The belief that planets, stars, and the Sun are fixed in the firmament, or a fixed celestial sphere, was a common view in ancient cosmology. This concept was prevalent in various civilizations and cultures, including ancient Greek, Mesopotamian, and medieval European cosmologies. It was based on observational evidence and the limited knowledge available at the time.
In ancient times, people observed that the celestial objects, such as stars and planets, appeared to move across the sky in a predictable manner. However, they lacked the understanding that these objects were actually moving in relation to Earth. To explain their observed motions, the concept of a celestial sphere or firmament was introduced. According to this idea, the celestial objects were thought to be embedded in a rotating, fixed sphere surrounding the Earth.
The fixed nature of stars and planets in the firmament was seen as consistent with the notion of a geocentric model of the universe, where Earth was believed to be at the center of the cosmos. This model placed Earth as the immovable point around which all celestial bodies revolved.
It wasn't until the scientific advancements of astronomers like Nicolaus Copernicus, Johannes Kepler, and Galileo Galilei in the 16th and 17th centuries that the heliocentric model gained acceptance. The heliocentric model proposed that the Sun, not Earth, was at the center of the solar system, and the planets orbited around it.
Through subsequent scientific discoveries and advancements, we now understand that the stars are distant suns, many of which have their own planets. The planets, including Earth, orbit around the Sun due to gravitational forces. Our current understanding of the universe is based on extensive astronomical observations, mathematical models, and scientific evidence, which have allowed us to develop more accurate explanations for the motions and behaviors of celestial objects.