More than 2300 years ago, in 350 BC, Aristotle noted that the Moon displays different shapes every month, but during an eclipse, the outline is consistently circular. This was one of the first observational evidences of the spherical shape of the Earth.
The photograph below, taken by Marco Bastoni, displays a sequence of images depicting the Moon’s transit within the Earth’s shadow cone during the lunar eclipse on 27 July 2018. The circular shape of the Earth’s shadow is distinctly visible.
A lunar eclipse happens when the Sun, Earth, and Moon are perfectly aligned, causing the Moon to be immersed in the Earth’s shadow cone. The Earth, being in the middle, blocks the Sun’s light and casts its shadow on the lunar surface.
Aristotle observed that during a lunar eclipse, the shape of the Earth’s shadow on the Moon is always circular. He concluded that the Earth must be spherical because only a sphere can cast a circular shadow from any direction of light. Other shapes, such as a disc, would only cast a circular shadow if the light beam was perpendicular to its plane.
Aristotle observed that during a lunar eclipse, the shape of the Earth’s shadow on the Moon is always circular, and discussed the deep implications of this in the “De Caelo” (“On the Heavens”), Aristotle’s chief cosmological treatise written in 350 BC. He concluded that the Earth must be spherical because only a sphere can cast a circular shadow from any direction of light. Other shapes, such as a disc, would only cast a circular shadow if the light beam was perpendicular to its plane.
