Friday, May 25, 2012
An annular solar eclipse occurs when the moon is at the more distant part of its earth orbit, passing between the sun and the earth so that its shadow crosses the surface of the earth.
Unlike a total eclipse when the moon appears large and covers the sun because it is closer to Earth, in an annular eclipse the moon appears smaller and leaves a solar ring around the moon.
To see the eclipse you must be within the moon's roughly 150-mile-across shadow. Since the earth and moon are in constant motion, the shadow travels at a speed greater than 1,100 mph across the surface of the Earth. An annular eclipse occurred Sunday.
I had never seen an annular solar eclipse, and this looked like my best chance to do so. The darkest part of the shadow would not cross Walla Walla but would pass about 400 to 500 miles south of here, between Crescent City and Susanville, Calif., in late afternoon.
The sun would be low in the western sky, making the clear view over the ocean at Crescent City my first choice. Clouds can be a problem on the coast, so my second choice was Susanville.
We cannot view the sun directly without proper eye protection, so I ordered eclipse glasses and viewing cards online. However, I wanted to get a larger view of the sun. (Without magnification, the sun appears to be only a quarter of an inch across.) I found a good solution in an article entitled "Build a Sun Funnel" in the June 2012 issue of Sky and Telescope magazine.
The sun Funnel is made from a standard Blitz Super Funnel with a telescope eyepiece at the small end and a piece of Da-Tex rear-surface projection screen material stretched over the open end. (Go to ubne.ws/KZKbjA for construction instructions.)
With a small refracting telescope, a projected image of the sun about three inches in diameter is created on the sun funnel screen. This provides a larger image of the sun that is easy for several people to see, and I could also take pictures of this image. I ordered the rear-projection screen online and then built the sun funnel. A test of the system on the normal sun showed that it worked well, with a great image. I was ready for the eclipse.
On May 19, I checked the weather: Crescent City did not look good, but Susanville was forecast to be clear. Saturday afternoon I left for Burns, Ore., where I spent the night. I met two university students, Shannon Regan and David Gross, in Burns, and they rode with me toward Susanville. Because of low clouds, we stopped just short of Susanville at Honey Lake State Wildlife Area, where the weather was clear.
Set-up and preparation are important. From start to finish, the annular eclipse would last less than five minutes, so we wanted to be prepared well before it began.
We arrived at 2:30, quickly set up our telescope with the sun funnel, and checked to be sure we would have a clear view of the path of the sun during the entire eclipse. We assigned roles: David and I would adjust the telescope to track the sun, while Shannon took pictures.
The partial eclipse started at 5:13 local time. The part of the eclipse of greatest interest to me, the annular eclipse, started at 6:26, with maximum eclipse at 6:29 local time. Because we were only 8 miles from the centerline path, the moon was very close to centered in the sun, with only 3 percent of the sun showing. This portion of the sun, visible all the way around the moon, is called the ring of fire.
As the moon covered the sun for these three minutes, the temperature at our site dropped a little more than 10 degrees F. We continued our observations until the end of the annular eclipse phase at 6:31. Then we packed up, and Shannon and David watched the end of the eclipse while I drove back to Burns for the night. Even though Shannon and David drove all night to get back to Walla Walla, they agreed it was well worth the drive.
Remember, another event -- the transit of Venus -- is coming up June 5. The sun funnel would be a good option for viewing this event if you have a telescope. Otherwise, go to ubne.ws/LzGnre and watch it online.
Note: Don Pettit took pictures of the shadow created by the eclipse from the International Space Station that NASA posted at ubne.ws/KH3aNB.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.