Lecture 8 - Solar Eclipses and Early Greek Astronomy (1/31/96)

Seeds: Chapters 3, 4

  1. First Order of Business
  2. Review of Lunar Eclipses - three types:
    • Total Lunar Eclipse - Moon entirely in Earth's Umbra
    • Partial Lunar Eclipse - Moon partially in Earth's Umbra
    • Penumbral Lunar Eclipse - Moon in Earth's Penumbra only
    • Shadow of Earth (umbra) is curved -> spherical Earth
  3. Solar Eclipses
    • Earth - Sun distance varies by 1.7%
    • Earth - Moon distance varies by 7%
    • Sun and Moon appear nearly equal angular diameter from Earth
    • Three types of Solar Eclipses:
      • Total Solar Eclipse - we see Moon totally block Sun's disk (an umbral eclipse)
      • Annular Solar Eclipse - Moon too small to cover Sun but is entirely in front of Sun
      • Partial Solar Eclipse - Moon covers part of Sun's disk (a penumbral eclipse)
    • Eastward speed of Moon 3400 km/hr - Eastward rotation of Earth at Equator 1670 km/hr = Eastward advance of shadow 1730 km/hr
    • Appearance of Solar Eclipse:
      • partial - Moon takes "bite" out of Sun
      • almost total - "Bailey's Beads" as sliver of Sun visible through lunar mountains and valleys
      • almost almost total - "Diamond Ring" Sun shines through deep lunar valleys only
      • totality - Suns photosphere (disk) blocked, corona and prominences visible
    • Observing Eclipses with a pinhole camera (and by eye during totality)
  4. Eclipses Elsewhere, Transits and Occultations
    • Transits of Mercury and Venus across the face of the Sun
    • Eclipses on Jupiter by its own moons
    • Occultations of stars by the Moon
  5. Early Greek Astronomy
    • Earliest Greek natural philosophers were Ionian school (in region thats now Turkey)
    • Pythagoas (d.497 BC)
      • known for Pythaogean theorem a^2 + b^2 = c^2
    • assumed Earth,Sun,Moon and other bodies were spherical (from curved shadow of Earth on Moon during lunar eclipse)
    • series of concentric crystalline spheres, 7 planetary + 1 stars
    • mystical, proposed the "music of the spheres"
  6. Aristotle (384 - 322 BC)
    • most famous of Greek natural philosophers
    • most like a modern scientist
    • knew Earth was spherical (from curved shadow in eclipse, and changing elevation of stars as move north or south
    • knew Sun was more distant than Moon (Moon eclipses Sun)
    • considered rotation of Earth (instead of heavens) but dismissed it
    • considered revolution of Earth about Sun (instead of the opposite) but lack of parallax led him to dismiss this
    • adopted Pythagoras' cosmology of the spheres
  7. Later Greek Astronomy - the Alexandrian school
  8. Aristarchus of Samos (see next lecture)

Next Lecture - Later Greek Astronomy

First Order of Business

Review of Lunar Eclipses

Solar Eclipses

The average angular diameter of the Sun as viewed from Earth is 31'59", and varies due to the ellipticity of the Earth's orbit by about +/- 1.7%. The average angular diameter of the Moon is 31'5", and is smaller than that of the Sun. Thus, in the absence of variation, the Moon could not cover the entire Sun. However, the ellipticity of the Moon's orbit (combined with the fact that we view the Moon not from the center of the Earth but from the surface) causes the apparent size of the Moon to vary by almost 7% --- the maximum angular diameter of the Moon is 33'16" and is larger than even the maximum angular diameter of the Sun. Thus, the Sun can be totally eclipsed by the Moon.

Eclipses Elsewhere, Transits and Occultations

Early Greek Astronomy

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Steven T. Myers - Last revised 01Feb96