Wednesday, August 15, 2007

Eclipses and Showers

Two big astronomy events are due to occur in just two weeks. One is an eclipse of the moon, that is when the moon passes through the Earth's shadow, and the other is an extraordinary meteor shower that has occurred only three times in the 20th century and will not occur again for another 50 years, the Aurigids. Both will happen in the early morning hours. The eclipse will occur on the 28th of August and the meteor shower on September 1st. For both of these events, the West Coast will get the best show.

The lunar eclipse will have 90 minutes of totality with the umbral portion beginning at 3:51 a.m. PDT, that's when the moon starts to pass into the shadow, and totality commencing at 2:52 a.m. when the moon will be totally in the shadow. Totality will end at 4:22 a.m. and the umbral eclipse will end at 5:24 a.m. It will be so long because the moon will pass more deeply into the Earth's shadow. The whole event will actually begin at 1:51 a.m. when the moon passes into the penumbra. Here in California, we will get to see the entire show, but further east, the moon will set while still in eclipse.

An eclipse of the Moon can only take place at Full Moon, and only if the Moon passes through some portion of Earth's shadow. The shadow is actually composed of two cone-shaped parts, one nested inside the other. The outer shadow or penumbra is a zone where Earth blocks some (but not all) of the Sun's rays. In contrast, the inner shadow or umbra is a region where Earth blocks all direct sunlight from reaching the Moon. If only part of the Moon passes through the umbra, a partial eclipse is seen. However, if the entire Moon passes through the umbral shadow, then a total eclipse of the Moon occurs.

I remember viewing a lunar eclipse some years ago when I was at a rehearsal. The conductor allowed everyone to take a break and go outside to view the event for awhile. The moon was a coppery red and an eerie glow covered the ground. It was quite an experience.

On the other hand, I have not had a good track record viewing meteor showers. We just had the annual Perseid shower early in the morning on August 13th. I saw nothing, well maybe three, but I couldn't swear to it. I had stayed up until 1:00 a.m. when the meteors were supposed to be at their peak. All evening, every half hour or so, I went out to check on things. We had a new moon, so the seeing was supposed to be excellent. It was a warm night with no clouds or fog—until 1:00 a.m. Where did those clouds come from? It's is frustrating enough trying to do any astronomy from a big city location like Los Angeles with its light pollution, but being near the coast, we get a lot of fog, too. This time the clouds were high and coming from the east.

So I am planning to try something different for the morning of September 1st when the Aurigid shower will take place. This rare shower was formed by the dust trail of the comet Kiess which first passed by the sun (as far as we know) in 83 B.C. (give or take a century as the article in EOS, a publication of the American Geophysical Union, says). The comet returned in 1911 after completing one orbit! The dust trail comes into Earth's path a little more frequently. The shower will be brief lasting about an hour with the peak at 4:36 a.m. PDT and will be visible with the naked eye from locations in the western U.S.

However, while brief, it should be spectacular. When viewed in 1935, 1986, and 1994, the zenith hourly rate was about 200 meteors per hour. That's three times the rate of the Perseid shower. And they have been very bright, ranging from -2 to +3. (For your reference, negative numbers indicate brighter magnitude and the planet Venus appears at -4. The naked eye limit is +6.) The moon will be just past full, but should not diminish the display much (it is hoped).

For best viewing: Keep Moon out of field of view (best to block behind obstruction such as telephone pole, then watch whole sky), avoid city haze that scatters moonlight Best direction: East and Northeast Best time: Start one hour before peak, then see the rate of meteors increase and decrease while Earth travels through the shower.
If you play around with the handy Fluxtimator Java Applet on the Ames Research site, you can find out how well you will be able to observe the shower from your location. I'm planning to leave town.

1 comment:

  1. Are you planing on coming out our way to see the shower?