ch 2.2 readings

Learning Objectives

  • By the end of this section, you will be able to:

    • Describe early examples of astronomy around the world.

    • Explain how Greek astronomers deduced that Earth is spherical.

    • Explain how Greek astronomers calculated Earth’s size.

    • Describe the motion of Earth called precession.

    • Describe Ptolemy’s geocentric system of planetary motion.

Astronomy Around the World

  • Ancient Cultures and Astronomy

    • Babylonians, Assyrians, and Egyptians:

      • Determined the length of the year; Egyptians created a 365-day calendar based on the star Sirius.

    • Chinese Astronomers:

      • Recorded comets, meteors, and solar spots, and developed their calendar.

      • Noted on transient stars ("guest stars").

    • Mayan Civilization:

      • Created a calendar based on Venus, with sites built for astronomical observations.

    • Polynesian Navigators:

      • Used star positions for navigation over vast ocean distances.

    • British Stone Circles:

      • Ancient peoples built structures like Stonehenge to track the motions of Sun and Moon (dating as far back as 2800 BCE).

Early Greek and Roman Cosmology

  • Cosmology:

    • Derived from Greek, it refers to the study of the universe's structure and origin.

  • Knowledge of a Spherical Earth:

    • Recognized over 2000 years ago; influenced by philosophers such as Pythagoras (circa 2500 years ago) who viewed spheres as perfect forms.

    • Aristotle (384–322 BCE):

      • Explained lunar phases as resulting from illuminated portions visible from Earth.

      • Noted that the Moon's shadow during lunar eclipses is always round—a key argument for a spherical shape.

      • Observed varying night sky visibility when traveling south, indicating Earth's curvature.

    • Aristarchus of Samos (310–230 BCE):

      • Proposed a heliocentric model (Earth orbits the Sun) but was largely dismissed.

  • Stellar Parallax:

    • The Greeks could not detect this shift in star positions due to Earth's movement, leading them to favor the geocentric model.

Astronomy Basics: Evidence of Earth’s Roundness

  • Observations:

    • Ships disappearing hull-first over the horizon.

    • Photographs of Earth from space show it as a sphere.

    • Interest in regional daylight variations at the same time, affirming sphericity.

Measurement of Earth by Eratosthenes

  • Eratosthenes (276–194 BCE):

    • First to calculate Earth's circumference using geometric methods and shadows.

    • Observations from Syene (directly beneath the Sun) and Alexandria (shadow cast by a column).

    • Calculated angle of 7° corresponding to 1/50 of Earth’s circumference, inferring total circumference as 250,000 stadia.

    • Accuracy of estimates varied based on the length of a stadium used.

Hipparchus and Precession

  • Hipparchus:

    • Lived in Nicaea and established a star catalog with 850 entries; introduced celestial coordinates.

    • Discerned changes in the north celestial pole's position—related to Earth's wobble, termed precession.

  • Precession:

    • The slow change of Earth's axial orientation, moving through a circular path over 26,000 years.

    • Historical reference points for the North Star changing over time.

Ptolemy’s Model of the Solar System

  • Claudius Ptolemy (circa 140):

    • Author of Almagest, consolidating astronomical knowledge, particularly from Hipparchus.

    • Developed a geocentric model predicting planetary positions using complex epicycles and deferents to explain retrograde motion.

    • Model lasted over a thousand years until challenged by Copernicus.

  • Retrograde Motion:

    • Observing outer planets appearing to reverse direction; explained in Ptolemy's model as due to combined motions of Earth and planets.

  • Complexity of Ptolemy’s Model:

    • Required numerous circles (epicycles) to explain planetary motion under the assumption of a stationary Earth.

    • Illustrated the mathematical genius of Ptolemy, despite the model's eventual obsolescence.