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Astronomical question and answer 247

 

Frank L. Preuss

 

What is the problem with this diagram on the previous webpage? This diagram showing how a solar eclipse works? Why does it make it so difficult to understand how a solar eclipse actually works and how the shadow moves and how to see, what influences the speed of the shadow moving over the surface of the earth?

 

The diaram gives the impression that the centre of the sun and the centre of the moon and the cente of the earth are or must be on one straight line.

It also does not show that the earth is revolving around its own axis and in what direction it revolves.

There should be a separation between two lines. The one line should be a stationary line and the other a mobil line.

The stationary line should be the one between the centre of the sun and the centre of the earth. And the mobile line should be the line between the sun and the moon. And the moon should not be shown being exactly between the sun and the earth, but slightly off, in order to indicate one of the positions the moon has during the course of the eclipse.

The sun should be assumed to be stationary and also the earth.

There are two movements involved. One is the movement of the moon on its orbit, and the other is the rotation of the earth around its own axis.

The moon moves from west to east, and that is indicated by he the red arrow. Now the rotation of the earth around its own axis should also have been indicated by an arrow, and this arrow should be in the same direction as the red arrow on the orbit of the moon.

The light cast by the sun upon the earth moves in the opposite direction. The surface of the earth moves from west to east, and this causes the light of the sun to move from the east to the west, therefore in the opposite direction of the the movement of the shadow of the moon.

Now during a solar eclipse the moon can be exactly between the sun and the earth. But that would be a rare case. It would mean that the course of the eclipse is on the equator. But normally the shadow of the moon can be anywhere on the surface of the earth.

The normal case is that the moon comes to its closest position to the line between the centre of the sun and the centre of the earth, in the course of the 29 days or so it takes to make one orbit.

And when then the shadow of the moon moves over the surface of the earth, then halfway it reaches its closest position, and that point of time is the clock time given in astronomical tables as the time of the new moon, and then the line between the sun and the moon is identical to the line between the sun and the earth.

And then the diagram would be all right. But then this line between the sun and the moon still does not go through the centre of the earth, but only through the axis of the earth. If it would go through the centre of the earth, then we would have this rare case, where the course of the shadow would be on the equator.

Then one would have the rare case, where the three centres of these three heavenly bodies would be on one straigth line and the time would be the clock time of the new moon.

The line from the centre of the sun to the centre of the moon crosses every month the axis of the earth, but only when there is a solar eclipse, the crosssing point lies in the interior of the earth, otherwise in the extension of the axis of the earth beyond the surface of the earth.

The blue arrow is correct, because it serves as information, serves the orientation. But for our contemplation it is of no influence, since the moon rotates around the sun together with the earth.

But this diagram has the advantage that I had to really make an effort to sort out the details involved in this process of a solar eclipse.

Now when one imagines to be above the earth and the moon, and one looks down to these two heavenly bodies, then there would be the same situation as when one is on the moon, space would be black.

And that would mean that one would just see half of the moon and half of the earth, those two halfs that are illuminated by the sun.

The other two halfs would not be seen. They would be as black as the surrounding space. May be the half of the moon would be illuminated by the earth shine, and that would then be the only time, this earth shine lit moon can be seen, because looking at the moon from earth does not allow the observer to see it, because the sunshine completely dominates everything.

 

This is the end of "Astronomical question and answer 247"
To the German version of this chapter: Astronomische Frage und Antwort 247

 

 

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