This is because the earth is a sphere. At noon on the summer solstice in Syene, the sun's rays shine at the zenith (directly above), so there are no shadows and the light reaches the surface of the water. If the earth were flat, the same light would shine on Alexandria. However, if the earth were spherical, the sun's rays would not reach the surface of the water because the sun's rays do not shine from the zenith. Eratosthenes used this difference to calculate the circumference of the earth.

Eratosthenes first measured the height (angle) of the sun at noon on the summer solstice in Alexandria. As a result, he found that the sun was visible 7.2° south of the zenith. Next, the distance between Syene and Alexandria was determined to be 925 km between the two cities. Alexandria was then assumed to be directly north of Syene. Given these three conditions, we can derive the circumference of the earth, or more precisely, the length of the meridian line connecting the North and South Poles.

Since 925 km is 7.2/360ths, or 1/50th, of the total circumference, the total circumference is 925 x 50 = 46,000 (km). We now know that the meridian is about 40,000 km long, but Erasthenes' calculation was only about 15% larger than that.

Ino Tadataka (1745-1818), a former merchant, astronomer, geographer, and surveyor of the Edo period, was the first person in Japan to measure the size of the earth. Ino, who is famous for creating the "Great Map of the Maritime Zone of Japan," was the first person to measure the latitude and longitude of the earth.^＊It is said that he went out to survey the earth to determine its entire circumference from 1°! ²⁾ The following is a list of the most common problems with the

Ino made surveying and astronomical observations while traveling straight north along the Oshu-kaido Road during the first survey, and determined the length of 1° latitude (arc length) to be 106 km. Based on this arc length, the length of the meridian is approximately 38,000 km. In the second and third surveys, the arc length was determined to be "28 ri 2 min" (110.7 km), from which the length of the meridian was calculated to be about 39,900 km. Ino's figure of "28 ri 2 min," the arc length he derived from his survey, had an error of 0. ^2%3 when converted to today's metric system.) This is astonishingly accurate. Incidentally, the distance Ino walked during his 15-year surveying trip was approximately 40,000 km, which is almost the same as the circumference of the earth.

Chikyu is written in Chinese characters as "earth sphere," but in reality it is shaped like a slightly squashed sphere, like a tangerine. Therefore, the length of the equator, which divides the earth into north and south, and the meridian are different. The length of the equator is about 40,75 km, and the length of the meridian is about 49,000 km.