Celestial Sphere

Borrowed from antiquity, a cornerstone to astronomy is the celestial sphere, which describes that the celestial bodies are projected to a sphere or dome around the observer. Astronomers use a spherical coordinate system to describe the sky. Indeed, a spherical celestial sky is often put forward as evidence for a spherical world.

Going much further than it being a mere representation of a universe spread out around us, astronomers also explain that straight lines in space will appear to us as curves -- as great circles on the celestial sphere. The Moon Tilt Illusion, in which the illuminated portion of the Moon often and paradoxically points upwards and away from the Sun, is attributed as an effect caused by the Sun and Moon resting at different angles upon the celestial sphere. The Milky Way, usually thought of as a flat entity viewed from the side, appears as a bending arch in the sky on the celestial sphere. The Sun's path bends and warps on the celestial sphere. So too do shooting stars and meteors curve upon the celestial sphere above us. We are told, essentially, that we observe the heavens as if we were inside of a planetarium, where straight lines become curved on a spherical surface around us.

Why should it be that a straight line in space is warped and curved? If a straight line was receding in distance from our position, at which point would that straight line become curved? The celestial sphere is proposed by conventional astronomy without a mechanism, and with only vague statements that it is natural to observe nature in this way. Actively ignored as a topic of discussion, the warping of lines upon celestial sphere showcases the weakness and untenability of conventional astronomy.

In contrast to this, the Flat Earth Theory's celestial model directly provides a mechanism for why straight lines appear curved in the sky and for our domed observations.

Description
The book General Astronomy from WikiBooks says:

An astronomy course at the University of Virginia describes:

The New Jersey Institute of Technology states: