Physical Science Semi-Final Lesson 2: Greek Terrestrial Motions

Terrestrial Motions in Ancient Greece

• Early people observed motions of objects on Earth, in the sky, and in the universe, which led to the concept of day and night.
• The Greeks' idea of terrestrial motion was heavily influenced by Aristotle's philosophy, which was more speculative than experimental.

Aristotle's Philosophy

• Aristotle believed that everything on Earth had an appointed place and function.
• He categorized objects into four elements: earth, water, air, and fire.
• Objects made of earth and water sought to move closer to the center of Earth.
• Objects made of fire and air sought to move to the outer spherical layers.
• Natural motion was the result of objects moving to their rightful place.
• Forced motion required an outside mover.
• Aristotle postulated that planets and stars were made of a fifth element called "quintessence" and moved in circular paths.

Diurnal Motion

• Diurnal motion refers to the apparent daily motion of the sky from east to west.
• This motion is caused by Earth's rotation from east to west.
• The axis of this motion coincides with Earth's axis of rotation.

Annual Motion

• Annual motion refers to the events or phenomena that occur once a year.
• Specifically, it refers to the motion of Earth in its orbit around the Sun, which takes a year to complete.
• The changing of the seasons is attributed to the revolution of Earth around the Sun.

Precession of the Equinoxes

• Precession of the equinoxes refers to the gradual shift in the orientation of Earth's axis of rotation.
• This motion is caused by the gravitational forces of the Sun and the Moon, and to a lesser extent, other celestial bodies on Earth.
• The cycle of precession takes approximately 26,000 years.
• Credit for discovering precession of the equinoxes is given to Hipparchus.

Concept of the Spherical Earth

• The concept of the spherical Earth dates back to around the 6th century B.C. in ancient Indian philosophy and ancient Mediterranean thought.
• Aristotle supported the idea of a spherical Earth, providing physical and observational arguments.
• He observed that every portion of Earth tends toward the center, forming a sphere.
• He also observed that travelers going south see southern constellations rise higher above the horizon, and the shadow of Earth on the moon during lunar eclipse is round.

Plato's Problem of "Saving the Appearances"

• Plato's problem was to reconcile the imperfect motions of the planets with the idea of a perfect universe.
• He believed that the erratic motions of the planets needed to be explained to "save the appearances" and create a logically integrated universe.
• He recognized that the planets do not move at a constant rate and move in irregular retrograde loops.

Eudoxus' Model of the Universe

• Eudoxus, a student of Plato, used geometry to envision a model of the universe.
• His model was homocentric and concentric spheres inside a sphere sharing a common center, which was Earth.
• Eudoxus' model explained the erratic motion of the planets while still allowing for perfectly uniform and circular motion.