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referring to this guide, there is a point in southern Illinois that has the longest duration eclipse and a point in western Kentucky where the eclipse is the "greatest".

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the first i understand, the second doesn't make sense to me. for anyone in the center of the path, the eclipse is as total as it can possibly be. at some moment, the moon is squarely in front of the sun.

can someone explain the difference?

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How do you mark an answer as an educated guess. Ah well, perhaps this will help.

Different points on the earth's surface must be at different distances from the moon. The angular diameter of the moon is greater at the point on the earth that is closest to the moon (nearer things are bigger). Is the greatest eclipse at this place? The transit time will also be impacted by the rotation of the earth, which will have the effect of shortening it as the moon takes aprox a month to get round and the earth surface only a day. In the location of the "greatest" eclipse where the surface of the earth intersects the shadow tangentially the rotation is in the most effective direction for a rapid transit. The longest transit time must be elsewhere; where the angular diameter of the moon is nearly largest, and the surface of the earth is cutting a slightly longer section through the shadow.

TBH It's an enhancement to the question, I suppose, given that I have been creative.

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  • $\begingroup$ well, if you're on the center of the path, there is a moment in time when the moon sits squarely in front of the sun. if there was no such moment, you're not on the center of the path. i understand what "Greatest Duration" is but i can't think of what other quantitative measure there is. how close the axis line connecting sun to moon swings near to the earth's center? $\endgroup$ – robert bristow-johnson Dec 23 '16 at 22:31
  • $\begingroup$ So what I am saying is that if the moon is squarely on the center of the sun, in some places it is bigger in area because you are closer to it. ...because the earths surface is curved. $\endgroup$ – JMLCarter Dec 23 '16 at 22:51
  • $\begingroup$ i think i get it. that would be where the axis line (at the center of the umbra) is most vertical with respect to the surface of the earth. okay. ya know, now that i think of it a bit, i think that is the same as the closest the axis line swing near to the earth's center. $\endgroup$ – robert bristow-johnson Dec 23 '16 at 23:04
  • $\begingroup$ yes. The magnitude of this effect could be significant. The other effect I suggest could be a factor is that the earth is spinning. So it is not just the shadow moving past the earth, you are moving through the shadow, and in a different direction depending on the angle of incidence of the shadow on the earth's surface (perfect sphere aprox) at your locaton. $\endgroup$ – JMLCarter Dec 23 '16 at 23:08
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    $\begingroup$ JML The phrase you may be looking for is "largest solid angle subtended". $\endgroup$ – dmckee Dec 24 '16 at 3:29

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