APOD: Millennium Annular Solar Eclipse (2010 Jan 22)

[APOD Robot]

Millennium Annular Solar Eclipse

Explanation: The Moon's shadow raced across planet Earth on January 15. Observers within the central shadow track were able to witness an annular solar eclipse as the Moon's apparent size was too small to completely cover the Sun. A visually dramatic ring of fire, the annular phase lasted up to 11 minutes and 8 seconds depending on location, the longest annular solar eclipse for the next 1,000 years. This picture of the Moon's silhouette just before mid-eclipse was taken within the eclipse path from the city of Kanyakumari at the southern tip of India. The telescopic image was made through a filter that blocks most visible light, but still transmits light from hydrogen atoms. As a result, detailed mottling, or granulation, caused by heat convection in Sun's atmosphere can be seen around the dark lunar disk.

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[ajbock]

In today's explanation of the "incomplete" solar eclipse it says "..the Moon's apparent size was too small to completely cover the Sun..". I just watched a great doumentary on Einstein which covered the decisive "proof" of the validity of his Relativity theory and accuracy of his formulas by measuring apparent displacement of stellar images. To do that, several teams of astronomers, at great expense, traveled to Australia to witness a "Total" eclipse of the sun (I think). How does one tell if a solar eclipse will be "Total" or "Annular"? When they say "apparent size" does that relate directly to the phenomonon that both the Sun and Moon appear much larger close to the horizon than at zenith?

[emc]

Hi ajbock,

Not sure if this answers all of your question but the apparent size of the Moon changes relative to its position during its orbit. The Moon is closer to the Earth at times. This back and forth motion is called libration. So depending on the Moon's relative position to Earth, the eclipse is "Total" or "Annular"



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

[ajbock]

Hi EMC,
Thanks a lot - that helps. I'm still abit vague tho on what effect the "horizon proximity" illusion would have on a "scheduled" total or annular eclipse. Since there is a gross diference between the distances to the sun and moon, I wonder if the horizon proximity effect would alter transit coverage area expectations and "actual" observered size ratios.

[neufer]

<<Kanyakumari is a town at the southernmost tip of the Indian Peninsula. In his work
on ancient India, Ptolemy had identified Kanyakumari (Cape Comorin) as a center for pearl fishery.

• Diagram comparing a cross-section
  
  of a cultured pearl, upper:
  
  with a natural pearl, lower

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According to Hindu legend, Kanya Devi, an avatar of Parvati, was to marry Shiva, but as he failed to show up on his wedding day, the rice and other grains meant for the wedding feast remained uncooked and remain unused thereafter. As the legend goes, the uncooked grains turned into stones as time went by. Some believe that the small stones which look like rice on the shore today, are indeed grains of the wedding that was never solemnized. Kanya Devi is now considered a virgin goddess who blesses pilgrims and tourists who flock the town.
-------------------------------------------------------------------
The Kanyakumari Thiruvalluvar Statue:


The Thiruvalluvar Statue is a stone sculpture of the Tamil poet and saint Tiruvalluvar, author of the Thirukkural. It was opened in January 1, 2000 and is located atop a small island near the town of Kanyakumari, where two seas and an ocean meet; the Bay of Bengal, the Arabian Sea, and the Indian Ocean. The statue stands upon a 38 foot pedestal representing the 38 chapters of "virtue" in the Thirukkural. The 95 foot statue represents the "wealth & pleasure" earned & enjoyed on the foundation of solid virtue [; e.g., Tamil Tiger Woods?]. The combined height of the statue and pedestal is 133 feet, denoting the 133 chapters in the Thirukkural.>>

[Chris Peterson]

I'm still abit vague tho on what effect the "horizon proximity" illusion would have on a "scheduled" total or annular eclipse. Since there is a gross diference between the distances to the sun and moon, I wonder if the horizon proximity effect would alter transit coverage area expectations and "actual" observered size ratios.

The illusion is just that- an illusion; a trick of human perception. So it would have absolutely no effect on anything. The atmosphere does refract light, however, and that means that the closer you get to the horizon the more error there is between an astronomical object's actual and observed position. Right at the horizon, the refraction is about 1/2°, or the size of the Sun and Moon. This means that when you see either just touching the horizon, the object itself is actually below the horizon. That is enough to cause several minutes of timing error. It also means that there may be a position shift of the eclipse path if part of that path is in a sunrise or sunset zone. So if you were planning on critically observing an eclipse right at sunrise or sunset, you would want to compensate your timing and position to allow for this effect. Refraction effects drop off quickly with altitude, so you probably would not find it necessary to compensate if the Sun were more than about 10° above the horizon.

I witnessed an annular solar eclipse at sunset back in 1992, and don't recall needing to compensate for position; the timing could have been slightly off and I wouldn't have noticed.

The difference in distance between the Sun and Moon isn't relevant to atmospheric refraction and distortion, since the rays from either are passing through exactly the same path. Even though the distances are very different, all the optical effects happen in the last few hundred kilometers of their light paths.

[ajbock]

I don't believe the horizon proximity illusion is related to the refraction effects. However, your refraction eclipse timing distortions are very interesting. -ajb

[Chris Peterson]

I don't believe the horizon proximity illusion is related to the refraction effects.

No, it isn't. That was what I tried to point out- it is an optical illusion only, and has no physical basis. So it won't affect the timing or position of an observed eclipse. Refraction is a physical effect, however, and does slightly affect the actual eclipse path.