Which of the following is no longer considered valid?
a. the ptolemaic system
b. the copernican system
c. Kepler’s laws of planetary motion
d. Newton’s law of gravity
The correct answer and explanation is :
The correct answer is: a. the Ptolemaic system
Explanation:
The Ptolemaic system, developed by Claudius Ptolemy in the 2nd century AD, described the universe with Earth at its center — a model known as geocentrism. In this system, the sun, moon, planets, and stars all orbited Earth in complex paths involving “epicycles” (small circles moving along larger circles) to account for observed planetary motions like retrograde motion. This model was widely accepted for over a thousand years, especially because it fit well with philosophical and religious views of the time.
However, with the scientific revolution in the 16th and 17th centuries, the Ptolemaic system was replaced. Nicolaus Copernicus proposed the Copernican system, which placed the Sun at the center (heliocentrism), simplifying the explanation of planetary motions. Later observations by astronomers like Galileo Galilei (who used telescopic evidence) and Johannes Kepler (who formulated precise mathematical laws for planetary orbits) provided strong support for heliocentrism.
Kepler’s laws of planetary motion (early 1600s) described how planets orbit the Sun in elliptical paths, not perfect circles, and matched observations with unprecedented accuracy. These laws remain valid and are still taught today as fundamental to understanding celestial mechanics.
Newton’s law of gravity, formulated in the late 17th century, described a universal force of attraction between masses. Although Einstein’s theory of general relativity later refined our understanding of gravity, Newton’s law is still extremely useful and accurate for most everyday applications and even for many space travel calculations.
Thus, the Ptolemaic system is the one that is no longer considered scientifically valid. It has been completely superseded by heliocentric and gravitational models based on modern observational evidence and mathematical theory.