As noted in the book "Huainanzi", in the 2nd century BC, Chinese astronomers effectively inverted Eratosthenes' calculation of the curvature of the Earth to calculate the height of the Sun above the Earth. By assuming the Earth was flat, they arrived at a distance of (approximately ). The "Zhoubi Suanjing" also discusses how to determine the distance of the Sun by measuring the length of noontime shadows at different latitudes, a method similar to Eratosthenes' measurement of the circumference of the Earth, but the "Zhoubi Suanjing" assumes that the Earth is flat.