Final answer:
Nicolaus Copernicus was the earliest astronomer to question the validity of the Ptolemaic model. He proposed a heliocentric model of the solar system, which was supported by Kepler and Galileo.
Step-by-step explanation:
The earliest astronomer to really question the validity of the Ptolemaic model from the Almagest was Nicolaus Copernicus. He proposed a heliocentric (sun-centered) model of the solar system, in which the Sun is at the center and the Earth and other planets revolve around it.
Copernicus's ideas were further supported by the precise measurements of planetary motions by Tycho Brahe and the analysis by Johannes Kepler and Galileo Galilei. Kepler discovered three laws of planetary motion, including the fact that the motion of each planet is an ellipse.
This challenge to the geocentric (Earth-centered) model of the Ptolemaic system marked the beginning of the Scientific Revolution in the 16th and 17th centuries.