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View Full Version : Q about Sky Illumination during Solar Eclipses



spaceboy0
2008-Jun-14, 07:02 PM
I'm interested in the Solar Eclipse August 1 as seen from northern Canada.

I will be in Yellowknife, NWT which is 7 degrees south of Cambridge Bay where the Sun will be on the horizon at Totality at 9:20 a.m. UT.

So from Yellowknife the Sun will be about 7 degrees below the horizon, so there will still be some twilight.

I checked Starry Night and the Moon completely covers the Sun's disc from Yellowknife at Totality also, except the Sun will be 7 degrees below the horizon at that location and moment.

So my question is, at Totality, even from Yellowknife where the Sun is 7 degrees below the horizon, can I expect the sky to go completely dark at Totality?

During Totality when the eclipse is up in the sky, the sky apparently goes completely dark at Totality except along the horizon where there is some illumination. This illumination is caused by sunlight hitting the upper atmosphere at points outside the Track of Totality.

However, in the case of Yellowknife, the Sun is 7 degrees below the horizon, yet if it was above the horizon at that location I would be right in the Totality track.

So from a point on the Earth 7 degrees below where the Moon's shadow touches the ground, what sort of sky illumination effects can I expect in Yellowknife at Totality? Complete darkness? Or still an illuminated sky with no variation in brightness from before Totality through past Totality (disregarding the fact the Sun will be slowly rising)?

jokergirl
2008-Jun-16, 08:29 AM
I've only seen one total eclipse and that was in the middle of the day, but there seemed to be quite a bit of stray light before and after. It was certainly not night-dark in any case. We were standing on a hill and the shadow was rather fuzzy on the landscape below...
There's a rather good picture of an eclipse from space floating around on APOD which shows just how fuzzy the shadow is. I'm sure somebody here can dig it up...

;)

mugaliens
2008-Jun-16, 12:34 PM
I've been precisely along the center track of just one total eclipse in my life, and while it was indeed dark, it was more like what's called "civil twilight," as the path of total obscuration isn't all that wide, and scattered light, mostly UV and near UV (deep blue and indigo) still made it to where I was.

dcl
2008-Jun-16, 03:39 PM
I've been at the centers of the paths of totality of several total solar eclipses and can attest to the fact that the sky is not as dark as night during totality. In one especially memorable case, I remember seeing the "shadow bands" moving over the flat ground around me, and the ground was dimly lit by the corona, which was remarkably bright. That and the residual light coming into the center of the zone of totality add up to enough illumination of the atmosphere around that site are enough to drown out the light from all except the brightest stars. I was in each instance so preoccupied with observing the eclipse itself that I didn't notice whether any stars were visible, but there couldn't have been very many. Otherwise, I believe I'd have noticed at least some. On the other hand, recall that Eddington measured the displacement of a star whose light passed close to the sun during an eclipse to confirm one of the predictions of Einstein's general theory of relativity. But I expect that that measurement must have been made with a telescope equipped with very special filtering to block entry of light from other directions into the telescope.

tony873004
2008-Jun-16, 07:56 PM
The eclipse path is 80 miles wide, and you are about 525 miles from it. I looked up the two cities on Google Earth, and NASA's eclipse page, and you're right, it's aimed straight at you. Therefore the Moon's shadow will appear about 80-100 miles wide. So you may see a dark patch similar to a crespular ray, about 10 degrees wide in the direction of the Sun, on the horizon. At 525 miles, when you look at the horizon in the direction of Cambridge Bay, you will be looking at the atmosphere nearly 200,000 feet (~38 miles) above Cambridge Bay. There's probably enough atmosphere at that height to catch some sunlight on either side of the Moon's shadow, to provide contrast to the shadow. But don't expect it to get pitch dark. As others have pointed out, even in totality, it's not pitch black. I was in Hawaii in 1991, in totality. Although the sky was cloudy and I couldn't see the eclipse, I did notice the light level. It was a little too dark to play golf, but not dark enough trip over your own feet. But in your situation, only 10 degrees of your horizon twighlight will be darkened, so it shouldn't make much difference to the overall amount of light.

Shortly after totality at Cambridge Bay, totality will reach you, but it will be 40 miles above you, stretching up an additional 80 miles to about 120 miles. That's a little too low for satellites, but I'd watch for them anyway. How cool would it be to see a satellite enter and re-emerge from the Moon's shadow?

I wish you clear skies. Horizons are often clouded even when it is clear overhead. Take a video or a series of still pictures (1 every 10 seconds for a few minutes) of the sunset twilight that you can assemble into an animation. That would be very cool.

spaceboy0
2008-Jun-16, 08:25 PM
I won't be able to afford the $ 1200 round trip to Cambridge Bay so I'll go to Yellowknife to see what I can see.

There's are no summer roads north of Yellowknife; it's flying only northbound

dcl
2008-Jun-16, 08:27 PM
tony873004, your experience in Hawaii reminds me of the worst experience I ever had in trying to observe a total solar eclipse. It was at the waterfront in Marblehead, Massachusetts. It happened barely ten minutes after sunrise in a pouring rain. I could tell when totality occurred because the sky darkened again briefly after sunrise had caused it to lighten slightly. I had another interesting experience with a total solar eclipse. I was on the Gaspe Peninsula in Canada, parked across the street from a farm house. There were several people on the porch. As totality approached and the sky darkened, the people on the porch went into the house!

Preparatory to observing a total solar eclipse in Maine, I fogged some black-and-white photographic film to enable me to safely look directly at the corona. It was widely rumored that infrared radiation from the sun would seriously damage vision even when viewing through fogged photographic film, so I had ran a s sheet of black and white film through a spectrophotometer to assure that the film had high enough opacity to radiation throughout the entire visual and infrared ranges of the spectrum to be safe for looking directly at the sun. I brought more than enough film strips for my entire family with several spares. I said to a stranger standing nearby with a magnifying glass and a sheet of white cardboard onto which to project an image, "Would you like to use this?" He had bought into the propaganda so thoroughly that he response was, "Of course not." My best total solar eclipse was in North Carolina in perfect weather. For the first time, I got a good look at shadow bands.

spaceboy0
2008-Jun-17, 12:56 AM
so what you're saying is that the sky glow level seen from Yellowknife will depend on how much sunlight is shining on the upper atmosphere? that's fascinating

kleindoofy
2008-Jun-17, 01:59 AM
I experienced a totality back in the 60s near Boston and seeing the corona suddenly 'explode' around the moon's disc was utterly fascinating.

However, night dark or near night dark, darkness is something we get once a night, every night, so it itself is nothing new to us.

I found the strange ongoing grayness of the ca. 4/5 eclipse I experienced in Germany a few years ago much more interesting than the darkness of the totality. It was a very strange light, almost 'eery'; nothing like twilight or the darkness under very thick rain clouds. It was like looking through a perfect light-grey filter, and it held on seemingly uniformly for quite a while, not like the fleeting dynamic shades of twilight.

Eery.

spaceboy0
2008-Jun-17, 06:15 AM
I took these pics from Vancouver of the sky illumination with the Sun 7 degrees below the horizon, the approximate position of the Sun during the Eclipse as seen from from Yellowknife at 3:23 a.m. August 1. The last shot is the Sun 9 degrees below horizon representing Sun at lowest point in track from Yellowknife on Aug 1:

http://farm4.static.flickr.com/3097/2585865721_d4a8a95cc1.jpg

http://farm4.static.flickr.com/3142/2586701202_f75a93e23e.jpg

http://farm4.static.flickr.com/3270/2585866815_2beb46dced.jpg

http://farm4.static.flickr.com/3176/2586702260_008a6fe1a0.jpg

tony873004
2008-Jun-17, 08:16 PM
Photoshopping your picture, here's my guess as to what you might see

http://orbitsimulator.com/BA/eclipsehorizon.JPG

spaceboy0
2008-Jun-17, 08:42 PM
that's cool! then again, it may not even look that dramatic as at 200,000 feet height there's not a whole lot of atmosphere density to scatter light.

do you think it's worth it to go all the way from Vancouver?

spaceboy0
2008-Jun-17, 08:47 PM
I used a more precise formula: h = R +/ - sqrt [{R^2 - d^2}] where R is 6391 km and d is distance from Yellowknife to the shadow cone touching Cambridge Bay which I think is 845 km.

So the shadow cone would pass 55 km over Yellowknife.

tony873004
2008-Jun-17, 09:19 PM
...do you think it's worth it to go all the way from Vancouver?
You run the risk that it will be:
a) much less dramatic than my image depicts
b) might not be noticable at all
c) might be cloudy

I hate to discourage you, because you might come back with some really nice photos, but the odds you'll come back disappointed are pretty high. How much is it costing you to go? Will you fly or drive?