Ancient Astronomy

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Ancient Astronomy

• Daniel High School
• Science Department

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China

• Developed the length of the year at 365.25 days
• See drawing of local meridian, zenith, and how
  length of year can be calculated
• Really big on eclipses

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Babylonians

• Tried using lunar cycles to split up the year but
  if you do it ends up 11.25 days short
• So they added an extra month whenever needed to
  bring calendar back in tune with seasons

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Assyrians 800 BC

• Used the stars as their guide and did a better
  job of keeping track of their movements than any
  previous people

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Egyptians

• Used constellations to help identify the coming
  of Spring

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Equinoxes

• Usually March 21 and September 21
• During the course of the year the sun appears to
  rise over different points along the eastern
  horizon

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Modern Calendar Timing

• Today we keep all this in track by using Leap
  Year.
• This is adding 1 extra day to our calendar every
  4 years. However, there is a longer cycle in
  place as well.
• Homework 1 Look up information about the Leap
  Year cycles.
• Homework 2 Stonehenge what is the latest
  theory (including solstice alignment and/or
  eclipse predictor)

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Early Greek views of Universe

• Thales of Miletus (636 546 BC) - believed
  water to be the primary principle of all things
  and saw earth as a flat disk floating on water.
• Anaximander (611 547 BC) - saw universe as
  infinite with the earth as a cylinder floating
  free in space.
• Anaximenes (585 526 BC) - saw the earth as a
  disk supported by air.

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More Greeks

• Anaxagoras (499 428 BC) - saw the earth and
  moon as earthlike in nature (having solid, crusty
  surface and shining by reflected sunlight)
• He also believed lunar eclipses to be a result of
  the moons moving into earths shadow (was exiled
  for such sacrilegious ideas).

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Still more

• Pythagoras of Samos (582 507 BC) - believed
  earth to be spherical and the curvature seen in
  lunar eclipse as being suggestive of the curved
  nature of earth itself.
• No mention at this point of earth rotating on
  axis or revolving around the Sun.

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More Greek Influence

• Philolaus (450 BC) - promoted the idea of a
  central fire around which revolved the earth,
  moon, sun, and planets
• His ideas did get people thinking about
  explanations of movement
• Democritus (250 BC) - Milky Way is really lots
  of distant stars which appear close together

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Aristotle

• Tutor of Alexander the Great
• Developed idea of spherical earth center of
  Universe
• Earth is stationary while sun, moon, planets,
  moved around the earth in circular orbits

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Ideas Developed in Alexandria

• Aristarchus of Samos (310 230 BC) - asserted
  that the sun was the center of solar system and
  that earth other planets revolve around the sun
  
• Apparent motion of objects due to result of
  earths rotation in circular orbit around sun
• Stars to be very distant in fixed positions
• Hard to see movement of stars (they are too far
  away) so his ideas were rejected idea of
  stellar parallax

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Hipparchus (died around 127 BC)

• Father of positional astronomy first to record
  positions of stars in sky
• Accurately stated position of more than 1000
  stars that were ranked according to their
  apparent brightness
• 6 categories brightest 1st magnitude stars
  while the dimmest (to human eye) 6th magnitude
  stars
• Sun appears to move faster through stars during 1
  part of year and slower during other part
• Result must be that orbit is not circular
  developed idea of epicycles

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Hipparchus cont.

• Compiled eclipse records from Babylonians (used
  them to predict eclipses hundreds of years in the
  future
• Partially successful attempt to determine
  distances between objects in solar system and
  their sizes (he concluded that the distance to
  the moon was about 71 times the radius of the
  earth which is close to accepted value of 60
  times earths radius)

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Hipparchus cont.

• Discovered the precession of equinoxes
• Earth revolves around an axis that tilts about
  23 from the plane of its orbit, called the
  ecliptic. The axis of rotation wobbles like a
  spinning top. Earth completes one wobble in
  about 26 000 years. This precession makes true
  north move away from the North Star. Polaris is
  our north star now but in 13 000 years it will be
  Vega. Also the position of the sun with respect
  to the stars at the spring and fall equinox
  appear to move around the sky every 26 000 years.

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Claudius Ptolemy

• Last astronomer of Alexandrian School
• Died sometime between 141 and 151 AD
• Published Almagest - a compilation of works
  of astronomy
• Used epicycles (no observations of time
  invalidated the model corrections were often
  needed
• Remained model for 1300 years (Dark Ages)

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Ptolemy cont.

• Gave the following reasons for proposition that
  earth is round
• Sun, moon, stars do not rise at same time
  everywhere on earth
• Lunar eclipses seen at different times at two
  distant locations
• Difference in local time between events is
  proportional to distance between them
• Constellations appearance changes with latitude
• Appearance of mountains changes as you sail
  toward them

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Ptolemy cont.

• Many translations by Arabic investigators
• Said that our planet is stationary center of
  universe

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St. Thomas Aquinas

• Elevated Aristotelian model to level of religious
  dogma with earth at center of Universe)
• If you went against these ideas Inquisition
  (could be put to death)
• In 1600 Giordano Bruno opponent of Aristotelian
  ideas was put to death

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1500 New Age Begins

• Experimentation beginning as Dark Ages approach
  end
• Education and communication (printing press)
  leaving the hands of the Church
• Homework Describe the changes in science that
  occurred as mankind left the Dark Ages

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Copernicus

• Published late in life (due to political and
  religious considerations)
• Idea at rest in middle of everything is the
  sun, with planets revolving around it
• Died within days of publication
• Homework look up retrograde motion

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Brahe (1546 1601)

• On island of Hven established observation and
  constructed instruments capable of measuring
  angles between stars quite accurately
• 800 stars measured to accuracy of 1/60 of a
  degree
• Known for accurate observations and keeping
  careful records

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Johannes Kepler (1571 1630)

• Attempted to fit various orbital shapes to
  observed motion of Mars, shapes that would
  provide for variation in speeds noticed
• Decided that best fit would be ellipse
• Best fit of observed motion of Mars is elliptical
  orbit with sun at one focus with the other focus
  being an imaginary point in space

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Keplers Laws of Planetary Motin

• Law 1 Each planet moves around the sun in an
  orbit whose shape is that of an ellipse with the
  sun at one focal point.
• Law 2 A straight line joining the planet and
  the sun sweeps out equal areas of space in equal
  intervals of time.
• Law 3 The square of the periods of any two
  planets have the same ratio as the cubes of their
  semimajor axes.

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Galileo
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Newton
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Cosmogony

• Definition a collection of ideas that gives an
  explanation of how the solar system began.
• The first physical cosmogony was proposed by the
  French philosopher Rene Descartes in 1644.
• Descartes envisioned a universe initially filled
  with gas. He believed that this gas would
  contain vortices (like eddies in a stream of
  flowing water).