162 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914, 
of stars as one travels in a north or south direction, must have given 
rise to a belief in the curvature of the earth in the minds of early 
astronomers. Thales, and after him Aristotle, is said to have taught 
the sphericity of the earth, and it is not surprising that we soon find 
attempts being made to determine its dimensions. 
To Eratosthenes is due the honor of being the first of whom we have 
any record to make an estimate of the circumference of the earth 
based upon measurement. He was born at Syene, in southern Egypt, 
in the year 276 B. C., and, his ability being early recognized by 
Ptolemy Euergetes, he was placed by him in charge of the Alexandrine 
library. His geodetic measures consisted in noting that at Syene, at 
the time of the summer solstice, the sun passed through the zenith 
of the place, as was shown by a vertical object casting no shadow; 
while at the same time at Alexandria such an object cast a shadow of 
such a length as to show that the sun’s rays made an angle with the 
vertical equal to one-fiftieth of a whole circumference. He concluded, 
then, that as the two places were nearly on the same meridian the dis- 
tance between them is one-fiftieth of the whole circumference of the 
earth. The distance being estimated at 5,000 stadia, the circumfer- 
ence of the earth becomes 250,000 stadia. As we do not know the 
precise length of his stadium, we are unable to estimate the accuracy 
of this determination. We now know that the longitudes of the two 
places differed by 3°; also the amplitude of his are was too small by 
15’. Notwithstanding these sources of inaccuracy, however, great 
credit is due to him for inaugurating a correct method for determining 
the dimensions of the earth. 
Cleomedes, to whom we are indebted for the account of Eratos- 
thenes’s operations, suggested that if two gnomons be set up at two 
places on the same meridian the lengths of their shadows on the 
same day would serve to determine the amplitude of the are joining 
the places. His suggestion thus contained the germ of the method 
used at the present day to measure the length of a meridian arc. 
According to the same writer, another determination of the earth’s 
circumference was made by Posidonius about a century and a half 
later. This observer noticed that at Rhodes the bright star Canopus 
just appeared in the horizon when at meridian passage, while at 
Alexandria it had an altitude equal to one forty-eighth of a circum- 
ference. As the distance between the two places was estimated 
to be 5,000 stadia, the whole circumference becomes 240,000 stadia. 
From this time interest in the sciences in Egypt and Greece appears 
to have languished, and during the Dark Ages the only country in 
which astronomical science was cultivated appears to have been 
Arabia. In the year 814 an Arabian caliph proposed to his astrono- 
mers the problem of measuring an are of the meridian. From a 
selected spot on the plain of Singar, near the Arabian Gulf, one party 
