An Introduction to Astronomy Part II: Historical Development of Astronomy

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An Introduction to AstronomyPart II
Historical Development of Astronomy

• Lambert E. Murray, Ph.D.
• Professor of Physics

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The Gift of the Greeks

• The Greek philosophers were the first to realize
  that the universe was comprehensible
• By careful observation of the motions of the
  stars and planets they developed a model for
  the universe that satisfactorily explained the
  known universe for nearly 1500 years.
• What facts can we learn about our universe by
  careful observation of the different objects in
  the day and night sky?

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What do you know about the Suns Motion?

• Where and when does the Sun rise and set?
• Are the days always the same length? Why?
• Why is it hotter in the summer and colder in the
  winter?
• How high does the Sun get in the daytime sky?
  Does this change during the year?
• Why dont you see the stars during the day?
• What causes solar eclipses?

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Facts about the Sun

• It rises in the East and sets in the West.
• It reaches different maximum heights in the
  summer and winter.
• It rises north of East in the summer and South of
  East in the winter.
• The length of day and night changes with the
  seasons.
• Sometimes the sun is blotted out a solar
  eclipse.

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What do you know about the Moons Motion?

• Where and when does the Moon rise and set? Does
  it rise and set at different times each night?
• What direction is the Moon moving relative to the
  stars?
• What causes the phases of the Moon? Is it the
  Earths shadow? Where would you expect to see a
  full moon?
• Can you see the Moon during the day?
• What causes lunar eclipses?

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Facts About the Moon I

• It has the same basic daily pattern as the Sun
  moving from East to West during the day/night.
• The moon changes its position relative to the
  stars (and Sun) each night moving slowly in an
  Eastward direction relative to the
  constellations.
• The moon passes through phases, completing one
  cycle about every 28 days.
• The moon can sometimes be observed during the
  daytime.
• The Full Moon is seen when it is opposite the Sun
  in the sky, while a New Moon is seen near the
  Sun.
• Sometimes the moon is blotted out a lunar
  eclipse.

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Phases of the Moon
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What do you know about the motion of the Stars?

• What is a constellation?
• Do you always see the same constellations at
  night? How do they change during the year?
• How do the Sun and Moon move relative to the
  stars?
• How do the stars appear to move during the night?
• Why cant you see the stars during the day? What
  if we had no atmosphere?
• What happens to constellations as you move north?
• Is everything that looks like a star a star? How
  can you tell which are stars?

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Facts About the Stars I

• There are a very large number of stars many are
  invisible to the naked eye.
• Most stars appear to move in fixed groups (called
  constellations) with the same basic daily motion
  as the Sun and Moon, moving from East to West.
• Stars are seen only at night (although the
  brightest ones are seen just before sunset and
  are still visible just after sunrise).
• The North Star is approximately fixed in the
  night sky.

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Facts About the Stars II

• Different constellations are visible at different
  times of the year, and these constellations
  appear to move Westward during the year.
• As one moves northward, the North Star appears to
  move upward in the night sky, while the stars in
  the south drop below the horizon.
• Some stars (the wandering stars) appear to move
  among the other stars. These stars sometimes
  move in a bizarre manner.

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Star Tracks
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Constellations
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Constellations and Asterisms

• Usually we think of a constellation as a
  particular grouping of stars that may look like
  some stick figure man, lion, etc. Many of these
  grouping of stars have been identified by various
  names in various nations over past history. To
  make things more uniform, the International
  Astronomical Union in 1928 divided the night sky
  into 88 well-defined regions (named
  constellations) associated with these well know
  star groupings.
• An asterism is a group of easily identifiable
  stars which may be a part of one or more
  constellations.

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The Winter Triangle An Asterism
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Early Models of the Solar System

• A simple model to describe the motion of the
  stars in a 24 hour period might be to picture the
  stars on a spherical shell which rotate around
  the earth.
• An alternate model, which works just as well, is
  for the earth to rotate inside the shell of
  stars.
• In order to explain the motion of all the other
  celestial bodies, more spherical shells must be
  added to this model.

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Aristotles Model of the Universe

• Aristotles Model of the Solar System was based
  upon celestial observations and upon terrestrial
  observations (fire and air always rise).
• This diagram of his model indicated very little
  detail in the actual way the planets moved, but
  the position of the sun and various planets could
  be modeled by having the different shells move at
  different rates and at slightly different angles
  to one another.

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A More Complex Model
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The Celestial Sphere

• The celestial sphere is a model of the night sky
  where we assume that all the stars in the heavens
  are attached to a sphere surrounding the Earth.
• Positions on the celestial sphere are designated
  in one of two ways
• Local Altitude and Azimuth angles
• Declination and Right Ascension angles
• Declination is like the latitude angle on the
  Earth, but measured from the Celestial Equator.
  This angle is measure in degrees.
• Right Ascension is like the longitude angle on
  Earth, but measured from the Vernal Equinox.
  This angle is measured in hours, minutes, and
  seconds.

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The Celestial Sphere
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The Celestial Sphere with Constellations
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Additional Facts Known to Early GreekAstronomers

• Aristotle had argued that the earth, moon, and
  sun were spherical objects based partly upon
  observations of eclipses (about 350 B.C.).
• Using geometric techniques, the early Greek
  astronomers had determined approximate values
  for
• the diameter of the earth
• the relative distances from the earth to the moon
  and to the sun.
• the relative diameter of the moon and the sun.
• They could accurately predict the occurrence of
  eclipses.

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Aristarchus Method for Determining the Relative
Distance to the Sun and Moon
Note This is a schematic. It is not totally
accurate for the Sun and Moon.
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Eratosthenes Method for Determining the Earths
Radius
Light from the Sun
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Aristarchus Proposal to Determine the Moons
Distance
If we assume the Earths shadow is approximately
the same diameter as the Earth, we can
approximate the diameter of the Moon (by seeing
how far the Moon moves through the Earths
shadow). Thus Distance to Moon
Diameter of Moon/Angular size of Moon
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Lunar Eclipse

• This sequence of photographs shows the shadow of
  the Earth projected across the path of the Moons
  orbit.

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Scientific Evidence for the Geocentric Model in
200 B.C.

• All things fall to the earth - even objects from
  space.
• The motion of the sun, moon, stars, and planets
  could be well explained using Ptolemys
  geocentric model.
• The model was based upon perfect circles.
• This model worked well for over a thousand years.
• We cant feel the earth move.

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Arguments for a Heliocentric Model in 200 B.C.

• Aristarchus proposed an alternate, heliocentric
  (sun-centered) model which could also explain the
  observed motions of the celestial bodies.
• His major reason for proposing this model was the
  enormous size of the sun.
• However, one observation decided against this
  model there was no observed parallax of the
  stars.

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The Failure of Parallax
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Ptolemy Refines the Model

• Ptolemys principle contribution to astronomy was
  his efforts in fine-tuning the geocentric model
  so that this model could accurately describe and
  predict the motions of the celestial bodies.
• His model was based upon the concept of perfect
  circles.

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Ptolemys Simple Model for Planetary Motion
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Ptolemys Model for Retrograde Motion
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Ptolemys Model for Mercury and Venus
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Ptolemys Complete Geocentric Model
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Ptolemys More Exact Model
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Timeline of Ancient Astronomy
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The Marriage of Aristotle and Christianity

• In the 13th century St. Thomas Aquinas blended
  the natural philosophy of Aristotle, which
  included the Ptolemaic model, with Christian
  beliefs.
• A central, unmoving Earth fit perfectly with
  prevalent Christian thinking, and various
  scriptures where found, whose literal
  interpretation, seemed to agree with this model.
• 1 Chronicles 1630 He has fixed the earth firm,
  immovable.
• Psalm 9610 He has fixed the earth firm,
  immovable ...
• Psalm 1045 Thou didst fix the earth on its
  foundation so that it never can be shaken.
• Isaiah 4518 ...who made the earth and
  fashioned it, and himself fixed it fast...

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Timeline of Renaissance Astronomy
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Copernicus Proposes a New Model

• A rebirth of astronomy occurred in the 14th
  century. As observations improved, continuous
  refinements to Ptolemys model were required.
• Finally, by the 16th century the corrected
  Ptolemaic model had become very complex.
  Copernicus suggested the heliocentric model as a
  simpler geometrical model which would produce
  the same observed results, but fewer circles were
  required.

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Copernicus Model
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Requirements of the Model

• To be a correct model of the Solar system,
  Copernicus model had to agree with observations.
• His model could explain retrograde motion as long
  as the inner planets had shorter periods than the
  outer planets (see next slide).
• However, there was still the problem with the
  lack of observable parallax.

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Copernicus Model for Retrograde Motion
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Galileo Father of Modern Astronomy
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Galileos Careful Observations Put an End to the
Geocentric Model

• Galileo was the first person to direct a
  telescope toward the heavens. His observations
  had a profound impact on astronomy (and
  religion).
• He observed the Moons of Jupiter
• He observed the phases of Venus
• He observed Sunspots on the Suns surface (and
  later went blind).

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Galileos Observations of Jupiters Moons

• This observation verified that not everything
  orbited the Earth.

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Galileos Observations of Venus
Like Ptolemys model Venus appears larger (thus
closer) when we view its dark side. However,
notice how much of Venus surface is illuminated
when it is far from us!
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Venus Phases in Ptolemys Model
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Venus Phases in Copernicus Model
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Galileos observations of the phases of Venus
indicated the Venus must orbit the Sun a major
modification of Ptolemys model and the end of
the geocentric model of the solar system.

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Tyco Brahe Faults Copernicus Model

• Copernicus originally utilized circular motion
  for the planets. But he found he could not
  reproduce the more accurate observations with
  such a model.
• Tycho Brahe, rejected Copernicus model because
  of the lack of parallax. He proposed a slightly
  different geocentric model in which the Sun and
  Moon orbit the Earth, but all the other planets
  orbit the Sun.

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Tychos Model
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But What about the Scriptural Evidence for the
Geocentric Model?

• As more and more evidence began to build which
  indicated the correctness of Copernicus model,
  faithful Christians had to ask some fundamental
  questions about their interpretation of
  scripture.
• By the end of the 17th century, most Christians
  had come to accept the heliocentric model.
• These Christians had to make adjustments to their
  interpretation of certain scriptures the Earth
  being fixed must be interpreted differently.

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Keplers Laws of Planetary Motion

• Based upon 50 years of careful observations by
  Tycho Brahe, Kepler, a mathematician, derived
  three laws of planetary motion
• All bound objects orbit the sun in elliptical
  orbits.
• As an object orbits the sun, it sweeps out equal
  areas in equal times.
• The square of the orbital period is proportional
  to the cube of the semi-major axis.

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Keplers Law of Equal Areas
A highly elliptical orbit such as this is
characteristic of comets.
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Keplers 3rd Law Orbital Periods

• Using Keplers 3rd Law, we can relate the orbital
  period of other planets to that of the Earth

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Bodes Law

• Bodes Law is a simple relationship which can
  be used to remember the approximate distance of
  each planet from the Sun.

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Orbital Periods of Visible Planets
Planet Approx Dist (Bode) A.U. Actual Dist A.U. Approx Period True Period
Mercury .4 .387 92 days 88
Venus .7 .723 214 days 225
Earth 1.0 1.0 365 days 365
Mars 1.6 1.52 739 days 687
Asteroids 2.8 (Ceres) 2.77 4.7 yrs Ceres 4.6 yrs
Jupiter 5.2 5.2 11.86 yrs 11.86 yrs
Saturn 10.0 9.54 31.62 yrs 29.46
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Newtons Laws of Motion

• Law of Inertia All objects remain at rest, or
  move with constant speed along a straight line,
  unless acted upon by some outside force.
• The acceleration of a body is proportional to the
  force applied and the mass of the body
• For every action, there is an equal and opposite
  reaction.

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Newtons Law of Gravity

• Any two objects in the universe experience a
  force of mutual attraction. This force is
  proportional to the product of the two masses and
  inversely proportional to the square of the
  distance between them.
• Based upon this law and the basic laws of motion,
  Newton was able to derive all of Keplers laws of
  planetary motion!

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Demonstration of Orbital Motion in Gravitational
Fields

• Simple Orbital Motion (Keplers three laws)
• Elliptical motion
• Equal areas in Equal times
• Circularizing Orbits
• Unbound Motion
• Multiple Planets Orbiting a Single Sun and
  Orbital Stability
• Gravitational Boosts
• Comets and Meteor Showers
• Multiple Sun Systems

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Eclipses and Eclipse Seasons
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Lunar Eclipse

• This sequence of photographs shows the shadow of
  the Earth projected across the path of the Moons
  orbit.

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Lunar Eclipses

• Lunar eclipses occur when the moon passes through
  the shadow of the Earth
• The location of the moon relative to the Earths
  shadow determines the type of eclipse that
  occurs.
• Recall that the size of the Earths Shadow is
  roughly three times the size of the Moon.

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Eclipse Geometry
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Types of Lunar Eclipses
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Solar Eclipse
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Solar Eclipses

• A solar eclipse occurs when the moon passes
  between the Earth and the Sun
• A movie of the motion of the Moons shadow across
  the Earth
• The area of total shadow is relative small
• The Earth rotates as the Moon passes by producing
  a curved path for the shadow.

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Total and Partial Eclipses
Those located at X observe a total eclipse,
while those located at Y observe only a partial
eclipse.
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Annular Eclipses
Those located a A observe an annular ecliplse,
while those located at P only observe only a
partial eclipse.
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Length of Eclipses

• The maximum duration of a total lunar eclipse is
  about 1 hour and 47 minutes, the time it takes
  for the Moon to pass through the Earths Umbra.
• The length to time for a solar eclipse can be
  anywhere from a few seconds, up to a maximum of 7
  and a half minutes, depending upon the size of
  the Moons Umbra at the Earths surface.

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Solar Eclipse Paths through 2017
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Eclipse Seasons

• Why dont a solar and a lunar eclipse occur every
  month?
• The Moons orbit around the Earth is tilted
  relative to the orbit of the Earth around the
  Sun.
• This means that there are eclipse seasons that
  occur about every 6 months. But even then
  eclipses do not always occur, because of the
  relative position of the Sun, Earth, and Moon.

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Eclipse Seasons
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Seasons of the Year and Time
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Seasons are Caused by the Earths Tilt
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Geocentric View
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Seasonal Heating Effects
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Time is Measured by the Earths Rotation and
Revolution

• The Solar Day and Time Zones
• The Sidereal Day (measured relative to the stars)
  23 hrs 56 min
• Sidereal Month (measured relative to the stars)
  27.5 days
• Synodic Month (Lunar Month) 29.5 days
• Solar Year 365.25 days

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Sidereal Day vs. Solar Day
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Synodic Monthvs.Sidereal Month
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Time Zones
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End of Part II