Talk:Spheroid/Archive 1

Archive 1

Untitled

Do someone know with which software the images in this article were created? GNU

Mathematica, I believe. ✈ James C. 05:15, 22 August 2004 (UTC)

Confusion over terms

The formula for the volume of a spheroid refers to a and b being the axes (a and b usually represent the semi-axes). The formula for the surface area also refers to a and b. In the formula for the volume, do a and b represent the axes or the semi-axes? In the formula for the surface area, do a and b represent the axes or the semi-axes?

Try a = b. Charles Matthews 16:50, 4 May 2005 (UTC)

The formula and the images

Unless I'm mistaken, the formula given is for a spheroid with the x-axis of the Cartesian coordinate system as the symmetry axis. The images, however, seem to have the z-axis as the symmetry axis. Shouldn't we change the formula accordingly to

${\frac {x^{2}}{b^{2}}}+{\frac {y^{2}}{b^{2}}}+{\frac {z^{2}}{a^{2}}}=1$

See also Oblate Spheroid and Prolate Spheroid at MathWorld. Or did I go wrong somewhere? Lupo 07:49, 11 October 2005 (UTC)

Prolate spheroid = rugby ball, oblate spheroid = Earth's shape (a little flattened at North and South poles). Charles Matthews 10:13, 11 October 2005 (UTC)

Yes, I knew that, but it doesn't answer my question above. Lupo 11:28, 11 October 2005 (UTC)

I fixed it.--Patrick 13:16, 11 October 2005 (UTC)

I think it is still wrong. Earth is an oblate spheroid: a = the equatorial radius/semi-major axis and b = the north polar radius/semi-minor axis and the south polar radius/semi-minor axis, thus the equation was right as it was

${\frac {x^{2}}{a^{2}}}+{\frac {y^{2}}{b^{2}}}+{\frac {z^{2}}{b^{2}}}=1$

AND, therefore, I believe it should really be "an oblate spheroid has the semi-major axis longer than the two semi-minor, (a > b), and can resemble a disk; a prolate spheroid has the semi-major axis shorter than the two semi-minor (a < b)" (though I did forgot to transfer the "and can resemble a disk" remark).

As for MathWorld, I think it leaves much to be desired! P=/

Thus--unless I'm reading it wrong--the ellipsoid page is also really screwed up. P=( ~Kaimbridge~ 14:14, 11 October 2005 (UTC)

No, x and y play the same role and can be interchanged, they are both divided by the same equatorial half-diameter a, for the Earth larger than the one polar half-diameter b (for only one of the three axes we have this smaller number).--Patrick 15:19, 11 October 2005 (UTC)

Are you sure about that? As I understand it--and I'll be the first to admit I quite easily get lost in any abstract analysis P=)--this is the relevant diagram and equation I understand (realizing that the picture is of an ellipse, not an ellipsoid--though I believe the same oblate/prolate effect applies): If a > b, then you have an oblate ellipsoid; if b > a, then it is prolate; the equation is three terms, to allow different values of b ("b_{north}" and "b_{south}")--? ~Kaimbridge~ 16:45, 11 October 2005 (UTC)

No, you have three, because you have 2 directions for a.--Patrick 20:46, 11 October 2005 (UTC)

The formula for Surface Area also suggests an imaginary value when the spheroid is oblate! (because the eccentricity would be a negative square root). — Preceding unsigned comment added by 128.120.163.171 (talk) 22:19, 27 February 2006 (UTC)

minor axis / major axis

I removed this sentence:

"A prolate spheroid has a semi-minor axis shorter than the semi-major axis (a > b); an oblate spheroid has a semi-minor axis longer than the semi-major axis (a < b) and can resemble a disk."

1. Because it's redundant (oblate and prolate are explained above)

2. Because it's completey wrong

The major axis is longer than the minor axis by definition - see ellipse.
Additionally, this sentence leads to the fact, that in the volume calculation always the shorter axis will be squared. This is makes absolutely no sense, it is always the axis not being the rotation axis which will be squared, as it exists in two directions - see ellipsoid

(sorry for my english, i'm no native speaker) --JogyB 15:08, 12 July 2006 (UTC)

Planet Earth as an example

I don't think Earth is a good example. While it is technically correct, most people think of it as a sphere, and it looks like a sphere to the naked eye. To ensure that everyone gets the point immediately, the article needs an example of something that is universally known for being a bit squashed. Nothing comes to mind at the moment unfortunately. Piccadilly 14:19, 6 August 2007 (UTC)

An M&M would work Dr d12 03:01, 23 September 2007 (UTC)

You can also use a pumpkin Eny 21 March 2008 —Preceding unsigned comment added by 76.110.171.34 (talk) 00:01, 22 March 2008 (UTC)

something is wrong here

A prolate spheroid has a>b, right? But then oe=arccos(a/b) is imaginary? —Preceding unsigned comment added by 193.174.246.179 (talk) 14:05, 29 April 2009 (UTC)

No, with a prolate spheroid, a<b——a>b on an oblate spheroid, like Earth. ~Kaimbridge~ (talk) 14:28, 29 April 2009 (UTC)

Blind

I guess "blinded researchers" probably means like "double blind", not with blindfolds on! Someone more familiar with experimental protocol should clarify this section. —Preceding unsigned comment added by 121.44.181.7 (talk) 12:17, 14 October 2009 (UTC)