What state has an arc of a perfect circle?

At the moment of a lunar eclipse... the state when the earth, moon and sun are in direct alignment with each other.

Details: At night, when a full moon is in the sky, and when the sun is directly behind the earth in line with the moon, and the shadow of the earth is projected onto the moon's surface....

"The earth together with its surrounding waters must in fact have such a shape as its shadow reveals, for it eclipses the moon with the arc of a perfect circle."~Nicolaus Copernicus.

A similar effect can be observed during an annular eclipse of the sun, when the moon comes between the earth and the sun, obscuring the sun except for a bright ring of sunlight around the circumference.