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  #1  
Old May 10th, 2012, 07:49 AM
Doug Kerr Doug Kerr is offline
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Default Annular solar eclipse on 2012.05.20

An annular solar eclipse will be visible (among other places) along a track across the western contiguous United States late in the day on May 20, 2012.

In an annular solar eclipse, the distance of the moon from the earth is so great that the moon's apparent diameter is less than the apparent diameter of the solar disk, leaving the outermost ring ("annulus") of the solar disk not blocked. The resulting effect is sometimes described as the "ring of fire".

In this case, the moon will be almost at apogee (at its greatest distance from the earth), a counterpart to its having been at perigee (closest to the earth) at the time of the recent "supermoon". Thus the unblocked annulus will be about as "wide" (radially) as is possible.

US cities from which the eclipse is "full" (in the sense described) include Medford, Oregon; Redding and Chico, California; Reno, Nevada; Albuquerque, Santa Fe, and Roswell, New Mexico; and Lubbock, Texas.

Here in Alamogordo, New Mexico we will see almost the full effect.

The track of visibility is shown here in a plot provided by NASA:

http://eclipse.gsfc.nasa.gov/OH/OHfi...2012-Fig02.pdf

The quasi-elliptical outlines show the extent of the moon's shadow on the Earth's surface at three different instants of time during the overall event.

At any place to the east of the line seen through Las Vegas, Nevada, the duration of the visible annular eclipse will be truncated by sunset.

Those interested in determining the exact circumstances at their location for any eclipse (over several millenniums) may wish to use this calculator (also provided by NASA).

http://eclipse.gsfc.nasa.gov/JSEX/JSEX-NA.html

Thanks to Fred Espenak of NASA for the wonderful reference materials.

In later days I will try and provide links to articles about safe visual observation of the eclipse and about photography of the event.

Best regards,

Doug
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  #2  
Old May 10th, 2012, 08:15 PM
Tom Robbins Tom Robbins is offline
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Thanks for the heads up and for the links, Doug. These events are rare enough that I can remember every one I've seen. It was interesting to determine the exact dates using the eclipse calculator.
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  #3  
Old May 11th, 2012, 08:22 AM
Doug Kerr Doug Kerr is offline
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In my ini6tial note on this topic, referring to the NASA "track" graphic, I said:
The quasi-elliptical outlines show the extent of the moon's shadow on the Earth's surface at three different instants of time during the overall event.
That description is a little imprecise.

First consider a "total" (as contrasted to annular) lunar eclipse. Here, from certain locations of observation, the entirety of the solar disk is blocked from view by the moon (perhaps "and then some"). The result is that (except for the effects of atmospheric scattering), no sunlight reaches the earth at those locations. They are in full shadow (called umbra, for "shadow").

At other locations, an observer will see only part of the solar disk blocked by the moon. (Part of the moon's disk lies outside the solar disk.) The result is that at such locations some sunlight, but not the "normal" amount, reaches the earth. They are in partial shadow (called penumbra, for "almost shadow").

For such a total eclipse, at any specific instant of time, we can plot on the surface of the earth a generally-elliptical figure that shows the extent of the umbra. Within that figure, at that instant (except for atmospheric scattering), no sunlight reaches the earth, and an observer will see the entire solar disk blocked out.

The outline is in effect the intersection with the earth's surface of a "cone of darkness" caused by the moon's presence.

But the upcoming eclipse is not total, but annular, and so the above does not exactly pertain. (The cone of darkness comes to a point before it reaches the earth's surface.) My guess is that the significance of the ellipse-like outlines is this. Anywhere within the outline, an observer would see the entire lunar disk within the solar disk. That is, the moon is blocking as much of the solar disk as it can. Nowhere is all sunlight blocked out.

In that case, the shadowing situation inside the outlines is called antumbra ("opposite the shadow").

This matter is nicely described in Wikipedia:

http://en.wikipedia.org/wiki/Eclipse

Best regards,

Doug
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Old May 19th, 2012, 01:14 PM
Doug Kerr Doug Kerr is offline
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A simple instrument for safely viewing a solar eclipse by projecting an image of the sun on a screen can be easily constructed. Here is one we made today, for use tomorrow (2012.05.20) for viewing the annular solar eclipse (visible almost fully here in Alamogordo, N.M.):



Douglas A. Kerr: Solar eclipse viewer by laboratories dak

The materials cost about $68.00 at Lowe's Home Improvement Center and included a free bathroom cabinet and some lovely Styrofoam blocks.

The viewer uses a pinhole aperture to create an image of the sun on a white paper screen. The image can be safely viewed. The box helps keep direct and incidental illumination off the screen



Douglas A. Kerr: Pinhole aperture

The pinhole aperture is a hole about 1.8 mm in diameter punched in a piece of heavy aluminum foil strengthened with packaging tape (also used to mount the aperture in an end flap of the carton) - the hole was punched in the foil-tape sandwich. The aperture was mounted on one end flap and the other flap closed over it; a 1" square hole had been cut in both end flaps.

Here we see Carla at the instrument, setting it up for a trial view of the full sun (nearly at zenith).



Douglas A. Kerr: Carla at the instrument

The image (only about 5/16" in diameter) can be seen on the white paper screen at the bottom of the instrument.

Here we see it close-up:



Douglas A. Kerr: Projected image of the full sun

A hand magnifier or loupe can be used to enlarge the viewed image.

Best regards,

Doug
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  #5  
Old May 19th, 2012, 05:59 PM
Bart_van_der_Wolf Bart_van_der_Wolf is offline
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Hi Doug,

Looking forward to the results.

Quote:
Originally Posted by Doug Kerr View Post
The materials cost about $68.00 at Lowe's Home Improvement Center and included a free bathroom cabinet and some lovely Styrofoam blocks.
Just wondering, wouldn't a black plastic bucket, filled with water, serve a similar purpose? The water surface serves as a mirror to the (subdued by reflection) incident light. Obviously it wouldn't come with a free bathroom cabinet, but still ...

Cheers,
Bart
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Old May 19th, 2012, 09:12 PM
Doug Kerr Doug Kerr is offline
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Hi, Bart,

Quote:
Originally Posted by Bart_van_der_Wolf View Post

Just wondering, wouldn't a black plastic bucket, filled with water, serve a similar purpose? The water surface serves as a mirror to the (subdued by reflection) incident light. Obviously it wouldn't come with a free bathroom cabinet, but still ...
Ooh, very clever.

Couldn't use it here for this event - the elevation of the sun when the eclipse fully hits is about 5!

Best regards,

Doug
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Old May 20th, 2012, 04:16 PM
Bart_van_der_Wolf Bart_van_der_Wolf is offline
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Quote:
Originally Posted by Doug Kerr View Post
Hi, Bart,

Ooh, very clever.

Couldn't use it here for this event - the elevation of the sun when the eclipse fully hits is about 5!
Hi Doug,

I didn't calculate it for your location. Five degrees indeed requires to fill the bucket (or dish) to the brim.

Cheers,
Bart
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Last edited by Bart_van_der_Wolf; May 21st, 2012 at 12:49 AM.
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  #8  
Old May 20th, 2012, 07:16 PM
Adrian Wareham Adrian Wareham is offline
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Got an okay shot with the 60D when I realized this was happening. oopse!



-Adrian
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  #9  
Old May 21st, 2012, 08:54 AM
Doug Kerr Doug Kerr is offline
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We are fortunate in that our new home town of Alamogordo lies just outside the band on the ground in which the full effect of the annular eclipse can be seen (that is, the moon's disk falls wholly within the sun's disk).

One of the two local astronomy clubs staged an eclipse viewing party in the parking lot of the New Mexico Museum of Space history, which is located on a high bluff at the northeast corner of Alamogordo. It affords a completely clear view to the western local horizon (which of course is itself constituted by a mountain range about 50 miles away).

(The other club had traveled to Santa Rosa, east of Albuquerque, very near the centerline of the ground track, where the full annular eclipse phenomenon could be seen.)

We had failed to make any provisions for actually photographic the eclipse (11-13 stop ND filter, for example) but we joined the party (taking along our pinhole viewing box, in case that turned out to be worth anything, which it was).

There was an excellent turnout, perhaps 125 people.

Here we see one of the club members setting up a large Celestron Schmidt-Cassegrain catadioptric telescope (I think 16" but I'm not sure). He had an eyepiece that made the solar disk almost completely fill the field of view. He had a "neutral" filter in the front (we would say "ND"). This instrument afforded a fabulous view of the sunspots as we viewed the un-eclipsed solar disk.



Douglas A. Kerr: 16" Celestron Schmidt-Cassegrain catadioptric reflector telescope

Here we see part of the crowd, at the edge of the bluff where numerous telescopes were set up, about half an hour before the eclipse began (which was at about 6:31 pm MDT):



Douglas A. Kerr: Crowd at the eclipse watching party in Alamogordo

A docent from the museum had set up his own personal solar observation instrument, a Sunspotter solar telescope. This is a clever folded Keplerian telescope that projects the image of the sun on a screen with a diameter of about 5". Its main purpose is to allow student to easily see the sunspots on the solar disk (and it did that just fine as we observed the eclipsed sun with it).



Douglas A. Kerr: Sunspotter solar telescope

Here a local math/science teacher helps her daughter observe the uneclipsed sun on the Sunspotter telescope, while the father shoots the image with his cell phone camera.



Douglas A. Kerr: A young astronomer

The docent had commented that the diameter of the sun is roughly 100 times the diameter of the earth. "Wow!", said the teacher without missing a beat. "A million times the volume".

You can see from the elevation of the instrument axis the low altitude of the sun (and this was a while before the eclipse actually began). By the time of maximum incursion, it was necessary to add a couple more books to the stack under the rear support of the telescope track to allow the telescope to attain the needed low altitude aim. There were plenty of Germans in attendance to give advice in this regard. (Recall that the Luftwaffe Advanced Flight Training School is here in town.)

[continued]
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Old May 21st, 2012, 09:03 AM
Doug Kerr Doug Kerr is offline
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[Part 2]

Here we see via the Sunspotter telescope the first incursion of the moon into the solar disk:



Douglas A. Kerr: First incursion

You can see the rendering of the sunspots here.

These show successive later phases of the eclipse:

[CENTER]
[CENTER]


Douglas A. Kerr: The annular eclipse progresses

This view was taken just after maximum incursion (at the instant of maximum incursion, I was observing through the 14" Schmidt-Cassegrain)



Douglas A. Kerr: Maximum incursion

The "horns" of the remaining crescent are a bit blocked by the inadvertent transit of an attendee in front of the instrument. Yes, professional astronomers do yell, "Down in front".

[continued]
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  #11  
Old May 21st, 2012, 09:10 AM
Doug Kerr Doug Kerr is offline
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[Part 3]

Although this is Mescalero Apache territory, the Cherokee are quite active as well.

Here Carla directly observes the eclipse through a pair of special viewing glasses, provided by an arm of NASA through the astronomy club:



Douglas A. Kerr: Carla views the eclipse directly

Here we see Carla observing the eclipse, shortly before the instant of maximum incursion, through the 16" Schmidt-Cassegrain:



Douglas A. Kerr: Carla at the 16" Schmidt-Cassegrain

Note the low elevation of the instrument axis at this time.

Overall, it was a lovely event, revolving around a truly unique occurrence.

Best regards,

Doug
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