Re: Semi-minor Axis
- From: "oriel36" <geraldkelleher@xxxxxxxxx>
- Date: 31 Dec 2005 02:43:53 -0800
JG 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.
>
> JG
Do what Newton did and transfer Flamsteed's axial rotational /stellar
circumpolar sidereal equivalency to a geocentric /heliocentric orbital
equivalency thereby getting your mean Sun/Earth distances.
http://www.pfm.howard.edu/astronomy/Chaisson/AT401/IMAGES/AACHCIR0.JPG
You get you stretching of distances from a Sun/Earth mean but you also
get the ugly spectacle of the Earth travelling faster at the aphelion
and slower at the perihelion.
Go ahead and fit the dumb sidereal .986 deg orbital displacement into
an elliptical framework and watch the Keplerian insight destroyed.
Of course nobody here is a heliocentric astronomer and they would not
know this.
.
- References:
- Semi-minor Axis
- From: JG
- Semi-minor Axis
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