Two More Eclipse Questions

While we are on the subject of astronomy, I’d like to share just a bit more about eclipses. Here are my questions (those who follow us on Facebook may have already seen these questions. Try not to blurt out the answer before your friends have had a chance to think about it):

First Question

Question:: If the sun and moon both travel from east to west, why was the total solar eclipse last August seen first in Seattle and last in Charleston?
Answer:: The simple answer is that as the sun and moon race across the sky, the sun (on the outer lane) overtakes the moon (on the inside lane). Here is an illustration of that.

Solar Eclipse Diagram — This drawing is not to scale

Since the surface of the Earth is moving from west to east as the Earth rotates, the big question for some of you is, “which is moving faster”? It turns out it is the shadow. (Actually, the Earth’s rotation is reflected by the sun’s movement in this picture, but the question is still valid).

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Here is an Earth-centric partial drawing of our solar system showing most of the details/numbers that one needs to consider in arriving at this answer. Solar System Diagram

The above ignores details like the fact that the Earth’s axis of rotation is not the same as the axis of its orbit around the sun. Nor is it the same as the axis of the moon’s orbit around the Earth. These factors affect the path of the eclipse across the Earth. Here is a map of the paths of all the total (and annular (defined in the following note)) solar eclipses crossing North America in this first half of the 21^st century.

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Because of the elliptical nature of the moon’s orbit, sometimes it seems larger than the sun and sometimes smaller. (Based on the average distances shown in the second drawing (hidden in the previous note), the moon would be smaller). If the moon appears larger than the sun and completely hides it during an eclipse, it is called a total eclipse. Its path is shown in blue on the map. When the moon appears smaller, the sun can peek out all around it, and it is called an annular eclipse. Those paths are shown in yellow.

I derived this from other maps found on a NASA website. This information is courtesy of Fred Espenak, NASA/Goddard Space Flight Center, from eclipse.gsfc.nasa.gov.

Last Question

Question:: Which side of the country would see a lunar eclipse first? Why?
Answer:: This could be considered a trick question. As the second drawing (hidden in the first note) suggests, the geometry of a lunar eclipse is totally different from a solar eclipse, so the relative sizes of the shadow and the object being shadowed are entirely different.

Our last animated illustration shows the relative size of the Earth and moon (shadow) from the sun (under specific conditions) during a solar eclipse. For a lunar eclipse, imagine the Earth is now the Earth’s shadow as the moon (in the place of the moon’s shadow) goes behind it. As the moon flies into the shadow, that event is visible simultaneously wherever the moon can be seen. For another (possibly better) view, see the second illustration in “A Nighttime Solar Eclipse?”.

Well, that should just about cover everything you ever wanted to know about an eclipse (and more). If you have any questions, you can ask in the comment section, or you may wish to consult an astronomer.

First Question

Last Question

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