The Think Tank

So most of you probably heard that there was a Lunar eclipse last Monday night, August 27th.  According to the news, those of us lucky enough to live here in Oregon and Washington were in the absolute prime viewing area, peak viewing for the whole planet!  (Or at least the half that was experiencing night).  It started a little before 2:00AM and finished up around 5:30AM, just before sunrise.  Mom and I decided to take a drive up to Crown Point and Vista House to catch a glimpse of this beautiful event and were shocked to find that nearly 100 other people had the same idea!

We took a bunch of photos and managed to get a few pretty good ones, including the one at the top of this post, where I arranged a series of shots into one image with the eclipsed moon at the center.

I had never seen a total Lunar eclipse before, and I was surprised to discover that it was nothing like I thought it would be.  Instead of the Moon disappearing altogether as I expected, or being darkened and wreathed in light like a total solar eclipse, Lunar eclipses only change the appearance of the Moon, leaving it a dark rust color and making it difficult to distinguish her familiar characteristics.  It was a fascinating experience, so I thought I’d share a few interesting facts with you.  Enjoy!

Total Lunar Eclipses happen at least twice per year and like Solar events are only visible to a portion of the world for prime viewing.  In the August 28th eclipse, the west coast, British Columbia, Alaska and the southeastern tip of Australia were the only places on Earth where the full effect could be seen.

Lunar eclipses occur when the Moon passes through the Earths shadow, and because of the physics involved can only occur during a full Moon.  Given that there is a full Moon every 29 days (plus some change), you’d think we’d see a lot more eclipses then we do.  The reason for this is that the Moons orbit does not match the orbital plane of the Earth (called the ecliptic): its off by about five degrees.  That means that Lunar eclipses most often occur near the two intersections where the Moons orbit comes close to the ecliptic.  There are other types of eclipses that happen away from these intersections, but they are extremely rare.

Lunar eclipses come in two varieties: Umbral and Penumbral.  Most of the time during a Lunar eclipse the Moon passes through the Penumbra (think of it as the outer edges of Earths shadow), resulting in a slight yellow color and very little change in the Moon’s luminance.  The other kind of eclipse is when the Moon passes through the very heart of the Earth’s shadow, called the Umbra, where it receives very little solar radiation and darkens dramatically.  Umbral eclipses usually cause the Moon to turn a dark rust or maroon color, although Umbral eclipses that occur on the outer edges closer to the Penumbra can cause the edges of the Moon to turn blue or turquise.  The Moon won’t disappear completely from the sky, however, as it still receives some refracted light from Earths atmosphere.  If the Earth were a dead rock floating through space without any gases surrounding it, total Umbral eclipses would make the Moon vanish.

As it stands, Umbral eclipses like the one on the 28th leave the Moon dark red, and very eerie looking.  The red color comes around for much the same reason that sunsets turn red: the longer wavelengths are reflected off of the atmosphere, leaving the shorter (redder) wavelengths to dominate our senses.  The luminance is rated on a scale called the Danjon Scale of Lunar Eclipse Brightness, named for the French astronomer Andre-Louis Danjon (1890-1967) who created it.  Using a telescope equipped with a prism which split the image in two, Danjon was able to dial down the intensity of the side which was sunlit to match the side which was earthlit, giving him a scientific scale on which to measure the brightness of Lunar eclipses. His scale was a five point system, where 0 equated to a nearly invisible Moon, 1 was very, very dark (red/brown) and 5 was very, very bright (white).  The 28th eclipse that I witnessed saw the Moon very deep into the Umbra and rated somewhere around a 2 on the Danjon Scale, with the Moon passing only a few degrees south of the absolute center of Earths axis.

Because the Moon was so far into the Umbra, and because it was near its apogee (farthest point from Earth in its orbit), the totality of the eclipse on the 28th lasted more then an hour and a half, longer then any eclipse visible to the Northwest in more then 150 years.  The longest lunar eclipse on record occurred in January of 2000, with totality lasting 108 minutes, but sadly totality was blocked for viewers in the US by sunrise.  The previous best record was in May of 1505 when totality lasted 107 minutes.

The next Lunar eclipse visible to North America will be on February 21, 2008, and the Northwest will again be in a very good spot to view it, though we may miss part of the start as it will be taking place during moonrise.  Sadly totality will only last for 25 minutes (as opposed to 91 minutes this month), since the Moon will only narrowly pass inside the edge of the Umbra.  On the upside, if the February 2008 eclipse rates higher on the Danjon scale, there is a better chance of seeing some blue flame up around the edges, which should make for some really interesting pictures!

Not that I have too much hope, though.  There have been 6 total Lunar eclipses visible to North America since 1997, and all of them have either been hidden to Oregon and Washington by poor weather or obscured by moonset or sunrise.