ANCIENT ASTRONOMY: THE GEOCENTRIC VIEW

MESOPOTAMIAN ASTRONOMY

MESOPOTAMIANS built observatories 6000 years ago:

  • the ziggurats had seven levels: one for each wandering object:
    Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn
  • tracked stars --- groups rising before sun at different times
    of year implied seasonal beginnings for planting and harvesting (zodiac).
  • divided circles in 360 degrees, each degree into 60 minutes
    and each minute into 60 seconds.
  • could predict planetary positions -- synodic periods,
    e.g., Mars returns to same location roughly every 780 days; 22 synodic periods = 47 years,
    so records of old planetary positions could give good locations.
  • knew about the SAROS cycle 2700 years ago:
    lunar eclipses occurred every 18.6 years.

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    GREEK ASTRONOMY: THE EARTH AT THE CENTER

    While they may have built upon Egyptian results (not preserved beyond
    calendars and orientation of temples), the Greeks tried to EXPLAIN and UNDERSTAND,
    not just PREDICT based upon cycles of motions.

    Thales (624--547 BCE) was claimed to have predicted a solar eclipse.

    Anaxamander (611--547 BCE) of Miletus (Asia Minor) produced a model: Earth as a cylinder,
    Sun, Moon and stars are fire filled wheels -- precursor of non-mythical explanations.

    Anaximenes of Miletus (585--526 BCE) believed stars were fixed to a solid,
    crystalline vault surrounding the Earth -- the concept of the Celestial Sphere.

    Pythagoras (582--500 BCE) and his students in Croton (S. Italy)
    argued the Earth and all heavenly bodies are perfect SPHERES.

  • All motions were perfect CIRCLES.
  • By then, it was understood that the Moon shine was reflected sunlight.

    Eudoxus (408--355 BCE) had planets moving on multiple spheres,
    all surrounding the Earth. These could explain RETROGRADE LOOPS in the
    orbits of MARS, JUPITER and SATURN
    -- but didn't account for diversity thereof or for variations in brightness of planets,
    since their distance from Earth was fixed.

    Aristotle (384--322 BCE) gave PROOFS that the Earth was SPHERICAL:
    objects all fell towards its center yet perpendicular to ground; implies it is a sphere (but it could still be a cylinder).

  • Noted shadows cast on moon during eclipse were always round --
    they sometimes wouldn't be if the earth were disk-like (or cylindrical).
  • But he also argued that since all stuff fell towards the earth,
    it was the heaviest thing around, therefore it shouldn't move.

    Aristarchus (310--230 BCE) of Samos applied Euclid's geometry to get the distance to the Moon.
    The angular diameter is measured directly; the linear diameter comes from seeing
    how much of the Earth's shadow the moon occupies during a lunar eclipse (about 3/8).
    If the Earth's diameter is known, this allows the Moon's to be found at about 3/8ths of Earth's.

  • He also used geometry to estimate that the Sun was 19 times further
    than the Moon (therefore 19 times larger, since angular sizes are the same).
  • Aristarchus then could estimate that Sun was about 7 times the diameter of the Earth (19 x 3/8)
    THIS LED HIM TO PROPOSE A HELIOCENTRIC COSMOLOGY ---
    with the BIG SUN at REST and the SMALL EARTH MOVING AROUND IT.
  • His lunar size was a little too big and his distance to the sun much too small,
    because of inaccurate measurements, but the techniques were clever.

    ANCIENT OBJECTIONS TO A HELIOCENTRIC PICTURE:

  • A moving earth should yield a powerful wind that would blow us off.
  • Stars didn't show measurable parallax (Greeks couldn't
    think of them be so much further away than planets).
  • It sure seems like we're standing still and everything is moving, doesn't it?

    Eratosthenes (276--195 BCE) used geometry and simple
    astronomy to make an accurate measurement of the Earth's radius.

  • He realized the difference in the altitude of the noonday Sun
    in Syene and Alexandria was the same as the difference in latitude between the cities.
    That gave the ratio:
    circumference of the Earth / 360 deg = distance / 7.2 deg
    Accuracy determined by distance in stadia --- measured by foot and
    uncertain, but around 40,000 km, and probably good to 10%
    (Correct value: 40,074 km or 24,890 miles)

    Hipparchus (190--125 BCE) utilized Aristarchus' method
    to get the Moon to be 59 Earth radii away (60 is correct!).

  • He made a better measurement of the length of the year.
  • Hipparchus also saw a NOVA and this caused him to make a
    CATALOG of bright stars.
  • Comparing his locations to those recorded about 170 years earlier
    he found a difference of about 2 degrees, and concluded that there was PRECESSION --
    his estimate of a 28,000 year period was very good.

    Ptolemy worked in Alexandria from 127--151 CE. As a geographer, he is
    the first one known to have used latitude and longitude on earth.
    His astronomy book, "mu epsilon gamma iota sigma tau nu" -- megiste -- or ``The Greatest''
    is usually known by its Arabic name Almagest.

  • Catalog of over 1000 stars with their brightnesses, using the MAGNITUDE SYSTEM.
  • Ptolemy's influence on astronomy was immense for he established a detailed
    GEOCENTRIC MODEL. He showed the simple system of Hipparchus,
    with just a DEFERENT and EPICYCLE was inadequate.
    His model added an EQUANT -- the motion of the center of the epicycle is
    uniform only if viewed from the equant.
  • However, Ptolemy's greatest contribution was the publication of his text,
    a summary of all earlier Greek astronomical knowledge.
  • While complex, IT WORKED (to the accuracy the Greeks could measure, anyway)
    and was used for 1500 years.