eratosthenes figured out the circumference of the earth.
eratosthenes figured out the circumference of the earth.
who figured out the circumference of the earth?
eratosthenes
posidonius calculated the earth's circumference by reference to the position of the star canopus. as explained by cleomedes, posidonius observed canopus on but never above the horizon at rhodes, while at alexandria he saw it ascend as far as degrees above the horizon (the meridian arc between the latitude of the two locales is actually 5 degrees 14 minutes). since he thought rhodes was 5,000 stadia due north of alexandria, and the difference in the star's elevation indicated the distance between the two locales was 1/48 of the circle, he multiplied 5,000 by 48 to arrive at a figure of 240,000 stadia for the circumference of the earth. it is generally thought that the stadion used by posidonius was almost exactly 1/10 of a modern statute mile. thus posidonius's measure of 240,000 stadia translates to , not much short of the actual circumference of . strabo noted that the distance between rhodes and alexandria is 3,750 stadia, and reported posidonius's estimate of the earth's circumference to be 180,000 stadia or . pliny the elder mentions posidonius among his sources and without naming him reported his method for estimating the earth's circumference. he noted, however, that hipparchus had added some 26,000 stadia to eratosthenes's estimate. the smaller value offered by strabo and the different lengths of greek and roman stadia have created a persistent confusion around posidonius's result. ptolemy used posidonius's lower value of 180,000 stades (about 33% too low) for the earth's circumference in his "geography". this was the number used by christopher columbus in order to underestimate the distance to india as 70,000 stades.
earth's circumference
in modern times, earth's circumference has been used to define fundamental units of measurement of length: the nautical mile in the seventeenth century and the metre in the eighteenth. earth's polar circumference is very near to 21,600 nautical miles because the nautical mile was intended to express of a degree of latitude (i.e. 60 × 360), which is 21,600 partitions of the polar circumference. the polar circumference is even closer to 40,000 kilometres because the metre was originally defined to be one 10-millionth of the circumferential distance from pole to equator. the physical length of each unit of measure has remained close to what it was determined to be at the time, but the precision of measuring the circumference has improved since then.
earth's circumference