From: Dan (dgray@justin-siena.napanet.net)
Date: Mon Aug 02 1999 - 11:33:38 PDT
Message-ID: <37A5E482.368DF481@justin-siena.napanet.net> Date: Mon, 02 Aug 1999 11:33:38 -0700 From: Dan <dgray@justin-siena.napanet.net> Subject: RE: eclipse shadow moves to the east
Gwen Roberts wrote:
> Why is the path of the solar ecllipse from west to east?
>
> Gwen
>
This is because the moon travels from west to east in its orbit around the
earth. This may still seem odd though when you consider that the earth turns
toward the east which in turn makes everything in the sky apparently move
toward the west. Shouldn't the eclipse move toward the west as well,
especially since the earth is spinning much faster (1 day per rotation) than
the moon is spinning around the earth in its orbit (about 1 month per orbit)?
But this is misleading. What is going on here? The moon's shadow travels
across the earth's surface. The moon's shadow forms a long cone stretching
out from the moon opposite to the side facing the sun. It may help to imagine
that the shadow is a long pen stretching from the moon to the earth and as the
eclipse comes and goes, the pen draws a line across the earth's surface. The
KEY to understanding this is that the pen does not point at the center of the
earth. If it did, then it would travel to the west as the earth turns under
it. If the earth did not turn, the pen (if it were pointing at the center of
the earth) would slowly draw a line toward the east and would take half of a
month (about) to draw the full eclipse path across one hemisphere from side to
side. But this is not what happens because the pen ALWAYS POINTS DIRECTLY
AWAY FROM THE SUN. This means that the shadow will cross the earth from side
to side in the time it takes the moon to travel a distance equal to the
diameter of the earth. The earth's diameter is equal to about 1/700 of the
moon's orbital path. So it would take about 1/700 of a month for the moon's
shadow to cross the earth from side to side. 1/700 of a month is about 1
hour. 1/700 part of an orbit is also about 1/2 degree of arc. This is the
angular size of the moon... so when you look at the moon you can think about
the duration of time that the path of totality is on the earth as lasting the
same amount of time that it takes for the moon to travel a distance in the sky
equal to its own diameter.
Dan Gray
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