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Hello again Kostronomers, it certainly has been a busy week for sky watchers! In case you didn't know there is an eclipse this month!

Some time ago, as you may know, I wrote a little diary that I will reprise today in light (or lack thereof as the case may be) of the forthcoming annular eclipse of May 9th/10th. I have to admit that I'm quite jealous of the lucky residents of a few western states will experience that eclipse in a couple of weeks!

To introduce you to the subject of eclipses I've embedded a video. If you've followed me at all you might know that I'm a big fan of Brady Haran the video journalist. Brady produces what could be described as back room interviews with scientists who talk about various subjects in their disciplines. I find them intriguing because we get a chance to know not only the men and women scientists but their sciences as well. In one of his films, produced for one the website Sixty Symbols Brady introduces us to astronomer Dr. Amanda Bauer during her trip to the University of Nottingham's Ningbo, China campus to observe the total eclipse on July 22nd, 2009.

Will the curly cloud disperse in time for totality?



Well I hope y'all won't feel like I tricked you. This diary may take a bit more than 15 minutes of your time. But I've warned you, the rest is up to you.

I'm sure you all have some understanding of solar eclipses, at least enough to know that making lots of noise will chase the sun eating dragon away. Although it may look as if the Dragon Moon swallows a helpless Sun during an eclipse the Moon is, of course, much much smaller than the Sun. It is actually a happy coincidence that in the epoch of man the Moon's apparent size nearly perfectly fits over the disk of the Sun.

The Sun's distance from the Earth is about 390 times the Moon's distance, and the Sun's diameter is about 400 times the Moon's diameter. Because these ratios are approximately the same, the Sun and the Moon as seen from Earth appear to be approximately the same size: about 0.5 degree of arc in angular measure.1

Yet the Moon moves in an elliptical orbit. Sometimes it is closer to Earth, sometimes farther away. The Earth itself also moves in an elliptical orbit with the same effect with respect to the Sun (though to a much lesser extent—relatively). Considering these two facts helps us understand that the apparent sizes of the Sun and Moon can vary. Essentially the type of eclipse depends on the Moon's apparent diameter.

There are four types of solar eclipses:
  1. A total eclipse occurs when the apparent diameter of the Moon matches or is greater than the apparent diameter of the Sun and totally covers the Sun. Only a small band of the surface of the Earth will experience 'totality'.
  2. An annular eclipse occurs when the apparent diameter of the Moon is less than that of the Sun. When the Moon fully covers the Sun a bright ring or annulus appears to the observer.
  3. A partial eclipse is where the Moon partially covers the face of the Sun. The Sun and Moon are not directly in line with respect to the observer.
  4. A hybrid eclipse are rare (A & B type) eclipses which occur when at some points of the transit of the Moon's shadow it appears as a total eclipse while at other points it appears as an annular eclipse.
Solar eclipse types
  1. Total eclipse in the umbra.
  2. Annular eclipse in the antumbra.
  3. Partial eclipse in the penumbra.
Figure 2
Picture not to scale
There can be between two and five eclipse in a year, usually two or three. There have been six five eclipse years since 1582 in 1693, 1758, 1805, 1823, 1870, and 1935. If you can hang on long enough there's another 5 eclipse year comming in 2206 2

Figure 3. displays the path of the July 22nd, 2009 (the eclipse in theSixty Symbols video).The area of total eclipse is displayed by the dark shadow traversing the globe. A Partial eclipse might be seen by observers in the lightly shadowed areas.
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Figure 3. Path of July 22nd, 2009
Total Eclipse.
Totality was visible in many large cities. The July 22nd, 2009 eclipse was visible all across India and China. Varanasi, for example, in India and Ningbo, Shanghai, Chapai Nawabganj as well as over the Three Gorges Dam in China. However, in Shanghai, the largest city in the eclipse's path, the view was obscured by heavy clouds. According to NASA, the Japanese island Kitaio Jima was predicted to have the best viewing conditions featuring both longer viewing time (being the closest point of land to the point of greatest eclipse) and lower cloud cover statistics than all of continental Asia.3
Brian Cox also went around to the other side of the world to view this eclipse for BBC 2's Wonders of the Solar System


The site of greatest eclipse for a total eclipse, mentioned above, is instant that the cone of the Moon's shadow comes closest to center of the Earth—the location of longest duration.4 The July 22nd eclipse (the eclipse in both the videos above) was the longest of the 21st century, lasting 6 minutes and 39 seconds and is a member of the same Saros cycle as the famous May 29th, 1919 eclipse—the Einstein eclipse. A Saros cycle describes a set of eclipses where the Sun, Moon, and Earth return to the same configuration thereby producing similar eclipses. As you can see below the tracks of Saros 136 are very similar. A Saros cycle has "a period of 6585.322 days (14 normal years + 4 leap years + 11.322 days, or 13 normal years + 5 leap years + 10.322 days)"Wiki
X

Of course, the further away you are, as in the case of STEREO-B the shadow gets smaller. Since STEREO-B is 4.4 times further away from the Moon than Earth the shadow the Moon casts against the Sun's disk is 4.4 times smaller as you can see from the instrument calibration video shown below.



STEREO-B has a sister ship named STEREO-A. Both are on a mission to study the sun. While STEREO-B lags behind Earth, STEREO-A orbits one million miles ahead ("B" for behind, "A" for ahead). The gap is deliberate: it allows the two spacecraft to capture offset views of the sun. Researchers can then combine the images to produce 3D stereo movies of solar storms.5
Over the next ten years there will be 6 total, 7 annular, 1 hybrid, and 10 partial eclipses.6



Photobucket
The May 9th/10th eclipse will be an annular eclipse—the moon will not cover the sun completely. It will begin in the state of Western Australia and finish in the eastern Pacific. It will be visible as a partial eclipse in Hawaii and possibly from Baja California.  The full track is shown in the graphic to the left. The race of the the shadow of the moon beginning at the terminator and ending at it as well is very interesting (at least to me it is ☺). We should thank Dr. Fred Espenak of the Goddard Space Flight Center. There is probably no greater expert on eclipses that I know of. Much of his work can be seen at the NASA Eclipse Web Site.

Photobucket Pictures, Images and Photos Photobucket Pictures, Images and Photos Data sheet for the annular eclipse of May 20-21, 2012.



So, when will the next total eclipse occur in the United States?


Photobucket Pictures, Images and Photos
The next total eclipse to occur over the US mainland will be on August 21, 2017. It will begin out in the Northern Pacific ocean midway between the Aleutians and Hawaii and then cross the entire continent starting near Corvalis, Oregon and pass out into the Atlantic near Charleston, South Carolina. The site of greatest eclipse will be in Christian County, near Hopkinsville, Kentucky at about 1:25pm on August 21st, 2017 and will last for about 2 minutes and 40 seconds. Imagine the hype, it will be tremendous, and I'm just 400 miles away. Party! I'll bet you can hardly wait now, too.

Photobucket Pictures, Images and Photos


Photobucket Pictures, Images and Photos
If all that beer and fireworks just isn't enough for you and you can wait a bit, in just seven more years another eclipse which will track an intersect with the 2017 eclipse within miles of Southern Illinois University (Go Salukis). In this case the time of longest eclipse will be near the little town of Lazaro Cardenas del Rio west of Monterrey. It will pass through Texas near Laredo, cross the track of the August 2017 eclipse and then on to Houlton, Maine.

Photobucket Pictures, Images and Photos


Footnotes
0Eclipse Photos: "Ring of Fire" Shines Over Africa, Asia
1Solar eclipse
2Solar Eclipse #Frequency
3Solar eclipse of July 22, 2009
4Fred Espenak;Glossary of Solar Eclipse Terms
5STEREO Eclipse - NASA Science
6Fred Espenak;Solar Eclipses: 2011 - 2020
7Helium
8Spectrograph of Helium
9Photographic plate of May 29, 1919 Eclipse
10Gravitational deflection of light
11Equivalence principle
12Lunar eclipse
I haven't included everything from my original diary. The sections on the discovery of helium, on Einstein and Eddington and the May 1919 eclipse, and about lunar eclipses. If you're interested in those topics click the link.

Thank you for taking the time to read this article. I look forward to your comments below.

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