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Pyriform
111024.  Thu Nov 02, 2006 6:44 am Reply with quote

The difference between the umbra and penumbra is due to the light source not being a point source (as with the Sun), so light may be blocked from one part of the source but not from another. In the penumbra of a solar eclipse you would see a partial eclipse, and in the umbra a total eclipse. Other stars are so far away that they are effectively point sources of light, so the shadow is all umbra.

edit: best explained by a diagram


Last edited by Pyriform on Thu Nov 02, 2006 6:55 am; edited 1 time in total

 
dr.bob
111030.  Thu Nov 02, 2006 6:47 am Reply with quote

costean wrote:
Perhaps a better way of phrasing the question would have been:

What is the causes largest shadow on Earth, (which is caused by the sun's rays being blocked), as visible from space?


But you'd have to carefully define how you measure "largest". After all, the shadow of the great wall of china, for instance, might not be as "high" as that of everest, but it might cover a greater area owing to its extensive length.

 
costean
111073.  Thu Nov 02, 2006 7:14 am Reply with quote

dr.bob wrote:
costean wrote:
Perhaps a better way of phrasing the question would have been:

What is the causes largest shadow on Earth, (which is caused by the sun's rays being blocked), as visible from space?


But you'd have to carefully define how you measure "largest". After all, the shadow of the great wall of china, for instance, might not be as "high" as that of everest, but it might cover a greater area owing to its extensive length.


Sorry, it would help if I didn't mangle the English language any more than normal, thus laying upon it a heavier semantic burden than it can reasonably be expected to bear; so reducing my somewhat jumbled collection of thoughts into a state of almost total incomprehensibility. The above should have read:

Quote:
What is the cause of the largest shadow on Earth, (by means of the sun's rays being blocked), as visible from space?


The answer being still the same as in the original. The Earth itself; the shadow of the Earth being 'night'. And from space it would be seen to cover none/some/all of the visible Earth depending on the direction from which it was viewed relative to the sun.

 
The Luggage
114035.  Tue Nov 07, 2006 10:05 pm Reply with quote

Umbra and Penumbra are used in French in every day language as Ombre (shade) and Penombre (Twilight).

 
Ian Dunn
371705.  Tue Jul 01, 2008 5:17 am Reply with quote

Here is something quite interesting from today's Times

Paul Simons wrote:
The Sun turned purple in California last week. The fantastic sight came from the smoke of hundreds of wildfires that broke out in tinder-dry conditions across the state. The tiny bits of soot that filled the air were just the right size to scatter the red colours of sunlight and let through shades of purple.

Scotland witnessed its own surreal spectacle on September 26, 1950, when the Sun turned blue. “All over the city people stopped to gaze at the sapphire sphere,” described The Scotsman in Edinburgh. The newspaper’s switchboard was jammed with callers, some of whom thought the end of the world was coming. And later that night the country was treated to the sight of a blue Moon.

An RAF jet fighter was sent up to investigate and reported that the Sun was a vivid blue up to a few miles high, where a layer of smoky brown haze was hanging. Above that the Sun looked normal. The smoke had come from massive wildfires in Alberta that turned the Sun purple or orange across much of North America. The smoke was then blown across the Atlantic.

Wildfires are not that unusual, so why is the proverbial Blue Moon, or Blue Sun, not seen more often? The smoke particles from the fires have to be the same specific size and that is quite rare.

 

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