ESC 1000

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ESC 1000

• Lecture 12
• Origin of Modern Astronomy

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• Ancient Greeks
• Used philosophical arguments to explain natural
  phenomena
• Also used some observational data
• Geocentric (Earth-centered) view of the universe
• Seven heavenly bodies (planetai)
• Changed position in sky
• The seven wanderers included the
• Sun
• Moon
• Mercury through Saturn (excluding Earth)

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Earth Basics

• Shape
• Pythagoras
• Aristotle
• Size
• About 200 BC Eratosthenes heard tales from
  travelers from Egypt that at noon on the longest
  day of the year you could look down a well and
  see the suns reflection. This never happened in
  the hometown of Eratosthenes so he assumed it had
  to do something with the curvature of the Earth.
  Thus, he measured the length of an obelisk and
  from this and the height of the monument he found
  that the angle the suns rays made with the
  vertical was about 7 degrees. From this he
  concluded that the distance all the way around
  the world was 25,000 miles.

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• Relative size of Earth, moon and sun
• Aristarchus- determined the relative size of the
  earth, moon and sun by the following
  observations
• Curvature of the earths shadow on the moon
  during lunar eclipse indicated that the earth was
  much larger than the moon (extended the shadow
  all the way around it implied an object 3 or 4
  times bigger.)
• Lunar eclipse it took the moon 30 min to enter
  total darkness and then 30 min to come out of
  the shadow.

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• Size of moon and sun
• Aristarchus observed that at half moon the line
  from the sun to the moon is perpendicular to the
  line from the moon to the earth.
• Heliocentric Theory
• Planets exhibit an apparent westward drift
• Called retrograde motion
• Occurs as Earth, with its faster orbital speed,
  overtakes another planet

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• Ptolemaic system
• A.D. 141
• Geocentric model
• To explain retrograde motion, Ptolemy used two
  motions for the planets
• Large orbital circles, called deferents, and
• Small circles, called epicycles

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Transition Period (1540-1630)

• Five noted scientists
• Nicolaus Copernicus (1473-1543)
• Concluded Earth was a planet
• Constructed a model of the solar system that put
  the Sun at the center, but he used circular
  orbits for the planets
• Ushered out old astronomy
• Tycho Brahe (1546-1601)
• Precise observer
• Tried to find stellar parallax

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• Johannes Kepler (1571-1630)
• Ushered in new astronomy
• Planets revolve around the Sun
• Three laws of planetary motion
• Orbits of the planets are elliptical
• Planets revolve around the Sun at varying speed

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• Galileo Galilei (1564-1642)
• Supported Copernican theory
• Used experimental data
• Constructed an astronomical telescope in 1609
• Four large moons of Jupiter
• Planets appeared as disks
• Phases of Venus
• Features on the Moon
• Sunspots

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• Sir Isaac Newton (1643-1727)
• Law of universal gravitation
• Proved that the force of gravity, combined with
  the tendency of a planet to remain in
  straight-line motion, results in the elliptical
  orbits discovered by Kepler

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Constellations

• Today 88 constellations are recognized
• Constellations divide the sky into units, like
  state boundaries in the United States
• The brightest stars in a constellation are
  identified in order of their brightness by the
  letters of the Greek alphabet alpha, beta, and
  so on

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Motions of the Earth

• Rotation
• Turning, or spinning, of a body on its axis
• Two measurements for rotation
• Mean solar day
• Sidereal day
• Revolution
• Perihelion
• Aphelion
• Astronomical Unit (AU)
• Precession

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• Phases of the Moon
• When viewed from above the North Pole, the Moon
  orbits Earth in a counterclockwise (west to east)
  direction
• The relative positions of the Sun, Earth, and
  Moon constantly change
• Lunar phases are a consequence of the motion of
  the Moon and the sunlight that is reflected from
  its surface

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• Lunar motions
• Earth-Moon
• Synodic month
• Cycle of the phases
• Takes 29 1/2 days
• Sidereal month
• True period of the Moon's revolution around Earth
• Takes 27 1/3 days
• Moon's period of rotation about its axis and its
  revolution around Earth are the same, 27 1/3 days
• Causes the same side of the moon to always face
  Earth
• This is called Synchronous Rotation

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• Eclipses
• Two types of eclipses
• Solar eclipse
• Lunar eclipse

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Solar Eclipse
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Lunar Eclipse