moon passes through some portion of Earth's shadow. That shadow is actually composed of two cone-shaped components, one nested inside the other. The outer or penumbral shadow is a zone where the Earth blocks part but not all of the Sun's rays from reaching the Moon. In contrast, the inner or umbral shadow is a region where the Earth blocks all direct sunlight from reaching the Moon.
Astronomers recognize three basic types of lunar eclipses:

      1. Penumbral Lunar Eclipse

      • The Moon passes through Earth's penumbral shadow.
      • These events are of only academic interest because they are subtle and hard to observe.

      2. Partial Lunar Eclipse

      • A portion of the Moon passes through Earth's umbral shadow.
      • These events are easy to see, even with the unaided eye.

      3. Total Lunar Eclipse

      • The entire Moon passes through Earth's umbral shadow.
      • These events are quite striking due to the Moon's vibrant red color during the total phase (totality).
Now you might be wondering "If the Moon orbits Earth every 29.5 days and lunar eclipses only occur at Full Moon, then why don't we have an eclipse once a month during Full Moon?". I'm glad you asked! You see, the Moon's orbit around Earth is actually tipped about 5 degrees to Earth's orbit around the Sun. This means that the Moon spends most of the time either above or below the plane of Earth's orbit. And the plane of Earth's orbit around the Sun is important because Earth's shadows lie exactly in the same plane. During Full Moon, our natural satellite usually passes above or below Earth's shadows and misses them entirely. No eclipse takes place. But two to four times each year, the Moon passes through some portion of the Earth's penumbral or umbral shadows and one of the above three types of eclipses occurs.
When an eclipse of the Moon takes place, everyone on the night side of Earth can see it. About 35% of all eclipses are of the penumbral type which are very difficult to detect, even with a telescope. Another 30% are partial eclipses which are easy to see with the unaided eye. The final 35% or so are total eclipses, and these are quite extrordinary events to behold.) Aren't you glad you asked?
So there you go, night-sky watchers, set your alarm clocks and get out there to enjoy the splendor of nature in all it's glory - dress warm!
Here's a nice little piano piece titled "Lunar Eclipse" to listen to while you try to understand that whole "umbra/penumbra" thing - in fact I recommend that you just close your eyes, enjoy the music and try to forget I ever brought it up (that's what I'm going to do):

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