darkoshi: (Default)
[personal profile] darkoshi
I thought of an easier way to explain why the eclipse shadow travels west to east, even though the moon travels east to west through the sky.

First, here's the general picture from the perspective of the sun, when looking down at the solar plane from above:
The earth revolves around the sun in a counter-clockwise direction, completing a full circuit about every 365 days.
The moon revolves around the earth in a counter-clockwise direction, completing a full circuit about every 28 days.
The earth rotates around its own axis in a counter-clockwise direction, completing a full turn every 24 hours.

Here's the general picture from the perspective of a spot on the earth at the equator, when looking up at the sky:
The sun revolves around the earth in an east to west direction, completing a full circuit every 24 hours.
The moon revolves around the earth in an east to west direction, completing a full circuit about every 24.5 hours (I hope I calcuated that right)

Now to explain why the solar eclipse shadow goes west to east:

Imagine you are standing on the north side of an east-west street, facing south.
The moon is a person walking on that street from east to west.
The sun is another person walking on that street from east to west, except that they are walking slightly faster than the moon, and emitting a bright light.


When the sun is still a fair bit behind the moon, the shadow that is cast from the moon due to the sun's light will point towards the west.
As the sun starts overtaking the moon, walking behind the moon compared to the observer, the shadow that is cast points towards west-northwest.
As the sun continues passing behind the moon, that shadow changes direction, towards to the northwest, then north, then northeast, then east-northeast.
So even though both the sun and moon are going east-to-west, the shadow goes west-to-east.

Maybe that is totally obvious to other people? I mean, it seems pretty obvious to myself now that I've explained it.

.

It's actually more complicated than that, of course.

The sun's path does go from east to west rather consistently, even though during the summer, the path is higher in the sky (northeast -> northwest) than during winter (southeast -> southwest).

But the moon's path is more dynamic, as it doesn't revolve in the plane of the equator. It may rise in the southeast and set in the northwest. Or it may rise in the northeast and set in the southwest. (right? I haven't ever really paid much attention to the moon's path, but I must have learned that somewhere.)

Because of that, based on the images I've seen, instead of the moon crossing the sun from right to left, during this eclipse, it will cross it from lower right towards the upper left.

So in the above example, the moon would be on a different street, at an angle to the other street, and the streets would happen to cross each other right at the point where the sun was walking behind the moon.
(although what angles the streets need to be at, and which direction the moon is going on its own street is a bit difficult for me to visualize right now.)
..

I guess it's time to create a new eclipse tag for all these entries, and to rename the eclipse tag I used on a single other post in reference to the software called "Eclipse".
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