Re: Semi-minor Axis

Wasn't it JG who wrote:
>Hi,
>
>I've been lurking and gaining a great deal of knowledge from this
>newsgroup for quite some time now but I have a very basic question that
>I have not seen asked.
>
>I used to think that Aphelion referred to the semi-major axis of the
>ellipse described by the Earth (or any planet) on its journey around the
>Sun and Perihelion was the semi-minor axis. Having read most of the
>planetary data from 'The Nine Planets' web site I now see that the sum
>of Aphelion and Perihelion is in fact the major axis of that ellipse.
>What I cannot understand is the statement in the Glossary that Aphelion
>is also the 'average' or mean distance of the planet from the Sun.
>
>Surely the maximum distance cannot also be the mean?
>
>What I really want to know is how to calculate the semi-minor axis.
>Given the 'Mean' and the eccentricity I can readily calculate the Major
>as a(1+e) and the Minor as a(1-e) but if the mean is also the Major then
>this doesn't make sense.

The aphelion is not the mean distance, and it doesn't say so in the nine
planets glossary:

http://www.nineplanets.org/help.html
aphelion
the point in its orbit where a planet is farthest from the Sun; when
refering to objects orbiting the Earth the term apogee is used; the
term apoapsis is used for orbits around other bodies. (opposite of
perihelion)

Nine Planets says that the *semimajor axis* is the average distance of
the planet from the Sun. (I'd claim that that depends what you mean by
"average". If you average over time, then you get a result that's longer
than the semimajor axis because the planet moves more slowly when
further from the Sun.)

--
Mike Williams
Gentleman of Leisure
.

Relevant Pages

  • Re: Semi-minor Axis
    ... > of Aphelion and Perihelion is in fact the major axis of that ellipse. ... > is also the 'average' or mean distance of the planet from the Sun. ... > What I really want to know is how to calculate the semi-minor axis. ...
    (uk.sci.astronomy)
  • Re: Semi-minor Axis
    ... >>of Aphelion and Perihelion is in fact the major axis of that ellipse. ... >>is also the 'average' or mean distance of the planet from the Sun. ... >>What I really want to know is how to calculate the semi-minor axis. ...
    (uk.sci.astronomy)
  • Semi-minor Axis
    ... of Aphelion and Perihelion is in fact the major axis of that ellipse. ... is also the 'average' or mean distance of the planet from the Sun. ... Surely the maximum distance cannot also be the mean? ... What I really want to know is how to calculate the semi-minor axis. ...
    (uk.sci.astronomy)
  • Re: Question about Keplers second law
    ... planet; however, the speed of the planet is not uniform: ... Take the pencil and draw a Sun at the location of one of the pushpins. ... Now draw a little circle at four or five places around the ellipse. ... motion, which is why it speeds up as it gets closer to the sun. ...
    (sci.physics)
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