Archaeoastronomy is the study of the astronomical knowledge of ancient peoples. Advanced technology is not necessary to have a sophisticated understanding of celestial cycles.
Astronomical alignments in structures are common around the world. Examples include Stonehenge (England), Newgrange (Ireland), the Temple of Isis at Dendera (Egypt), and Sun Dagger (New Mexico).
Most early Greek astronomers and philosophers believed that we inhabit a geocentric universe. They also believed in the principles of perfection of the heavens and uniform circular motion. They believed that the Earth did not move because they could not observe any parallax, the apparent motion of an object because of the motion of the observer. The motions of the Sun, Moon, and planets (including apparent retrograde motion) were explained by models of nested sets of rotating crystalline spheres (which carried these objects around Earth).
Aristarchus proposed that Earth rotates on its axis and orbits the Sun, but the Aristotelian geocentric universe was accepted for almost 2,000 years. Ptolemy ensured this continued acceptance by transforming it into a sophisticated mathematical model. He used epicycles to explain planetary motions.
Nicolaus Copernicus‘s book, De revolutionibus orbium coelestium, was published in 1543. In it, Copernicus proposed a heliocentric universe, although he (incorrectly) retained uniform circular motion and included his own epicycles. The Copernican Revolution is the paradigm shift from Earth being at the center of the universe to the Sun being at the center of the Solar System.
The puzzle of planetary motion was solved by Tycho Brahe and Johannes Kepler. Tycho made highly accurate measurements of the positions of celestial objects and Kepler analyzed the observational data to derive laws. Kepler’s three laws of planetary motion state that 1) the orbit of a planet is an ellipse with the Sun at one focus, 2) a planet sweeps out equal areas in equal intervals of time (a planet moves faster when closer to the Sun and slower when farther away), and 3) the square of the orbital period of a planet is proportional to the cube of the planet’s average distance from the Sun. The geometry of an ellipse is described by two simple numbers: 1) the semimajor axis (half of the longest diameter) and 2) the eccentricity (half the distance between the foci divided by the semimajor axis). The eccentricity of an ellipse tells you its shape; if it is nearly equal to one, the ellipse is very elongated, and if it is close to zero, the ellipse is more circular.
Galileo used the newly invented telescope to discover mountains on the Moon, the phases of Venus, and the Galilean moons. These discoveries undermined the perfection of the heavens and provided evidence for the Copernican hypothesis. He was condemned by the Inquisition not for heresy but for disobeying orders not to hold, teach, or defend Copernicanism.
The transition from ancient to modern astronomy (and the birth of modern science) began with the replacement of the Ptolemaic model of the universe by the Copernican model (in the years between the publication of De revolutionibus in 1543 and Galileo’s death in 1642).
—July 2020
—January 2021
—January 2023
Lance Jones 