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During the late afternoon and evening of Sunday May 20, many locations in the western third of the U.S. will be treated to an eclipse of the sun. Yes. You will need protection to view with the naked eye. This diary will present some information and links so you can enjoy the event. The eclipse is also related to the SuperMoon event, half of a lunar cycle before. The diary will explain how the two events are related.

This is not a total eclipse of the sun. It is called an annular eclipse. Annulus means ring. The apparent diameter of the moon will not quite be large enough to cover the entire face of the sun. As a result, some of the sun will still be exposed. For people located along the centerline of the eclipse, this will mean they get to see the entire ring. For people away from the centerline, you get to see the sun with a bite taken out of it. In 2005, some locations in Spain saw this annular eclipse. It is a good example of what you might see if you are fortunate to live along the centerline.

Please join me below for more details about this upcoming event.

The Lunar Orbit is Not Circular

The orbit of the moon is not quite circular. When it is closest to earth, it is called perigee. It appears larger due to its closeness. The full moon SuperMoon occurred at this time. It appeared larger than normal. The farthest point is known as apogee when the moon appears smaller. The difference between perigee and apogee is about 14% as illustrated here.

When the moon aligns directly between the earth and the sun at perigee, a solar eclipse occurs. This image shows the moon diameter larger than the sun diameter and can cause a total eclipse of several minutes with the sun completely blocked during that time.

When the moon aligns directly between the earth and the sun at apogee, a solar eclipse occurs. But, the moon diameter is not quite large enough to completely cover the sun. This is illustrated by the middle part of the following diagram. In that case, the sun is still able to shine around the perimeter of the moon and illuminate the earth. This effect shows a ring of the sun, or an annulus. Those of you living along the centerline will see this effect. The first image above from Spain is an example. Areas outside of the centerline will see a portion of the sun blocked as shown in the bottom part of the diagram.

Where is the Centerline of the Eclipse?

The centerline where annularity occurs is along a narrow band from the coast of China to the United States near Lubbock, TX. Shown here in the chart from NASA. It passes near Japan and the Aleutians before reaching the U.S. in the late afternoon and evening.
The annular ring is quite thick because the Moon's apparent diameter is only 94% that of the Sun. Traveling with a velocity of 1.1 kilometres/second, the antumbral shadow leaves Japan and heads northeast across the Northern Pacific. The instant of greatest eclipse occurs at 23:52:47 UT when the eclipse magnitude reaches 0.9439. At that instant, the duration of annularity is 5 minutes 46 seconds, the path width is 237 kilometres and the Sun is 61° above the flat horizon formed by the open ocean.

For the U.S., this chart shows the centerline from Eureka to Reno to Albuquerque to Lubbock. The sun will be low in the sky approaching evening. If you live within the gray band, you will have a chance to see the annular eclipse if weather permits. If you live outside of the gray band, you will see an eclipse with a large chunk of the sun darkened.

You can use this online tool to calculate your specific local conditions. The tool allows you to select a city, or a location from latitude and longitude, and find the details of the eclipse for your location. For an example, I chose Albuquerque as the city. Below that, I chose the correct century of 2001-2100. That brings up a table in part 3 with the 4th row for May 20. It shows the beginning of partial eclipse as 17:28:24 local time. That is 5:28 pm local time on the 12 hour clock. The annular phase lasts from 18:33:38 to 18:38:04, or 6:34 to 6:38. That is only about 4 minutes. Then, the eclipse goes back to a partial phase. Sunset occurs during this phase.

In Summary...

Most areas of the western U.S. will be able to see a partial eclipse. It will last all evening until sunset. Some areas along a narrow band will have the added opportunity to see the annular phase lasting only about 4 minutes. You will probably notice a subtle darkening of the sky. But, because of the effects of the evening time, this might not be something that would get your attention. It is interesting to watch for it.

Viewing Tips and Links

• Local astronomy clubs will be offering safe viewing opportunities. Over 700 clubs are listed here by state. The Albuquerque example listing reached this page.

• Pinhole viewers are safe and easy to make. This from the Exploratorium shows a couple of examples. You can also find others with a Google search.

• Some locations with the National Park Service offer programs such as this one with Glen Canyon.

• Some sites might be showing it live online. At this time, I don't have a listing. I did try entering 'live annular solar eclipse' into Google with very limited success.

• It is possible to project through one side of a pair of binoculars onto a light colored surface. I caution against that because the heat might damage the optics of your eyepiece. If you have an old cheap throwaway pair, maybe you can justify trying it.
Do NOT attempt to look through toward the sun.

• Eclipse viewing glasses are available. Many will be provided free by astronomy clubs. Be certain you are using certified safe solar viewing glasses. You risk eye injury with anything less. Some specialty astronomy stores might be in your area and will sell them. They can be bought online, but it is late to order now.

I hope you have perfect seeing conditions for your area. This is a unique type of eclipse. Get out that evening and end the daylight hours with a truly spectacular solar event. Happy viewing.

Originally posted to SciTech on Tue May 15, 2012 at 07:42 AM PDT.

Also republished by Astro Kos and J Town.


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