Who was the first person to get a more accurate measure of the earth's circumference than Eratosthenes?

When researching a paper on Eratosthenes for class I saw that one of his major accomplishments was the first more or less accurate measure of the earth's circumference,only between −2.4% and +0.8% off. This led to me researching a little on measures of the circumference during antiquity however it seemed even by the Islamic Golden Age nobody had made a more accurate measure. So my question is who created the first measure better than Eratosthenes?

• There are number of estimates which are arguably better or worse depending on how the various measures involved in their and Eratosthenes' estimates are interpreted. Wiki has a number of estimates here - en.wikipedia.org/wiki/History_of_geodesy - do you want to know the first person certain to have produced a better estimate? Or the first person who might have? Or a list of possibly better estimate? Apr 5 at 12:52
• The first person certain. Apr 5 at 13:30

Wikipedia articles on the "History of Geodesy", "Earth's circumference", and so on give a thorough summary of potential candidates, but suggest that this question isn't going to be answered decisively without a major discovery of new evidence.

Henry's answer raises the main problem: ancient units are imprecise. Arab astronomers under the Caliph al-Ma'mun confronted this problem by making their own measurements (c. 820 AD) which were very possibly more precise than the Greek ones. But in making this comparison, we confront the same problem again. Just as they didn't know enough about Greek stadia to rely on earlier measurements, we don't know precisely enough how long their cubits and Arabic miles were to say with confidence what they found.

When we get to early modern Europe, then there are clear and decisive breakthroughs. New instruments like the telescope and advances in trigonometry solved the fundamental problem. Jean Picard (c. 1670) is widely credited with making the first modern arc measurement.

Here we run into the further complexity that there simply is no single precise number even today. Sir Isaac Newton correctly predicted that the earth is an oblate spheroid, not a perfect sphere, as confirmed by measurements made in the 1730s. The equatorial circumference is about 40 miles longer than the meridional circumference (that is, through the north and south poles).

• I tihnk your conclusion that there is no single precise number is a bit to simplistic. Once you know that the earth is not a perfect sphere you can give a single precise number for a specific circumference, either equatorial or meridional. The measurements taken around 1700 seem to be the first ones where that happened. Apr 7 at 8:59

The accuracy of Eratosthenes's calculations of the circumference being 252,000 stadia depends on assumptions about the modern equivalent of length of the stadion.

The same issue applies to Posidonius's calculation of the circumference of 240,000 stadia.

In any case, their calculations contained errors which were substantially larger than modern claims of their final accuracy. For example Alexandria is not due north of Syene (Aswan) and Syene is not on the Tropic of Cancer, while Rhodes is not 5,000 stadia north of Alexandria and the star angles Posidonius measured were about 50% bigger than modern measurements.

Some of the errors offset each other, and some modern popular interpretations have had a tendency to choose lengths of the stadion which make the final results appear close to modern measurements while ignoring these larger errors.

• Excellent answer! Their results were extraordinary and deserve praise, but they were not as good as some would like to believe. Apr 5 at 11:57
• That is interesting to know but doesn't answer the actual question. Reading the rest of your linked wikipedia article, later estimates in the Islamic golden age had similar problems and hence where not noticeably better. So when did humans first get measurements that are correct within say 1% of the current known values? Apr 5 at 12:03
• As stated by others, useful, but doesn't really answer my question. Apr 5 at 12:40
• @John Wants to find the Meek I believe the point of this answer is that we don't know how accurate Eratosthenes measurement was, but it was definitely a lot less accurate than commonly reported. Until you know the accuracy of Eratosthenes' measurement, no one can answer your question as stated. Apr 5 at 16:51
• The question is "who created the first measure better than Eratosthenes?" and I think it's reasonable to answer with the next measure that was demonstrably better. That would mean ignoring the quoted uncertainties, finding the uncertainties introduced by the length of stadia and the other issues you raised, and then finding the next measure that was demonstrably better. You've merely reported that the quoted uncertainties were incorrect which, while a valid critique of the question, do not raise to the level of an answer. Apr 5 at 17:47