［十月新番］《地。-關於地球的運動-》(2)

The ancient Greeks had good measurements of the obliquity since about 350 BCE, when Pytheas of Marseilles measured the shadow of a gnomon at the summer solstice.[10] About 830 CE, the Caliph Al-Mamun of Baghdad directed his astronomers to measure the obliquity, and the result was used in the Arab world for many years.[11] In 1437, Ulugh Beg determined the Earth's axial tilt as 23°30′17″ (23.5047°).