Talk:Earth radius

I believe that in the comment where A is the authalic surface area of Earth, the word authalic is not needed. As the term means equal area, the "authalic surface area" of the Earth is identical to the surface area of the Earth! Making the distinction between the two may confuse people into thinking that they do not really understand what authalic means. If anything, make it clear A_r is the authalic radius.

Question: Why is this comment

Note: Earth radius is sometimes used as a unit of distance, especially in astronomy and geology. It is usually denoted by RE.

part of the "Volumetric radius" section?

Should it be perhaps "Earth volumetric radius is sometimes used...", or does this comment refers to the general case, situation in which it should be moved somewhere in the main text?

Radius vs. Sea Level

"The radius of Earth (or any other planet) is the distance from its center to a point on its surface at mean sea level." This doesn't seem right. Why is the mean radius of a planet exactly the sea level? The sea level of the earth is rising, does this mean the radius of the earth is increasing too? Why would the melting of the ice caps cause the land to rise? Anyone have a cite? -anon

Radius of curvature

I have changed the formula for Rc near the end of this section to reverse M and N, since when traversing along a meridian (with alpha = 0, i.e. facing North) one is moving along "M" the meridianal radius and when moving (e.g at the equator) eastward (for alpha = + 90 degrees) one is moving along "N" the radius of curvature in the prime vertical. JimC (C&B) 17:57, 7 January 2007 (UTC)

You just described it right——facing north-south the RoC = M, while facing east-west = N——so cos(0)M = M and sin(90°)N = N, which is how it was (your change went and inverted it!

Whoops, okay, I see the problem: Presenting it with MN as the numerator cancels and reverses the placement of M and N in the denominator. I've changed it to make it clearer. P=) ~Kaimbridge~ 15:21, 8 January 2007 (UTC)

"Neridional" radius of curvature???

The heading Neridonal appeared in a 1 April 2007 edit. Surely this should be Meridional. I don't think neridional is even a real word. Certainly it's not in any online dictionary. Also a Google search for it only gives 149 hits, and almost all of those hits are either this article or people who have copied this article verbatim (like answers.com). I'm going to edit this. If someone feels there is some really burning reason why it should be 'neridional' and wants to revert, please add a note here why such an obscure word that doesn't appear in dictionaries should be used. I'm looking at you, User:Kaimbridge, since you made that 1st April edit ... surely you wouldn't be playing an April Fool's joke on Wikipedia ;). Dr algorythm 03:19, 18 June 2007 (UTC)

LOL!!! Sorry about that——I'm blind in one eye and the other eye's no bargain, with an acutely advancing cataract further complicating things. So, needless to say, a few typos may escape me! P=) ...Hmmm, how come it took someone two and a half months to catch?!? P=/ ~Kaimbridge~14:27, 18 June 2007 (UTC)

ERROR?

On this page the polar radius is derived as being larger than the equatorial radius. Please check this derivation. —The preceding unsigned comment was added by 216.59.226.224 (talk) 04:48, 25 April 2007 (UTC).

What do you mean, a (equatorial radius) = 6378.135 and b (polar) = 6356.75? ~Kaimbridge~10:49, 25 April 2007 (UTC)

Regarding the above given values a and b, I think they should be corrected and precised as follows: a=6378.137 km (according to GRS80 and GRS84) and b~= 6356.752 km (as a derived value). Gil

Latitude dependent radius error?

When using the formula *RADII WITH LOCATION DEPENDENCE* I've had a not logical result, for a 43ºS. calculation. My carefully result was no logical because it was 5417.374347 Km., not between the equatorial and polar radius, can spmebody explain this abnormal fact, I will be very happy if somebody explains me this anomaly, Tks. Carlos J.J.Vial <carlosjjv@terra.cl>Carlos J. Jiménez Vial 21:02, 3 August 2007 (UTC)

Well, if its the actual radius at 43º, it should be about 6368 km (depending on the actual values of a and b); but if it involves cos(43º), 5417 is actually too big——it should be somewheres between 4633 and 4681 km (based on the arcradii extremes of 6335 and 6399 km)! What values does it give for 0 and 90°? ~Kaimbridge~17:37, 4 August 2007 (UTC)

SI Units

Why are we not using SI units here (radii in meters)? MeddlerOnTheRoof (talk) 02:21, 9 May 2008 (UTC)

Changes to section on authalic radius

I removed this text:

This number is derived by square rooting the average (latitudinally cosine corrected) geometric mean of the meridional and transverse equatorial, or "normal" (i.e., perpendicular), arcradii of all surface points on the spheroid

It may be that the number approximates the root of the average. I don't find evidence that this average is used to derive the exact expression. One derivation is here.

I also modified this text

the radius of a hypothetical perfect sphere which has the same, geometric mean oriented surface area as the spheroid.

The "mean oriented surface area" of a perfect sphere is zero, as each elemental vector area has an equal antipodal counterpart. The vector sum of the oriented area is zero. -Ac44ck (talk) 02:43, 1 November 2008 (UTC)

Approximation

The original Meridional definition of a meter was one ten-millionth the distance from the north pole to the equator. So a nice, and easy to remember approximation for the average radius of the earth is (2/pi)*10^7 meters. This is accurate to at least 0.1%. Pulu (talk) 05:52, 12 November 2008 (UTC)