Talk:Earth radius

Untitled

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

Moreover many planets (most?) don't have a sea. Rich Farmbrough, 14:28, 28 February 2011 (UTC).

Mean sea level establishes the hydrostatic equilibrium more accurately than more rigid land can. If sea level rises, then mean radius increases. I am not sure what melting ice caps have to do with the remainder of your question, but land beneath and around ice caps rise after melting due to isostatic rebound. For bodies that have no fluid surface, a fictitious one is established, typically as an atmospheric pressure. See, for example Mars#Geography. Strebe (talk) 19:14, 28 February 2011 (UTC)

The Elevation of Mean Sea Level was lower in the Past when the climate was cooler, and there was more ice in all of the Glaciers, this includes Continental Glaciers, Glaciers that started on Continents, and ended in Oceans, and Glaciers that filled Mountain Valleys, and slowly moved down hill. Mean sea level was a very useful reference elevation when surveying equipment consisted of Theodolites, Chains, Wye Levels, Compass, Clocks, Transits, and similar devices to determine X, Y, and Z. Z is the vertical elevation above Mean Sea Level, set at a reference time period. Having a universally accepted reference elevation keeps things a lot simpler so that surveyors around the world can work together using the same system.

The Fact that sea level is rising is only an inconvenience if you happen to live too close to sea level, but it is not really an inconvenience for surveyors as long as every body agrees not to change the previously agreed upon reference elevation. Fortunately we now have the Global Positioning System ( GPS ), and the people who control that system take about 1700 continuously monitored reference points and so with the help of lots of big computers, they keep the reference elevation and the change in the reference such that the change sums up to zero. This is very good for Surveying as it makes one of the Three " Delta Z " = zero.

The other Two Delta X, and Delta Y are not so co-operative as Continents like to wander in various directions, and they even spin around about some axis. Fortunately, in most locations this motion is very small. The movement is around 1 mm per year in each of the two directions, Longitude, and Latitude. The actual movements only cause problems over very long periods of time, like millions of years when you are trying to find out where Dinosaurs actually lived, but the Location that was around the Equator 65 Million years ago is now over 20 degrees North of the Equator. That is Chicxulub, and The Deccan Traps were both near the Equator between 65 MYA and 66 MYA, and both are now around 20 degrees North of the Equator. The Impact location that is now at Chicxulub was Anti-podal to the Energy focal Point on the opposite side of the Globe at what is now the Deccan Traps in India. So millimeter per year movements really add up over millions of years to hundreds of Kilometers of movement. Mike Clark, Golden Colorado, 63.225.17.34 (talk) 04:05, 6 September 2016 (UTC)

Apparently, I was wrong about how fast Plates Move. Look at UNAVCO Plate Motion Calculator. Denver is moving around 16.35 mm per year in the AZ 237.00 direction. Elko NV is moving about 18.14 mm per year in the AZ 225.00 direction, and Las Vegas NV is moving about 17.00 mm in the AZ 222.5 direction. Australia is booking North and a little East at around 7 cm per year. There is a whole lot of Plate Movement going on. In 2003, The GPS guys recalibrated the reference points on the Earth's surface. All are moving somewhere, and all are moving vertically as well. About 80 % of 1200 points moved between - 8 mm per year to + 16 mm per year, with the mid point at plus 4.15 mm per year. For simplicity, they chose only 157 points between -4 mm per year, and + 4 mm per year to constrain delta Z to zero.

This simplifies the reference system for surveying purposes, and calibrating satellite height, But...... I wonder what errors this simplification creates when viewed over very long time periods ? 98.245.216.62 (talk) 23:26, 18 April 2022 (UTC)

Polar radius

The current value in the article for polar radius is b = 6,356.7523 km, which would seem to more than accurate enough for an average visitor.

But I have a consistency problem: The value is attributed to "WGRS 80/84" in the "Fixed radii" section. It is attributed to WGS (E2008) in the "Mean radii" section.

In following links to here, it appears that WGS (E2008) is using the values of a and f from WGS 84.

The table here says those values make b ≈ 6,356,752.314,245 m. It uses the " ≈ " symbol, but reports the value to the nearest micron (1e-6 m)!

NOTE: Take 6,371,008.8 X 3 = 19,113,026.4 minus 6,356,752.314 245 = 12,756,274.09. Dividing by 2 gives 6,378,137.043 m. I do not think the 43 mm is all that important when compared to the 6,378,137 meter radius at the equator. 98.245.216.62 (talk) 23:24, 18 April 2022 (UTC)

For comparision:

Item	Size
Grain of Sand	100 to 2000 microns

So the precision of b in WGS (E2008) is adequate to make a topographic map of surface features on grains of sand.

What I have read suggests that the value for a does well to match an observable value for MSL by +/- 2 meters. Applying a flatness factor to it wouldn't seem to justify six decimal places in the result. The article also notes that the geoid heights at the poles are 1) unequal, and 2) "off" by at least 13 meters. Stating the value of b to the nearest micron seems absurd.

But I am uncomfortable with rounding "for" the visitor without telling them, though I did it myself in this edit, where I rounded to the nearest millimeter because the values for both GRS 80 and WGS 84 were the same through that precision here.

The value of b was further rounded to the nearest 0.1 meter for unstated reasons in this diff.

So I have three questions:

1. When might one want the value of b to the nearest micron?
2. What reference "should" we cite for the value of b?
3. If we cite a source but round its value, do we need to tell the visitor that we rounded "for" them?

-Ac44ck (talk) 01:01, 30 December 2008 (UTC)

1. Surely the only point of that much precision would be for use in other calculations, such as recovering the flattening value or computing the eccentricity. Sub-meter precision does not say anything useful about the "actual" polar radius, of course.
2. http://earth-info.nga.mil/GandG/publications/tr8350.2/tr8350_2.html
3. I would think so, along with the reason.

Strebe (talk) 03:13, 30 December 2008 (UTC)

Actual values

In the table ("Published values") only 2 look (to me) like actual measurements.

Can you provide a few more?

In particular: distance to the shores in the Arctic and Antarctic. But also some "hot spots" like Everest, Malaysia, South Pole. (NB: I couldn’t find it with Google).

MBG02 (talk) 23:30, 6 April 2022 (UTC)

Mean Earth Radius exact where ?

With e.g. the WGS84 ellipsoid the mean Earth radius is 6,371 km.

Per the Mean value theorem is follows that there are points where this radius is exact.

Where are they? (Should be added to the article, e.g. in the form of a lattitude). Lklundin (talk) 18:46, 4 September 2023 (UTC)

In relation to the earth's radius, this is not anything that appears in reliable sources, which suggests it’s not important. Without a citation, a calculation would just be original research. Strebe (talk) 15:54, 5 September 2023 (UTC)

OK, thanks for checking. Lklundin (talk) 16:19, 5 September 2023 (UTC)