Total eclipse, so why dawn glow? (APOD 05 Aug 2008)

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Expand view Topic review: Total eclipse, so why dawn glow? (APOD 05 Aug 2008)

by JohnD » Thu Aug 07, 2008 7:27 pm

Chris, thnaks again!

Not usual to achieve such complete explanation!

John

by Chris Peterson » Thu Aug 07, 2008 5:41 pm

JohnD wrote:The shadow of the Moon in the sky above - was that what you mean, Chris?
The shadow cast by the moon is a cone that points away from the Sun, and comes to its vertex at about the same distance as the Earth is from the Moon. A solar eclipse happens when that vertex intersects the Earth. So there is a cone of darkness extending from the Earth, up towards the Moon. Because the line between eclipsed points, the Moon, and the Sun isn't usually perpendicular to the Earth's surface, the cone doesn't point straight up. But the video clearly shows that the shadow is broad at the top and narrow at the bottom.
Strange, but I assume an aratefact of the video, the Moon over the Sun appears as a fixed diameter disc, about half the size of the Sun throughout the eclipse.
It's not an artifact. During the eclipse, the Sun is completely covered. What you are seeing around the eclipsed Moon is the corona, which can extend out several solar diameters.

by JohnD » Thu Aug 07, 2008 5:01 pm

Gosh, bystander, that video makes th whole thing clear!
The shadow of the Moon in the sky above - was that what you mean, Chris?
And the false dawn either side - extraordinary! A splendid video!

Strange, but I assume an aratefact of the video, the Moon over the Sun appears as a fixed diameter disc, about half the size of the Sun throughout the eclipse.

John

by Chris Peterson » Thu Aug 07, 2008 2:23 pm

JohnD wrote:That picture certainly shows two 'false sunsets', but surely the dark, truncated cone shape, from 6 to 8 o'clock, is the wing of the aircraft from which the photo was taken?
Yes, the shadow at the bottom is the plane's wing. The eclipse shadow is the inverted cone that covers the sky at the top, and converges to a narrow zone at the horizon.

by bystander » Thu Aug 07, 2008 12:33 pm

JohnD wrote:That picture certainly shows two 'false sunsets', but surely the dark, truncated cone shape, from 6 to 8 o'clock, is the wing of the aircraft from which the photo was taken?
It is indeed the wing of an aircraft as this video shows.

http://uk.youtube.com/watch?v=kP4QD8952g4

by JohnD » Thu Aug 07, 2008 8:22 am

Chris Peterson wrote: Check out this image on Spaceweather.com showing the actual cone shaped shadow of the eclipse, with two false sunsets.
Chris,
That picture certainly shows two 'false sunsets', but surely the dark, truncated cone shape, from 6 to 8 o'clock, is the wing of the aircraft from which the photo was taken?

John

Movies and stills of solar eclipses

by henk21cm » Wed Aug 06, 2008 7:51 pm

JohnD wrote:Chris, Thank you! I was hoping for someone with actual experinece of a totality to say if that is what happens. So theory and experience agree - you do see dawn's early light!
Never witnessed a full solar eclipse, so can't offer you an eyewitnessreport. Have to do with satelite images. Some taken by the satelites operated by Eumetsat can be found via http://www.eumetsat.eu/Home/Main/Media/News/005559?l=en

Last fridays eclipse was beyond reach of current MET-9.

by Chris Peterson » Wed Aug 06, 2008 6:44 pm

JohnD wrote:Chris,
Thank you! I was hoping for someone with actual experinece of a totality to say if that is what happens. So theory and experience agree - you do see dawn's early light!John
Check out this image on Spaceweather.com showing the actual cone shaped shadow of the eclipse, with two false sunsets.

by JohnD » Wed Aug 06, 2008 10:40 am

Chris,
Thank you! I was hoping for someone with actual experinece of a totality to say if that is what happens. So theory and experience agree - you do see dawn's early light!

John

Re: Total eclipse, so why dawn glow?

by henk21cm » Wed Aug 06, 2008 8:33 am

Chris Peterson wrote:but the actual situation is much simpler. There's no requirement that we be viewing perpendicular to the path of the eclipse. ... So the shadow edge may only be a few kilometers away.
Right you are, the shadow edge might be that close, i forgot to mention the words "worst case".

Re: Total eclipse, so why dawn glow?

by Chris Peterson » Tue Aug 05, 2008 10:02 pm

henk21cm wrote:There are three kinds of dawn...
Your analysis is fine, but the actual situation is much simpler. There's no requirement that we be viewing perpendicular to the path of the eclipse. In fact, the image is taken to the west, and the path was northwest to southeast. Near the end of totality, the shadow was approaching the camera from approximately the direction the camera is pointing. So the shadow edge may only be a few kilometers away.

Having seen several solar eclipses, I can state that this effect is completely normal. You get a false twilight effect near both the beginning and end of totality. When the shadow is small, you can see it all around the horizon at mid-eclipse.

Re: Total eclipse, so why dawn glow?

by henk21cm » Tue Aug 05, 2008 9:01 pm

JohnD wrote: Oh, say can you see that the dawn's early light is showing over the mountain range in the distance. If the Sun is high in the sky, with the Moon in front of it, why this dawn light?
There are three kinds of dawn:
  1. Civil dawn: between sunset and the sun is 6° below the horizon
  2. Nautical dawn: the sun is between 6° and 12° below the horizon
  3. Astronomical dawn: the sun is between 12° and 18° below the horizon
During civil dawn you can unaidedly move around without stumbling against objects
During nautical dawn a seaman can still find the horizon to shoot a star with his sextant
During astronomical dawn long exposed photographs get white due to too much stray light

Now lets do a simple "back of an envelope" calculation. The maximum width of the eclipse band was according to Fred Espenak236 km. Suppose the observer is right in the middle of this band. The nearest point where the sun is still visible is 118 km. Just a tiny limb of the sun, but that suffices to spoil the fun of totality. Now lets move to the equator, in your thoughts. When the sun travels 6°, to the end of civil dawn, the terminator has traveled 6°. 1° on the equator is 60 nautical miles (per definition) and that is approximately 111 km. So compared to the observer in the middle of the totality zone, the sun has set for just over 1° When the sun is 1° below the horizon, the sky is still lit a lot. And that is the glow around you, near the horizon, the dawn conditioned light you see in the image.

by bystander » Tue Aug 05, 2008 6:26 pm

Wikipedia - Total Solar Eclipse of August 1, 2008 with picture gallery.

by Arramon » Tue Aug 05, 2008 4:39 pm

That resembles a setting sun horizon. Looks like the edge of the moons surface is only allowing rays of light to flood the far horizon, so our atmosphere is doing its affects like normal as if the sun were blocked by the earth's bulk (instead its being blocked by the moon's bulk).

But just a guess on my part. =b

by npsguy » Tue Aug 05, 2008 3:34 pm

JohnD you beat me to the question. At first glance I thought the pic was a fake. Then I thought the reason might be what bystander said, that the glow is from the areas that are not eclipsed. It is possible that a longer exposure was used and that picked up the sunlight shining off the mountains in the distance.

However I do not recall ever seeing a solar eclipse picture quite like this before.

by bystander » Tue Aug 05, 2008 12:36 pm

http://antwrp.gsfc.nasa.gov/apod/ap080805.html
APOD: 2008 Aug 5 - A Total Solar Eclipse Over China
JohnD wrote:Oh, say can you see that the dawn's early light is showing over the mountain range in the distance. If the Sun is high in the sky, with the Moon in front of it, why this dawn light?
http://eclipse.gsfc.nasa.gov/SEmono/TSE ... -fig06.GIF

The path of totality is fairly narrow. I'm guessing that the area beyond the mountains is outside the path of totality, so that what you are seeing is the mountains backlit by partially eclipsed (or non-eclipsed) sunlight.

by orin stepanek » Tue Aug 05, 2008 12:14 pm

It would be tough to look at Venus and Mercury with the corona showing so bright. The best way would be to block out the sun completely to prevent damage to your eyes. An awesome picture at any rate.
Orin

by emc » Tue Aug 05, 2008 12:14 pm

Hi JohnD,

I am less than an amateur, but I wanted to test whether I understand the effect you are questioning.

Isn't the "dawn glow" appearance of the horizon due to the 360 degree shadowing effect from the eclipse and the resulting distance to the horizon? If the shadow wasn't present, then the sun light would be scattered all around in close proximity as blue. But the further the sun light travels through our atmosphere the more scattering/reflection/filtering occurs due to the composition of our air.

BTW - Is the eclipse boundary effect more of a "sunset" or a "dawn"?

Total eclipse, so why dawn glow? (APOD 05 Aug 2008)

by JohnD » Tue Aug 05, 2008 11:08 am

All,

APOD 5/8/8 : http://antwrp.gsfc.nasa.gov/apod/astropix.html

Oh, say can you see that the dawn's early light is showing over the mountain range in the distance. If the Sun is high in the sky, with the Moon in front of it, why this dawn light?

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