Re: Solar Eclipse Report 8/1/08

Robert Carnegie <rja.carnegie@xxxxxxxxxx> wrote:

On Aug 5, 10:37 am, nos...@xxxxxxxxxxxxxxxx (J. J. Lodder) wrote:
PS IIRC there has been an eclipse
that changed from total to annular
(or vica versa) as it happened.
So somewhere, some time,
the width of totality must have been exactly zero.

I was trying to figure out if there is another case. But... the
eclipse consists of the sun-side of the earth being traversed by the
moon's shadow, which is always there in space. So basically the
shadow passes on at the edge of the earth and passes off elsewhere on
the edge, there isn't a point on the surface where the total eclipse
stops happening. Except that there can be, because in the eclipse
scenario, earth locations at morning and night are farther away from
the moon than noon is, and so the umbra (total shadow) may be non-
existent when the eclipse hits twilight, although the eclipse was
there at noon - which is what you /said/, it becomes an "annular
eclipse"... wait, am I correctly recalling that moon and sun are
heading opposite directions during an eclipse? So a really good solar
eclipse would start in the evening and finish in the morning (i.e.
morning the opposite side of the world)?

The other way round:
the moon's shadow moves from west to east. [1]
Most of the -surface- 'moving' is done by the earth's rotation,
not by the orbital motions. [2]

So an eclipse could start at dawn in America for example,
hit Europe near noon,
and end at sunset in the Far East.
(all local times of course, a few hours in real time)

[1]
Reports from the 1999 eclipse in France
said it was particularly impressive to those standing on the cliffs,
seeing the moon's shadow rushing towards them across the sea.
(at about 800 m/s iirc)

[2] Geocentrism really helps here:
the sun goes round the stationary every 24 hours,
moving from east to west. The moon only once a month.
As the sun sweeps past, behind the (almost) stationary moon
the shadow cone sweeps the earth, going in the opposite direction.

.

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