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Astronomical question and answer 249

 

Frank L. Preuss

 

What is the main cause of a solar eclipse?

 

The main cause of a solar eclipse is the movement of the moon.

A solar eclipse happens at new moon. The moon is on its orbit around the earth. When it comes between the sun and the earth it comes from the west and moves towards the east and then its shadow is cast upon the surface of the earth.

The webpage before the last one dealt with a diagram that makes it difficult to understand the process und so I sat down and made my own diagram. Here it is:

MySolarEclipse30degrees

The moon moves on its orbit and comes from the west and moves towards the line between the sun and the earth.

At a certain distance from this line the shadow of the moon is cast upon the surface of the earth and that is the beginning of the course of the shadow. This shadow then wanders from the west to the east, just like the moon itself.

The diagram shows, as an example, a situation more at the beginning of the eclipse, therefore before the exact clock time of new moon, which is right in the middle of the eclipse

The shadow then reaches the plane between the sun and the axis of the earth und also its halfway. This happens at the time of new moon. The rest of the course of the shadow takes the same time as the first.

The line from the centre of the sun via the centre of the moon and then to the centre of the shadow, goes, when extended, through the axis of the earth, at new moon. And that is within the earth. If it would go through the axis of the earth outside of the earth, then there would be no eclipse. That is what normally happens at new moon. If it would go through the centre of the earth, then the eclipse would be on the equator.

The fact that the earth rotates around its own axis, and also influences the eclipse, is not actually a cause of the eclipse, just decreases the speed of the shadow.

The speed of the earth on its orbit around the sun does not influence the eclipse, because the earth and the moon are one system that traveles together around the sun, on the same orbit.

So the two important rotations are the one of the moon around the earth and the one of the earth around its own axis, and are therefore shown in red.

 

This is the end of "Astronomical question and answer 249"
To the German version of this chapter: Astronomische Frage und Antwort 249

 

 

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