Chapter 10
10
The chapter explores the well-supported reasons behind the Aristotelian worldview's beliefs that the Earth was spherical, stationary, and at the center of the universe, primarily drawing from Ptolemy's Almagest and Aristotle's On the Heavens.
The Earth as Spherical
Contrary to popular misconception, educated people since ancient Greek times believed the Earth was spherical due to several observations:
Differential Rise/Set Times: The sun, moon, and stars rise and set at different times for observers at different longitudes (earlier in the east, later in the west). Eclipse times also vary proportionally to distance. This observation is consistent with a uniformly spherical Earth and inconsistent with concave, flat, pyramidal, cubic, or other polygonal shapes.
Varying Star Visibility: Different stars become visible or hidden as one moves north or south, such as Polaris in the northern hemisphere or the Southern Cross in the southern hemisphere. This indicates curvature in the north-south direction, disconfirming cylindrical or other non-spherical shapes.
Approaching Land: When sailing towards land, the tops of high places like mountains are seen first, with lower areas gradually becoming visible. This is explained by the curvature of the Earth's surface submerging distant objects.
The Earth as Stationary
Prior to the 1600s, there were strong, though ultimately mistaken, reasons to believe the Earth was stationary:
Common-Sense Arguments:
Lack of Observable Motion Effects: If the Earth were rotating (over 1,000 mph at the equator) or orbiting the sun (around 70,000 mph), one would expect to feel significant effects like powerful winds or vibrations, similar to what is experienced in a fast-moving vehicle. Since no such effects are observed, it disconfirmed Earth's motion.
Inertia: Just like a large boulder, the Earth, being massive, would not move or maintain motion without a continuous and immense force. No such force was apparent, making a stationary Earth seem more reasonable.
The Argument from Objects in Motion: This argument posits that if the Earth were moving, an object thrown straight up would land behind the thrower, as the Earth would move out from underneath it. Since thrown objects land largely in the same spot, this was taken as strong evidence for a stationary Earth. (The flaw lay in the incorrect auxiliary hypothesis about the behavior of objects in motion, which was later corrected by the laws of inertia).
The Argument from Stellar Parallax: Parallax is the apparent shift in an object's position due to the observer's motion. Ptolemy noted that the "angular distances of the stars appear everywhere equal and alike," meaning no stellar parallax was observed. If the Earth were moving (either daily rotation or yearly orbit, covering vast distances), an observable shift in star positions relative to each other should occur. Since no such parallax was detected, it strongly supported a stationary Earth. (The key auxiliary hypothesis here was about the distance to stars; they were not believed to be inconceivably far away. Stellar parallax was eventually observed in 1838, proving the Earth's motion and the immense distances to stars).
The Earth as the Center of the Universe
This belief was a natural fit with a spherical and stationary Earth and aligned with Aristotelian physics:
Apparent Revolution: The moon, sun, stars, and planets all appear to revolve around the Earth, making an Earth-centered view the most straightforward explanation.
Natural Place of Elements: In the Aristotelian worldview, the earth element naturally tends to move towards the center of the universe. Since the Earth is primarily composed of this element, its natural place would be at the universe's center.
Perpendicular Fall of Objects: Heavy objects dropped from various points on the spherical Earth fall perpendicular to its surface, with their paths converging at the Earth's center. This implied that the center of the Earth must also be the center of the universe, as objects move towards the universe's center.