The Science of Astronomy

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The Science of Astronomy

• Ancient Civilizations
• Ancient Greek
• European Renaissance
• Modern Science

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Ancient Astronomical Knowledge

• Many of the surviving ancient structures have
  obvious astronomical purpose. These ancient
  structures clearly demonstrated that all ancient
  civilizations developed extensive knowledge of
  the celestial objectsmost likely because of the
  need to predict the seasons due to the
  development of agriculture. Astronomical
  knowledge are also very useful tool for
  navigation. Usually knowledge of mathematics and
  geometry were usually developed at the same time.

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Marking the Seasons

• In Hawaii, the first rise of the star cluster
  Pleiades is used to mark the beginning of the
  year.

Sun Dagger at summer solstics at Chaco Canyon,
New Mexico
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Navigating the World

• If you are sailing in the open sea from Tahiti to
  Hawaii in one of those voyaging canoe, how can
  you tell where you are at on Earth?
• Longitude?
• Latitude?

A Polynesian navigational instrument
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Ancient Greek Science

• The Ptolemaic Model of the Universe
• Earth is at the center of the universe
• All the objects move in perfect circular orbit
• Planets moves in small circles upon larger
  circles to explain the retrograde motion.

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Minority Opinion

• Pythagoras (582500 BC)
• The astronomy of the Pythagoreans marked an
  important advance in ancient scientific thought,
  for they were the first to consider the earth as
  a globe revolving with the other planets around a
  central fire.
• Aristarchus (310-230BC) Aristachus sought to
  explain the apparent retrograde motion of planet
  with a Sun-centered model.

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Missed Opportunity

• Although Pythagoras and Aristarchus proposed
  heliocentric model of the cosmos, their ideas
  were not widely accepted by their contemporaries,
  probably because
• Aristarchus model could not predict the
  retrograde motion any better than Ptolemaic
  model.
• If the Earth is revolving around the Sun, the
  stellar parallax must exist, but the ancient
  Greeks were not able to detect any stellar
  parallax.
• The ancient Greeks believed that the heavens must
  be geometrically perfect heavenly objects must
  move in perfect circles and must reside on huge,
  perfect sphere encircling Earth.

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What would you believe if you lived around 200
B.C. in Greek?
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Although the majority of ancient Greek
philosophers arrived at the wrong conclusion
about the model of the universe, they did so
based on sound logical reasoning processes, good
(albeit crude in todays standard) observational
data, (no stellar parallax, apparent retrograde
motion of planets), and good modeling efforts
(Ptolemaic geocentric model and Aristarchuss
heliocentric model). They followed a very
rigorous scientific method, and their failure was
not the failure of the scientific method. It was
due to the limited technology. They couldnt have
done better! The ancient Greeks were the first
to rely on logical thinking to explain the
natural phenomena. This is the same principle
that was followed by the scientist of the 15th
and 16th century to proof the validity of the
heliocentric model of the solar system, and is
the foundation of modern science.
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The Dark Ages

• During the dark ages of Europe, the rest of the
  world continue to develop. But the knowledge of
  the Greeks were preserved in the city of
  Alexandria, in Egypt.

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The Copernican Revolution

• Copernicus (1473-1543) revived the idea of a
  Sun-centered solar system model
• However, like Aristarchus, Copernicuss model
  was not accurate enough to convince many people.
• Tycho Brahe (1546-1601) made accurate (arc
  minutes) naked-eye measurement of planet motion
• Tycho believed that planets must circle the Sun,
  but his failure to detect stellar parallax forced
  him to put the Earth at the center of the system,
  with the Sun orbiting the Earth, and the planets
  orbit the Sun.
• Johannes Kepler (1571-1630) were able to make
  accurate prediction with his heliocentric model
  of planetary orbits, agreeing with Tycos
  observation
• Keplers initial failure (using prefect circular
  orbits) to match Tycos observation led him to
  adopt a model with elliptical planetary orbit.
• Galileo Galileis (1564-1642) telescopic
  observations helped solidify the heliocentric
  view of the solar system.

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Keplers Reformation
In attempting to explain Tychos observation of
the planetary motion, Kepler concluded that
planets do not orbit in perfect circles. Instead,
the planets travel around the Sun in elliptical
orbit.
The ellipse The distance from one focus to a
point on the ellipse to another focus is a
constant
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Keplers First Law
The orbit of each planet about the Sun is an
ellipse with the Sun at one focus.
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Keplers Second Law
As a planet moves around its orbit, it sweeps out
equal areas in equal time.
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Interesting Properties of Elliptical Orbits

• Keplers second law states that as a planet
  moves around its orbit, it sweeps out equal areas
  in equal time.
• It also means that the orbital speed is not
  constant like in a circular orbit. It is depends
  on its distance from the Sun.
• It is slower when it is further away for the Sun
• It is faster when it is closer to the Sun.

Click image to start animation
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Keplers Third Law

• More distant planets move more slowly in their
  orbit
• The planets orbital period is related to the
  average distance to the Sun
• (Orbital period in years ) 2 (average distance
  in AU) 3
• or
• p 2 a 3
• where p is the orbital period measured in year,
  and a is the average from the Sun to the planet
  in AU.

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• A Sun-centered solar system model with the
  planets moving in elliptical orbits allowed
  Kepler to make accurate predictions of the
  planets positions in the sky. So, now the
  heliocentric view has a better model than
  geocentric view. But there were other
  questions/objections to the heliocentric model
  that need to be answered

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The Challenges

• Major Objections to the Sun-centered solar
  system model
• If Earth is moving, then objects such as birds,
  falling stones, and clouds would be left behind
  as Earth moved along its path.
• The heavens must be perfect and unchanging.
• If the Earth is orbiting the Sun, then stellar
  parallax must be detectable.
• These objections must be addressed before the
  Sun-centered model can be accepted.
• Galileos observation with the new telescope
  helped to answer these questions

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Galileos Telescopic Observations

• Venus goes through the phases like the Moon
• Venus must be orbiting the Sun, not the Earth!
• This implies that not everything orbits Earth!
• The Four Moons of Jupiter
• Satellites can follow a moving planets
• This proofs that not everything orbits Earth!
• Sun has sunspots, and Moon has mountains and
  valley
• The heaven is not perfect!

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Answering the Critics

• If Earth is moving, then objects such as birds,
  falling stones, and clouds would be left behind
  as Earth moved along its path.
• Galileo showed that a moving object remains in
  motion unless a force acts to stop it.
• Newtons first law of motion
• Galileo saw through his telescope that there are
  four Moons orbiting Jupiter, not Earth.
• Objects can orbit a planet, thus the Moon can
  orbit the Earth without been left behind.

• 2. The heavens must be perfect and unchanging
• Tychos observation of supernova and comets
• Heaven can be changing.
• Galileos telescope showed that the Sun has
  sunspots, and the Moon has mountain and valleys
• Heaven can be imperfect.

3. If Earth is orbiting the Sun, then why
couldnt we observe any stellar parallax? Stars
are too far away. We do measure it today!
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In Hawaii, the linear speed of Earths rotation
is about 1,566 km/hr 0.435 km/sec, or 435
m/sec. If I drop a stone from a height of 1.25
meter above the ground, it is going to take
approximately 0.5 second to reach the ground. The
ground moves 217 m during the time it takes the
stone to fall to the ground. How comes the stone
does not get left behind?

• While we hold the ball before releasing it, the
  ball is also traveling with 1,566 km/hr. It is
  traveling with the same speed as the ground does
  while it is falling to the ground, because no
  force was applied to it to stop its motion in
  this direction. So, it does not got left behind!
• Newtons First Law of Motion! Chapter 4

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Summary

• The ancient Greeks were the first to use logical
  scientific method to try to explain the nature.
• The same scientific method was used by the
  scientists of the 15th and 16th century to
  finally establish the heliocentric model of the
  solar system.
• Tyco obtained very precise observations of
  planetary motion.
• Kepler was the first to device an accurate
  planetary model capable of predicting the
  position of the planets with great accuracy.
• Galileos telescopic observation helped to
  disprove many of the ancient believes, and firmly
  established the sun-centered model of the solar
  system
• .
• ? It is interesting to note that up to this
  point, there were still no discussions on why the
  planets should move in elliptical orbits, or on
  what is keeping the planets from running away
  from the Sun?

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Measures of Angles

• One complete circle can be divided into 360
  degrees.
• One degree is divided into 60 arc minutes.
• One arc minutes is further divided into 60 arc
  seconds.
• One complete circle has 1,296,000 arc seconds.

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