In the next several weeks we will have two eclipses, one lunar and one solar. One of them is rather ho-hum, but the other is so unique you will want to read about it, paying attention at the end for important safety information.
But first I wanted to report that last month's night launch of NASA's LADEE mission to the moon from Wallops Island, Va., was very spectacular. I observed it with some members from the Westminster Astronomical Society Inc. From our viewing location along the salt marshes, we had a good view of the rocket only 2.1 miles away. It was quite a spectacle as it leaped into the night sky on a thundering column of pinkish-orange flames.
Less spectacular was the launch several days later of the Antares rocket from the same location. That is because it was a daytime launch, and this time I observed it from the backyard, 144 miles from the launch pad. As this column is being written, its Cygnus payload has docked with the International Space Station, providing precious supplies to the astronauts on board.
Let us now talk about eclipses. A penumbral lunar eclipse occurs on Friday evening, Oct. 18. But just what is a penumbral eclipse, and is it even visible?
A total lunar eclipse occurs when the moon passes completely into the Earth's umbral shadow cast by the sun. What is an umbral shadow? The best way to explain it is to imagine an observer located inside of the earth's umbral shadow. From there a direct view of sun is completely blocked by the earth. The only sunlight reaching the observer is light that gets refracted by the atmosphere around the earth.
This refraction is what gives the moon its reddish tinge during a total lunar eclipse. But this month's eclipse is not a total lunar eclipse.
A partial lunar eclipse occurs when only a portion of the moon enters into the umbral shadow. That region would be very dark in comparison to the rest of the visible moon that lies inside the penumbral shadow. An observer located inside of the earth's penumbral shadow would see the sun partially blocked by the earth. The deeper one goes into the penumbra, the closer one gets to the umbra itself, and the less one would see of the sun due to its increasing blockage from the earth.
This month's eclipse is not a partial lunar eclipse because no portion of the moon touches the umbra.
Due to the geometry, any lunar eclipse occurs only at full moon. When it reaches the full stage this month, the moon will be sliding rather deeply into the penumbral shadow, and the darkening should be noticeable.
How noticeable? On its eclipse web site NASA estimates that "the beginning and end of a penumbral eclipse are not visible to the eye. In fact, no shading can be detected until about 2/3 of the Moon's disk is immersed in the penumbra." For the observer, this means that the nominal period of eclipse visibility on Oct. 18 extends from about 7:30 p.m. to 8:10 p.m.
Optimal time aside, the eclipse begins actually at 5:51 p.m. Thus, when the moon rises at 6:18 p.m. the eclipse is already in progress. The penumbral eclipse ends at 9:50 p.m.
The next eclipse is the unique one. Please keep reading for important safety information at the end. On Sunday, Nov. 3, a partially eclipsed sun will rise that morning for observers in Carroll County.
Maximum eclipse occurs at 7:10 a.m. But that's prior to sunrise and so will be blocked by earth's horizon. But when the sun does rise at 7:39 a.m., it will appear to have a big "bite" out of it amounting to 37 percent of its visible surface caused by the moon temporarily passing in front of the sun.
The partial eclipse will be visible from sunrise until it ceases at 8:10 a.m. In the intervening time the "bite" will continue shrinking as the moon ceases its occultation of the sun.
If you hope to see the November eclipse, you will need three things: 1) clear skies, a location such as a hill with an unobstructed view of sunrise and a safe method for observing the eclipse.
Great care must be taken when observing the sun. A rising sun will appear dimmed and reddened by the thick atmosphere near the horizon. The sun may not hurt to look at but is still a danger. Although its visible light is dim enough to cause no eye discomfort, harmful infrared radiation can still damage your vision.
One option is using projection. Directions for building an eclipse projector can be found on starpoints.org. Scroll down to the link for "Solar Eclipse Theater."
I would recommend getting a pair of black polymer eclipse glasses available at several October WASI activities. The first is a public star party at Soldiers Delight Nature Center in Baltimore County Saturday, Oct. 12 starting at 8 p.m. You can also pick up your eclipse glasses the following Saturday at the recently renovated Bear Branch Nature Center planetarium. The show starts at 7:30 p.m. Oct. 19.
For directions and registration information for these and other events, click on the calendar tab at the westminsterastro.org website. Eclipse glasses will be available for a $2 donation or three pairs for a $5 donation.