The sun rises in the east, the moon rises
in the east, and the stars rise in the east... but solar eclipses, oddly, come from the west,
like the April 2024 North American eclipse, or the August 2027 North African eclipse. Except, not all total eclipses come from the
west - a few near the north and south poles actually head west for a bit before turning
around and then heading east - all of which seems very weird. If eclipses are caused by the sun and the
moon, why don't they behave like the sun and the moon? The key to the explanation is that the paths
of the sun and moon through the sky depend on the rotational speed of the objects involved,
while the paths of eclipses depend on just the plain old straight-line speed of the moon
above the earth's surface. Viewed from the north pole, the earth and
moon both rotate counterclockwise - that is, towards the east. The path of the moon through the sky is determined
by the line of sight from the earth's surface to the moon, and because the earth is rotating
faster than the moon is orbiting, the sight line (and the moon) starts off pointing to
the east at moonrise, then as the earth rotates the moon appears to pass overhead and set
in the west (even though the moon is traveling towards the east the whole time). In contrast, the path of an eclipse is determined
not by the direction from us to the moon, but by where the moon's shadow falls on the
earth's surface, and the moon's shadow just points away from the sun. The moon is traveling "eastwards" around the
earth at just over 2000 miles per hour, and its shadow travels at basically the same speed
- eastwards at just over 2000 miles per hour. The earth's surface is also moving to the
east, but not nearly as quickly - the surface at the equator is only moving around 1000
miles per hour, and slower the closer you get to the poles. The moon's shadow easily outpaces the earth's
surface's eastward motion, which means eclipses appear to move from west to east. Put another way, the face of the earth is
about 8000 miles across, so the moon (and its shadow) cross the earth in around 3 and
a half hours (and less near the poles), while any point on the earth takes 12 hours to cross
the earth - it takes half a day to rotate halfway around the earth. It's kind of weird that the moon can orbit
slower than the earth rotates but travel faster; but the moon has a long way to travel: nearly
one and a half million miles over its month-long orbit, which equates to around 2000 miles
per hour. In contrast, a point on the equator only travels
25 thousand miles each day, or around 1000 miles per hour. There's no cosmic reason that eclipses on
earth travel west to east - it's essentially just a coincidence. If the earth were twice as big, or the moon
were half as far away (and therefore the circumference of its orbit half as long), then the length
of a month or day wouldn't change (and neither would the direction of moonrise), but the
relative linear speeds of the surface of the earth and the moon WOULD change, and eclipses
might move from east to west. In fact, if the earth and moon's sizes were
adjusted so that the moon was traveling slower than the earth's surface at noon but faster
at other times of day, it would be possible for an eclipse to move east, then west, then
east again... geometry is weird! Speaking of weird, those weird west-moving
eclipses near the poles happen because the earth's axis of rotation is tilted, so it's
possible for the moon's shadow to be moving to the east but hitting part of the earth
on the "night-time" side of the planet. You can get a rough idea by opening up google
earth and drawing a bunch of straight west to east arrows across the earth to represent
eclipse paths. If you look at these arrows from another vantage
point, oops suddenly the eclipse paths look way more wonky, and some of them go "backwards"! And while we're in google earth, if you tilt
the earth like it is during the spring and fall and draw some horizontal lines, you can
get a sense of why eclipse paths follow the curvy shapes they do (though actual eclipse
paths are more complicated because the earth is also rotating at the same time). In summary, even though the moon orbits to
the east "slower" than the earth rotates, in the sense that a month is longer than a
day, the moon also orbits to the east "faster" than the earth, in the sense that the moon
is literally traveling at a faster eastward speed than the surface of the earth - and
that's what determines the direction of an eclipse.