 The length of the tropical year was determined to 365 d 5 h 49 m 20 s, a value in accord with the value of the Gregorian Calendar, but obtained 300 years earlier.In 1787, Laplace applied these measurements to check his calculations on the secular changes of the obliquity of the ecliptic and the eccentricity of the Earth's orbit.It is the first in a series of 27 observatories built in the early Yuan dynasty.