|Why are some solar eclipses longer than others?|
|Question Date: 2017-08-25|
The duration of an eclipse depends on several
factors. First, it depends whether it is a
partial or total eclipse. Then it depends
on how close the moon is to Earth in its
elliptical orbit (affecting the apparent size
and speed of the moon). And lastly, it depends
on how close Earth is to the sun in its orbit
(affecting the apparent size of the sun). On
average a total eclipse lasts a couple hours from
start to finish, with totality (when the sun
is completely obscured) lasting only a couple
The moon's orbit isn't perfectly circular.
When the moon is closer to the earth, it appears
larger in the sky, and when the moon is farther
away, it appears smaller. If the moon is close to
the earth, then the size of the shadow that it
casts on the earth is bigger, and so takes longer
for the moon to move out of the way in its orbit.
The opposite happens if the moon is farther away,
and if it is really far away, then the apparent
size of the moon is smaller than that of the sun,
and so you get an annular eclipse that is
never total because you can see the edges of the
sun around the moon.
This NASA website says the eclipse duration is
shorter when the shadow falls on the curve of the
earth, instead of straight down on the earth:
duration of eclipse
Because of the rotation of the earth, the
region on Earth in the umbra will not stay
in the umbra forever. That's why the total
solar eclipse usually lasts for a few minutes.
To have the longest duration of the total solar
eclipse, the moon must be as close to Earth as
possible. This will make the region in the umbra
created by the moon larger, which means longer
total solar eclipse. You can think of this in
another way, you are walking in the shadow of a
building under the sun. If you walk slower or if
you can make the building larger (larger shadow),
then you will stay longer in the shadow. So
when the Earth-Moon distance is smallest, the
shadow projected on Earth is larger, then longer
total solar eclipse. There are some other
factors that will also affect the length of the
total solar eclipse, such as the Earth-Sun
distance and at which point or region the totality
happens on Earth, etc.
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