Talk:History of geodesy
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This text is taken from chapter 1 of the public domain resource Geodesy for the Layman at http://www.ngs.noaa.gov/PUBS_LIB/Geodesy4Layman/TR80003A.HTM#ZZ0 – please Wikify as necessary.
Riddle me this : how come Eratosthenes's estimation of the circumference of Earth, accurate to 0,3% in 200BC, is "non remarkable", while Aryabhata's estimate, accurate to 1% in 500 AD, seven hundred years later, is remarkable? 22.214.171.124 (talk) 19:42, 15 January 2011 (UTC)
- I agree. The paragraph that begins "Had the experiment been carried out as described, it would not be remarkable if it agreed with actuality" perm is written with such arrogance. Not to mention that some of its oh-so-clever statements may well be wrong - such as claiming that Syene "is actually...22 miles north of the Tropic of Cancer"... Well, today, yes, but if the user who wrote it is so well-informed as his insolent tone suggests, maybe he'd have known that with nutation, Syene may well have been on the tropic 2200 years ago. It'd be good if someone rewrote the entire Eratosthenes section with a bit more neutrality. BigSteve (talk) 07:35, 1 March 2014 (UTC)
He did not need to know the direct distance between Syene and Alexandria, which would require him to travel across the desert. He only needed to know the distance in the N-S direction between them as long as he made his measurement of the angle of the Sun on the summer solstice, which he purportedly did. (126.96.36.199 (talk) 16:49, 9 August 2010 (UTC))
Stadia, not miles
The text judges the accuracy of Eratosthenes' estimate of the size of the Earth based on a number expressed in miles. This is nonsense because E. did not use miles, the type of mile is not specified, the number is rounded to the nearest 1000 miles, and the relation between E's original stadion and the mile that was used for the conversion is not correct. It says that 1 stadion is 1/10 of a statute mile. This is inaccurate by default, because the statute mile did not exist in E's time and it is totally unlikely that E's stadion was a neat fraction of the much later unit. So dump the miles and give E's results in stadia; then worry about the length of the stadion that he may have used. Tom Peters 22:57, 26 August 2006 (UTC)
Someone has a vandetta against Archimedes ? They inflated the values 1000%, the edit by BDPATTON2 was, ONLY, to make the quoted result 10 times too large,and leaving the text saying Archimedes was quite accurate, a little under the true size.
Figures for Geodesy for Layman
The figures in the NOAA and other versions of _Geodesy for the Layman_ are very bad in direct scans. While teaching at the Naval Postgraduate School I had these updated and placed on the Web there for my students. (With the permission of NIMA now NGA.) They are still there. A version of _Geodesy for the Layman_ with the DMA/NIMA/NGA text and new figures can be found at
Jim Clynch (JimC728@gmail.com)
ps - More public domain geodesy items are also there. Some have been updated and expanded on my site clynchg3c.com —Preceding unsigned comment added by 188.8.131.52 (talk) 15:50, 11 November 2008 (UTC)
This section contains the following: "It was not until the 15th century that his concept of the earth's size was revised. During that period the Flemish cartographer, Mercator, made successive reductions in the size of the Mediterranean Sea and all of Europe which had the effect of increasing the size of the earth." This must be either incorrect or worded poorly, as Mercator's lifespan was exclusively in the sixteenth century.
- Agreed, and while I don't have access to suitable refs to improve the section, the notion that Mercator (or anybody else) changed the size of Europe, the Med and the Earth is bizarre; presumeably the passage means that he revised his estimates of these sizes. -- Timberframe (talk) 12:27, 18 January 2010 (UTC)
There appears to be no connection between the formulae quoted in this section and the adjacent "explanatory" diagram. What are "θ1" and "θ2" in relation to the angle "α" in the diargram. Is the Earth's radius "R" - or is it "r"? What is "d"? —Preceding unsigned comment added by 184.108.40.206 (talk) 18:43, 30 April 2011 (UTC)
I found this article very interesting, but I am surprised that the author fails to mention, in the sources of error discussion, the difficulty of synchronizing clocks between the two locations. The article states "At the same time, he observed the sun was not directly overhead at Alexandria", as if all he had to do was look at his watch. I realize that it has to do with knowing that both cities were at the same longitude, but even so, the means for knowing the time accurately is not discussed. Were there mechanical timekeepers in his time? Did he use a sundial to know when it was noon? How accurately could he measure the time? How sensitive is his calculation of the earth's circumference to an error in the time? I don't mean to critisize the article, I just think a few words on the subject of time synchronizing would be helpful. — Preceding unsigned comment added by Rbarline (talk • contribs) 18:51, 16 June 2014 (UTC)
No the issue is the shadow toward the north or south, NOT the east /west component. .. You can tell the time from the shadows .. When he says there was no shadow, he could say that it was definitely the solstice at noon, as the place was on the tropic. So the shadow was TELLING the time, no chronograph needed.
The shadow a little further north would always have a northward component, but as long as there was no east or west component, then it was noon. The error introduced by being able to tell midday precisely, eg a sixth of an hour in error, does not then lead to an error worth calculating, when the measurement of the long distance between the two observation points has much more error.