2.2 Ancient Astronomy

Astronomy Around the World

• Early civilizations developed sophisticated astronomical systems:

  • Babylonian, Assyrian, and Egyptian Astronomers: Knew the approximate length of the year. The Egyptians, 3000 years ago, used a 365-day calendar and tracked the rising of Sirius, which corresponded with the Nile's flooding.

  • Chinese Astronomers: Also determined the length of the year around the same time as Egyptians. They recorded comets, bright meteors, dark spots on the Sun, and "guest stars" (suddenly visible stars) whose records are still used for studying ancient stellar explosions.

  • Mayan Culture: Developed a sophisticated calendar based on the planet Venus and made astronomical observations from dedicated sites a thousand years ago.

  • Polynesians: Navigated by stars over hundreds of kilometers of open ocean, enabling island colonization.

  • Ancient Britons: Used stone circles, like Stonehenge (dating back to 2800 BCE), to track the motions of the Sun and Moon.

Early Greek and Roman Cosmology

• Cosmology: The concept of the cosmos—its basic structure and origin.

• Spherical Earth: Educated people in the eastern Mediterranean knew Earth was round 2000 years before Columbus.

  • Pythagoras (2500 years ago) suggested Earth should be a sphere, believing circles and spheres to be "perfect forms."

  • Aristotle (384-322 BCE) provided convincing arguments for a spherical Earth:

    • The shadow seen on the Moon during a lunar eclipse is always round (Figure 2.9). Only a spherical object always produces a round shadow.

    • Travelers moving south observe stars not visible farther north, and the height of the North Star decreases. This implies movement over a curved surface.

  • Additional proofs for Earth's roundness:

    • Ships disappearing over the horizon: The hull disappears before the mast.

    • Photographs from space: The International Space Station and satellites show Earth is round from all perspectives.

    • Time zones: People in different time zones experience different positions of the Sun simultaneously.

Measurement of Earth by Eratosthenes

• Eratosthenes (276-194 BCE) made the first fairly accurate determination of Earth's diameter around 200 BCE.

• Method: Geometric, based on Sun's parallel rays.

  • On the first day of summer at noon in Syene, sunlight struck the bottom of a vertical well, indicating the Sun was directly overhead.

  • At the same time in Alexandria (north of Syene), a column's shadow indicated the Sun was not directly overhead, but its rays made an angle of about 1/50 of a circle (7°) with the vertical.

  • Reasoning: Because the Sun's rays are parallel, the 7° angle in Alexandria meant that surface of the round Earth had curved away from Syene by 7° (or 1/50 of a full circle).

  • Calculation: Alexandria was measured to be 5000 stadia north of Syene. Therefore, Earth's circumference must be 50 \times 5000 = 250,000 stadia.

  • Accuracy: His result was within 1% of the correct value of 40,000 kilometers if he used a stadium of 1/6 kilometer.

Hipparchus and Precession

• Hipparchus (around 150 BCE) was a pivotal ancient astronomer.

  • He compiled a star catalog with 850 entries, designating celestial coordinates and dividing stars into apparent magnitudes.

  • Discovery of Precession: By comparing his observations with older data, he found that the position of the north celestial pole had altered.

  • Deduction: This meant Earth itself must be wobbling.

  • Precession Defined: The slow but regular change in the direction of Earth's axis.

    • Similar to a spinning top wobbling, Earth's axis describes a path in the shape of a cone (Figure 2.12).

    • Caused by the gravitational pulls of the Sun and Moon on Earth's equatorial bulge.

    • Cycle Length: Takes about 26,000 years for Earth's axis to complete one circle.

    • Effect: The star closest to the north celestial pole changes over time (Polaris is current, Vega will be the North Star in 14,000 years).

Ptolemy’s Geocentric System of Planetary Motion

• Claudius Ptolemy (around 140 CE) wrote Almagest, a compilation of astronomical knowledge, which includes the work of Hipparchus and describes his own cosmological model.

• Ptolemy's Cosmological Model: A geometric representation of the solar system that predicted planetary positions and endured for over a thousand years.

• Challenge: Explaining "retrograde motion"—the temporary apparent westward motion of planets in the sky—while assuming a stationary Earth.

  • Retrograde Motion: Occurs when a faster-moving Earth overtakes a slower-moving outer planet, making the outer planet appear to drift backward against the background stars (Figure 2.13).

• Ptolemy's Solution: Used a system of circles:

  • Epicycle: Each planet revolved in a small orbit called an epicycle.

  • Deferent: The center of the epicycle then revolved about Earth on a larger circle called a deferent (Figure 2.14).

  • By combining the motion on the epicycle and the deferent, Ptolemy could replicate the observed retrograde motion.

  • The system was complex, requiring dozens of circles, and was not perfectly centered on Earth. He also introduced an offset point (equant) to account for observed speeds.

• Impact: Ptolemy's geocentric model dominated Western thought for nearly two millennia, despite its complexity, due to its ability to predict planetary positions.