Discussion in 'Astronomy, Exobiology, & Cosmology' started by Dinosaur, Mar 31, 2006.

1. DinosaurRational SkepticValued Senior Member

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4,885
Suppose you were on a high peak and the shadow caused by an eclipse was approaching across a desert or an ocean. How fast would the shadow be moving?

Would you be able to see the edge of the shadow approaching or receding?

3. leopoldValued Senior Member

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17,455
and this is the first entry

The speed of the shadow at the equator is about 1706 km/h (about 1060 mph); near the poles, where the speed of rotation is virtually zero, it is about 3380 km/h (about 2100 mph).
http://encarta.msn.com/encyclopedia_761568140/Eclipse.html

5. tsmidRegistered Senior Member

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368
A more intuitive figure, which sets the speed of the shadow in better context, is roughly 1 km/sec.

I photographed the shadow actually from a peak looking out over the coast and ocean in Cornwall, England during the 1999 eclipse. See my webpage Solar Eclipse 1999 in Cornwall for a series of photographs that managed to show the effect of the moon's shadow despite an overcast sky. If you have a closer look at the image '30 seconds before end of totality', then you can see that on the left hand side the sea surface brightens already up again, whereas on the right hand side it is still dark. With a larger magnification one can clearly see the diagonal line that the 'edge' of the shadow makes across the sea (in a couple of the other images as well, although it does not come out clearly in the scans here).

In order to do this, you must be at least 200 m above the sea (or desert); 200 m puts the horizon at a distance of about 50 km (which is roughly the radius of the shadow cone) (see the map on my page http://www.openviews.org.uk/eclpath.htm for an illustration ).

Thomas

7. LensmanRegistered Senior Member

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88
That's 3600 km/hour, faster than the figure given for the poles. A more reasonable figure would be 1 km per
1 1/2 to 2 seconds, depending on latitude.

8. tsmidRegistered Senior Member

Messages:
368
For the 1999 eclipse, at the moment as shown on the map http://www.openviews.org.uk/eclpath.htm , the speed was actually 0.9 km/sec (the two shadow cones shown are 270 km separated in distance and 5 min =300 sec in time) although the latitude was nowhere near the poles (latitude 50 deg).
The figure usually mentioned (1700-3400 km/sec) (see http://star-www.st-and.ac.uk/~awc/eclipse.html ) assumes actually that the shadow moves exactly from west to east which is usually not the case (for the 1999 eclipse the shadow ended actually up at about 20 deg latitude). In general you have therefore also to take the elevation of the sun into account as the projected speed will be higher than for an overhead sun. For the 1999 eclipse the speed went even up to 2km/sec in the final stages (see http://sunearth.gsfc.nasa.gov/eclipse/SEmono/TSE1999/T99PathDesc.html (towards the end of the penultimate paragraph).

So depending on the eclipse and the location, the speed can be anything between about 0.5 - 2 km/sec.

Thomas