Posted by: T. Boyd | January 27, 2009

Monday’s (Jan. 26) Solar Eclipse

annulareclipse
Annular Eclipse photo from Wikipedia

A solar eclipse occured on Monday, visible from the Indian Ocean, so most of us missed it.  But someone was there and took excellent pictures (see link below).  This was an annular eclipse, which means that the moon was not large enough to cover the sun, but, instead, left a ring of fire around the shadow of the moon at “totallity”.

My wife asked, “Why aren’t all eclipses like that?”  And I explained that because the moon-earth distance varies quite a bit, it appears larger at some parts of the year than at other times.  The astonomers use the term eccentricity for describing this.  The moon’s orbit about the earth is not in a circle, but an ellipse.  When the eccentricity of an ellipse is zero, then the ellipse is equal to a circle.  But when it is greater or less than zero, it becomes egg shaped.

The eccentricity of the moon’s orbit is only .054, so the “egg shaped” orbit looks the same as a circle to the eye.  But the tiny  non-circular part causes a slight variation in the size of the moon as seen from the earth – enough to cause the different type solar eclipses.

To be exact, the eccentricity of the earth’s orbit about the sun ( eccentricity = .0167 ), a smaller effect, also affects the eclipses, but to a smaller amount.

You can see the photos at Pictures of Eclipse from Indonesia and read more about annular eclipses at this Wikipedia link.

Boyd


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