Mechanics of a Solar Eclipse

mechanics of a solar eclipse - nashville in the moon's shadow

I have seen people recently bring up some interesting questions about solar eclipses, like the one we saw in Nashville, Tennessee on August 21, 2017.  For instance, why is the shadow of the moon smaller than the moon itself?  Why did the eclipse move from West to East when the moon normally rises in the East and sets in the West?  You know, things like that.  I am going to try to explain a few things about solar eclipses in this post.

How Do We Know When a Total Solar Eclipse Will Happen?

A total solar eclipse happens roughly every 18 months somewhere on the Earth.  A date can be calculated based on several variables.  For instance, the Moon must be in new moon phase.  The distance to the Moon must be less than a certain distance in order to cover the Sun, as the Moon’s orbit is not a perfect circle.  Sometimes it is closer and sometimes further away.  I will post a link at the end of this post that describes in more detail how total solar eclipse dates are calculated.

How Do We Know Where a Total Solar Eclipse Will Happen?

A tougher question for me to answer… where?  Well, all I can say is that it can be calculated and I will include a link at the end of this post on how it is calculated.  But the good news is that the dates and places in the near future are already done for us and we can count on those calculations.  Ultimately the Sun, Moon, and Earth are like a big clock and everything happens in a defined pattern.

Explaining the Clockwork

The Earth completes a full rotation on its axis every 24 hours or so.  The Moon circles the Earth every 28 days or so.  The Earth circles the Sun every 365 days or so.  We have 4 seasons in a year because the Earth tilts on its axis, and sometimes the north pole faces away from the Sun, sometimes it faces the Sun, and sometimes it is somewhere in between.  That is why we have calendars, seasons, Moon cycles, etc.  And why we can calculate with surprising accuracy where and when a total solar eclipse (or any eclipse) will happen.

Why Is the Shadow of the Moon Smaller Than the Moon?

Usually when we shine a light at an object, the object’s shadow is bigger than the object itself.  This is because the source of the light is smaller than the object.  If the source were bigger, the shadow might also be bigger.  Whoops, did I just make a mistake there?  No, it really depends on the vantage point of the shadow.  Is the light source bigger or smaller than the object from the vantage point of the shadow?  If bigger, there will be no shadow.  If smaller, the shadow will be bigger than the object.

In the case of a total solar eclipse, the Moon is nearly the same size as the Sun from the vantage point of the Earth.  Actually the Moon is slightly bigger, which is why it takes 2.5 minutes (+/-) for the shadow to pass.  So the length of the Moon’s shadow is a product of how much bigger the Moon is than the Sun from the vantage point of the Earth.

Or put better yet, you are in the shadow of the Moon only when the Moon is completely blocking out the Sun.  If the Sun is peeking out at all, no shadow.

Why Did the Eclipse Move from West to East?

Every day, the Sun rises in the East and sets in the West.  The Moon does the same thing at night.  You would think that the solar eclipse would start in the East and move West right?  Let’s talk a little bit about why the Sun and Moon rise in the East and sets in the West.  It all has to do with the rotation of the Earth.  The Earth rotates from West to East, making the Sun and Moon rise in the East and set in the West.  The Sun is clockwork, sunrises happen almost perfectly every 24 hours.  No matter where you live.  The sunrise changes with the seasons but from day to day it is almost the same.

The Moon, on the other hand, rises later and later each night.  You can check, it really does.  So in relation to the Sun, the Moon is moving from West to East.  That’s the way it circles the Earth, which it does every 28 days.  So on total solar eclipse day, what matters is not where the Sun and Moon rise and set, it matters where the Moon is coming from and going to in relation the Sun, which is West to East.  The Moon is literally moving in front of the Sun and has no relation to the rotation of the Earth.

Now where you see the total eclipse also moves West to East because of the angles of the Moon and Sun to the Earth.  In the West, the eclipse happens closer to sunrise and in the East the eclipse happens closer to sunset.

Let Us Know What You Think

Hopefully this answered some questions.  If you have any others, feel free to comment below.

References and Further Reading:

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