Question 548404
I have an astronomy program that provides information on known comets.  They provide only two pieces of information:  Perhelion and eccentricity.

 I have gone through a number of books and web pages both for astronomy problems and math problems trying to find out how to find out the length of the major axis.
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The perihelion is the distance from one of the foci to the intersection of the ellipse its major axis.
The distance from the center to the focus is a*e, where a = 1/2 the major axis and e is the eccentricity.
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The perihelion, p = the distance from the focus to a, p = a - f
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f = a*e
p = a-f --> a = p+f
a = p + a*e
a - a*e = p
a(1 - e) = p
a = p/(1 - e)
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major axis = 2p/(1 - e)
That what you're looking for?
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PS  Did you see the comet whose perihelion was only 120000 km from the Sun's surface?
Comet Lovejoy.