Transcript Early Astronomers File

Western Scientific Astronomy
begins with the ancient Greek civilization about 600 BCE
Slide 1
Early Astronomers
• Unfortunately, there are no written
documents about the significance of
stone and bronze age monuments.
• First preserved written documents
about ancient astronomy are from
ancient Greek philosophy.
• Greeks tried to understand the motions
of the sky and describe them in terms
of mathematical models.
Slide 2
Ancient Greek Astronomers
Models were generally wrong because
they were based on “first principles”,
believed to be “obvious” and not to be
questioned:
1. Geocentric Universe: Earth at the
Center of the Universe.
2. “Perfect Heavens”: Motions of all
celestial bodies described by motions
involving objects of “perfect” shape, i.e.,
spheres or circles (Plato).
Slide 3
Ancient Greek Astronomers
• Eudoxus (409 – 356 B.C.):
Model of 27 nested spheres
• Aristotle (384 – 322 B.C.),
major authority of philosophy
until the late middle ages:
Universe can be divided in 2
parts:
1. Imperfect, changeable Earth,
2. Perfect Heavens (described
by spheres)
• He expanded Eudoxus’ Model to use 55 spheres.
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Using the distance between the Earth and the Moon as a baseline
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Eratosthenes of Cyrene
Born: 276 BC in Cyrene, North Africa (now Shahhat, Libya)
Died: 194 BC in Alexandria, Egypt
Measured circumference of the Earth
He got R = 6739 km
Actual radius 6378 km
Slide 8
Eratosthenes (~ 200 B.C.):
Calculation of the Earth’s radius
Angular distance between
Syene and Alexandria:
~ 70
Linear distance between
Syene and Alexandria:
~ 5,000 stadia
 Earth
Radius ~ 40,000
stadia (probably ~ 14 %
too large) – better than
any previous radius
estimate.
Slide 9
The problem of retrograde motion
Slide 10
Retrograde motion
• Backward travel of planets through the
moving night sky.
Slide 11
Later refinements (2nd century B.C.)
• Hipparchus: Placing the Earth away from the centers of the
“perfect spheres”
Slide 12
• Ptolemy: Further refinements, including epicycles
Claudius Ptolemy 85-165 AD
Mathematical Syntaxis (Almagest)
Slide 13
Epicycles
Introduced to explain retrograde
(westward) motion of planets
Slide 14
The Ptolemaic system was considered
the “standard model” of the Universe
until the Copernican Revolution.
Epicycles
Slide 15
The Copernican Revolution
Nicolai Copernicus (1473 – 1543):
Slide 16
Heliocentric Universe (Sun in the Center)
Born: 19 Feb 1473
Died: 24 May 1543
Church cleric, but rejected a 2000-yr old paradigm
Seven axioms written in a pamphlet “Little Commentary” (1514)
1. There is no one centre in the universe.
2. The Earth's centre is not the centre of the universe.
3. The centre of the universe is near the sun.
4. The distance from the Earth to the sun is imperceptible compared with the
distance to the stars.
5. The rotation of the Earth accounts for the apparent daily rotation of the stars.
6. The apparent annual cycle of movements of the sun is caused by the Earth
revolving round it.
7. The apparent retrograde motion of the planets is caused by the motion of the
Earth from which one observes.
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Slide 18
Copernicus’ new (and correct) explanation
for retrograde motion of the planets
Retrograde
(westward)
motion of a
planet occurs
when the
Earth passes
the planet.
Slide 19
This made Ptolemy’s epicycles unnecessary.
Slide 20
Slide 21
Tycho Brahe (1546-1601)
Contributions to Astronomy:
•Tycho was the first to suggest a non-circular orbit for a celestial body (a
comet).
•Used calibrated and bigger instruments, new techniques to measure angles
(similar to a sextant).
•Built an observatory (remember - no telescopes yet) and made accurate
and continuous measurements for 20 years. His measurements helped to
prove that planets orbited the sun.
•Accurate map of the stars with 777 stars.
•Observed “a new star” – supernova 1572 (Tycho’s Supernova).
•Measured length of the year to within 1 second.
•Was still unable to choose between the geocentric and heliocentric model.
He had his own model with the Earth at the center, orbited by the sun and
the moon, with planets orbiting the sun. Never worked out the mathematical
details, and his model was never accepted.
•Using Tycho's data, a German astronomer (Kepler) was able to refute the
geocentric model
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Use of Parallax
Slide 23
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Johannes Kepler (1571 – 1630)
Slide 26
Kepler hypothesized that a physical force moved
the planets, and that the force diminished with
distance.
Planets closer to the sun feel a stronger force and
move faster.
Elliptical orbits – key to the problem of the planetary
motion
Slide 27
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Kepler’s Laws of Planetary Motion
1. The orbits of the planets are ellipses with the
sun at one focus.
c
Eccentricity e = c/a
Slide 29
Elliptical orbits
Vocab: perihelion, aphelion, semimajor axis,
eccentricity
Ra - Rp
e=
Ra + Rp
x2 y2
 2 1
2
a
b
Slide 30
Eccentricities of Ellipses
1)
2)
e = 0.02
e = 0.1
e = 0.2
5)
4)
e = 0.4
Slide 31
3)
e = 0.6
Eccentricities of Planetary Orbits
Orbits of planets are virtually
indistinguishable from circles:
Most extreme example:
Earth: e = 0.0167
Slide 32
Pluto: e = 0.248
LAW 2: A line joining a planet/comet and the Sun
sweeps out equal areas in equal intervals of time
The closer to the sun, the
larger the orbital velocity
Slide 33
Planetary Orbits (2)
• A line from a planet to the sun sweeps
over equal areas in equal intervals of time.
• A planet’s orbital period (P) squared is
proportional to its average distance from the
sun (a) cubed:
Slide 34
Py2
= aAU
3
(Py = period in years;
aAU = distance in AU)
LAW 3: The squares of the periods of the planets are proportional
to the cubes of their semimajor axes:
2
1
2
2
3
1
3
2
P
a

P
a
For the Earth P2 = 1 yr, a2 = 1 AU
P ( yr )  a ( AU )
2
1
Slide 35
3
1
Note units!!
Johannes Kepler (1571 – 1630)
• Used the precise observational tables of
Tycho Brahe (1546 – 1601) to study
planetary motion mathematically.
• Found a consistent description by
abandoning both
1. Circular motion and
2. Uniform motion.
• Planets move around the sun on elliptical
paths, with non-uniform velocities.
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Galileo Galilei (1564-1642)
Was the first to report using the telescope to view the heavens.
Telescope invented in 1604 by Hans Lippershay.
Galileo used the telescope in 1609. Built his own. Two lenses in a metal tube
about 4 feet long, diameter = 4 cm (1.6 inches). Magnification 3X to 33X.
His observations between 1609 and 1612 changed our ideas about the
universe.
What did he see?
New stars (Milky Way made up of stars)
Mountains and valleys on the moon
Four moons orbiting Jupiter (now called Galilean moons)
Phases of Venus
Sunspots (rotating around the sun about once a month)
The rings of Saturn (sketches. was puzzling; not identified as rings until about
50 years later.)
Planets are disks, not pinpoints of light like the stars
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32X power
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Major Discoveries of Galileo
• Moons of Jupiter (4 Galilean moons)
• Rings of Saturn
(What he really saw)
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Major Discoveries of Galileo (2)
• Surface structures on the moon; first estimates
of the height of mountains on the moon
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Major Discoveries of Galileo (3)
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• Sun spots (proving that the
sun is not perfect!)
Major Discoveries of Galileo (4)
• Phases of Venus (including “full Venus”),
proving that Venus orbits the sun, not the Earth!
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The significance of what he saw:
Cast doubt on the view of the "perfection of the heavens" (of Aristotle and
Plato)
Showed deficiencies of the geocentric (Ptolemaic) model
Rotation of sunspots around sun suggested that if the sun could rotate,
perhaps the Earth could too.
Phases of Venus would be a natural consequence of the heliocentric model.
Jupiter's moons showed that centers of motion other than Earth existed.
Galileo published in Italian, not Latin. Widely read. Language of the people,
rather than language of the scholars.
Arguments against the geocentric model were so forceful that he came under
fire from the Catholic Church and was forced to give a public denial of the
heliocentric/Copernican system, and was placed under house arrest for the
last 10 years of his life. Was not pardoned by the Church until 1992.
Science in Italy was dealt a severe blow. The center of scientific investigation
shifted to northern Europe.
Many scholars refused to believe his ideas and a few even refused to look
through the telescope. Many clung to old ideas.
Slide 45
Galileo Galilei (1594 – 1642)
• Invented the modern view of science:
Transition from a faith-based “science” to
an observation-based science.
• Rejected the “old” view (still alive now!)
that the only path to true understanding is
through religious faith
• Observations are correct even if they
contradict the Scripture
• “The Bible tells us how to go to heaven,
not how the heavens go.”
Slide 46
Isaac Newton
English scientist
Sir Isaac Newton
(1642—1727)
explained gravity as
the force that holds
planets in orbit around
the Sun.
Slide 47
Early Astronomy
The Birth of Modern Astronomy
 Sir Isaac Newton
• Although others had theorized the existence of
gravitational force, Newton was the first to formulate and
test the law of universal gravitation. The universal law of
gravitation, helped explain the motions of planets in the
solar system.
 Universal Gravitation
• Gravitational force decreases with distance.
• The greater the mass of an object, the greater is
its gravitational force.
Slide 48
Gravity’s Influence on Orbits
Slide 49
Newton’s Laws of Motion
• 1st Law
– A body at rest, or in uniform motion, will
remain so unless acted upon by an
unbalanced force.
• 2nd Law
– The change in motion (acceleration) is
proportional to the unbalanced force
• 3rd Law
Slide 50
– For every action there is an equal and
opposite reaction
Gravity
• Gravity is the force that
– holds us to the Earth
– causes a rock to fall towards the ground
– causes the Earth to go around the Sun
– causes the Sun to be pulled towards the
center of the Milky Way galaxy
• Gravity acts between any two objects
even if they are far apart.
– “action at a distance”
Slide 51
Historical Overview
Slide 52