Transcript lecture6 - UMass Astronomy

Origin of Modern Astronomy
Earth's place in the cosmos
The nature of planet motion
How we understand “the truth”
Addressing these three key issues led
to the birth of science .
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Assigned reading:
Chapter 4
When reading a chapter, make sure you study
the Guidepost, the Connections, the Portfolio
inserts and the Summary at the end
Archeoastronomy
• There is evidence that Human kind has paid very closed
attention to the sky, its motions and periodicity.
• Humans always tried to explain them
• Archeoastronomy a blend of superstition, astrology,
religion, but also common sense
• Greeks were the first to try to understand (logically) the
universe.
• Thales of Miletus: first, true scientific attitude, humans
can understand
– Very different from previous “mysteric” attitude: humans cannot
comprehend mysteries
– Unknown vs. Unknowable
• Pythagoras: universe ruled by geometrical, mathematical
relationship
– This notions still fundamental today
The birth of modern science
• Thales: Universe can be understood
• Pythagoras: Observations show that rules are
mathematical
• Plato: observations imperfect, pure thought can achieve
the truth
• All ingredients there for the methods of modern science:
have a theory and test it against reality
– Science: test theory with observations
– A theory is good only if it (or its consequences) explain the
observations/experiments
• Often, the survival of a (wrong) theory due to not
sufficiently sensitive observations
• Ultimately, Greek “science” still a blend of scientific
method and “philosophical” believes.
• True scientific method had to wait for Galileo
The Old Astronomy:
Geocentric Universe
Aristotle (Greek, 384-322 BC)
Not to scale
Three Basic Assumptions
• The Earth was at the center of the
Universe.
• The only motion in the heavens was uniform
circular motion
• The heavens were more perfect than the
Earth, and objects in the heavens were
eternal.
Very different from methods
of modern science
• Build a theory on some hypothesis
• Test the predictions of the theory against
phenomenological reality
• Modify, update, generalize or discard and replace
the theory if *any* of its predictions or
consequences fail to reproduce reality
"Common Sense"
• If the Earth actually spun
on an axis, why didn't
objects fly off the spinning
Earth?
• If the Earth was revolving
around the sun, why didn't
it leave behind the birds
flying in the air?
• If the Earth were actually
on an orbit around the sun,
why wasn't a parallax
effect observed?
Two problems for the
Geocentric model:
(1) Change of
brightness
(2) Retrograde Motion
Ptolemy Solution:
add a special fix:
epicycle
or circle on circle
Brightness changes
because of distance
change
Ptolemic Model
The Copernican Revolution:
The Heliocentric System
Nicolai Copernicus (1473-1543).
The Sun, not the Earth, was the
center of the Solar System.
The Earth is just another planet
(the third outward from the Sun),
and the Moon is in orbit around
the Earth, not the Sun.
The stars are distant objects that
do not revolve around the Sun.
Retrograde Motion and Varying
Brightness of the Planets
The planets in such a system naturally vary in brightness
because they are not always the same distance from the Earth.
The retrograde motion could be explained in terms of
geometry and a faster motion for planets with smaller orbits.
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Copernicus’ ideas remained rather obscure for about
100 years after his death. Later work of Kepler,
Galileo, and Newton would build on the
heliocentric Universe, leading to the birth of
modern astronomy and natural science.
This sequence is commonly called the Copernican
Revolution.
Key Observations of Tycho Brahe
He made the most precise
observations of planetary motion,
particularly that of Mars.
However, Tycho was a firm believer
of the geocentric universe.
Danish nobleman
(1546-1601)
Johannes Kepler:
The Laws of Planetary Motion
Unlike Brahe, Kepler believed firmly in
the Copernican system. Based on
Tycho's data on Mars, Kepler
concluded that the orbits of the planets
were not circles, but were instead
ellipses
(1571-1630), German
Kepler’s Laws of Planetary Motion
1
The orbits of the planets are ellipses, with the Sun
at one focus of the ellipse.
2
Planets move proportionally faster in their orbits
when they are nearer the Sun.
3
More distant planets take proportionally longer to
orbit the Sun
Calculations Using Kepler's Third Law
The ratio of the squares of the revolutionary periods
for two planets is equal to the ratio of the cubes of
their semimajor axes.
R(AU)3=P(years)2
As an example, the "radius" of the orbit of Mars
(the length of the semimajor axis of the orbit) is:
R=P2/3=(1.88)2/3=1.52 AU
Galileo Galilei
Galileo discovered that our Moon has
craters, that Jupiter has it's own
moons, that the Sun has spots, that
Venus has phases like our Moon, and
many more discoveries.
(1564-1642), Italian
These discoveries confirmed the
Copernican hypothesis that the Earth
was just another planet.
The Phases of Venus
Venus went through a complete set of phases, just like the
Moon. This was the first empirical evidence that allowed a
definitive test of the geocentric and heliocentric models.
Imperfect and Changing
Universe
Sun had dark patches on Sun. The
motion of such sunspots indicated that
the Sun was rotating on an axis.
The Milky Way was composed of
enormous numbers of stars that had not
been seen before.
The planet Saturn had "ears".
The Moon was not smooth, but was
covered by mountains and craters.