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 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 from your 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 the world in this way. Actively ignored as a topic of dicussion, the warping of lines upon celestial sphere showcases the weakness and untenability of conventional astronomy.

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

Description

General Astronomy from WikiBooks says:

“ The stars appear to move together across the sky during the night, rising in the east and setting in the west, as if they are affixed to the inside of a dome. Because of this, many ancient civilizations believed that a dome really did enclose the Earth. Only a few centuries ago astronomers came to realize that the stars are actually very far away, scattered throughout the Milky Way Galaxy, rather than attached to the inside of a vast sphere.

The old idea remains useful, however. The concept of the celestial sphere provides a simple way of thinking about the appearance of the stars from Earth without the complication of a more realistic model of the universe. Working with the celestial sphere offers a convenient way of describing what we see from Earth. When we refer to the celestial sphere, we are imagining that everything we see on the sky is set on the inside of a huge spherical shell that surrounds the Earth. We will use the reference points of the celestial sphere as the basis for several coordinate systems used to place celestial locations with respect to one another and to us. ”

An astronomy course at the University of Virginia describes:

“ Everything appears to move around us as though we were at the center of the Universe.

The Celestial Sphere is an imaginary concept that is a useful tool for understanding some workings of the sky: - Sphere of infinite radius with Earth at center. - Stars on "surface of sphere".

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Even though we now know that this ancient model of a stationary Earth surrounded by a rotating sphere of stars is incorrect, we still use this model because it is a convenient way to predict the motions of the stars and planets relative to a location on the Earth. ”

The New Jersey Institute of Technology states:

“ Think of the sky as painted, or projected, onto the inside of a dome--like a planetarium dome. ”