Transcript History of Astronomy

History of Astronomy - Part I
• Astronomy is a science that has truly taken shape
only in the last couple centuries
– Many advances have been made in your lifetime
• However, astronomical concepts and ideas have
been around since the birth of man
• The next two lectures will lead us through the
development of astronomy from ancient times
through the modern age
Ancient Astronomy
• Astronomy held a special
significance for ancient
cultures for different
reasons
Stone Age England
– Very important navigation
tool
– Mark the seasons
– Religious implications
• Many erected structures in
order to track celestial
events
• Ancient records are used
to track supernovae and
comets
Native American
Mayan
Ancient Greece
• The Greeks were one of the first cultures to concentrate and
debate the nature of the universe
• The matter was chiefly philosophical, but did seek to match
the observed properties of the stars and planets with a
suitable model
• The study of the universe as a whole, its structure, and its
evolution is now known as cosmology
• For the Greeks, the universe was rather smaller than today,
limited only to the Earth, Moon, Sun, planets and the
celestial sphere
The Greek Frame of Mind
• Much of the Greek method of thinking revolved
around philosophy instead of scientific
reasoning
• Greeks valued perfection and therefore any
model of the universe should involve the
perfect shape, the circle
• Greek also had no reason to believe that the
Earth was not the center of the universe.
Egotistical, yes - but completely reasonable at
the time
• The only 'scientific' data they had available to
them was the motion of the Sun, Moon, and
planets, which were monitored heavily at the
time
Ptolemy ~140 AD
The Motion of the Planets
Retrograde Motion
• A model of the universe
would be very simple except
for the fact that the planets
undergo a looping motion in
their orbits
Retrograde Motion
• Remember, in one night, all
planets still rise in the east
and set in the west
• However, if you keep track of
the planet's position versus
the background stars night to
night, you will see the planet
'move'
Jupiter and Saturn (6/2000 - 5/2001)
• The word 'planet' means
wanderer in Greek
Ptolemaic Model
• In order to produce the
retrograde motion of the planets,
Ptolemy created a model with
epicycles
• All the planets orbited the Earth
in a perfect circle
• The planet itself made a smaller
orbit centered upon the larger
orbit around the Earth
• With the right timing, this model
can reproduce the retrograde
motion seen from Earth
Deferent = larger circular orbit around Earth
Epicycle = smaller circular orbit around the deferent
Ptolemaic Model
• In Ptolemy's complete model,
each planet had its own orbit
around the Earth with its own
epicycle
– By changing the period of the
orbit and the epicycle, the model
could match observations
relatively well
• The Sun and the Moon
traveled around the Earth in
perfect circles
• The entire model was
composed of more than 80
circles and was very
complicated
Simplified Ptolemaic Model
The Ptolemaic Model Survives
• Since the Ptolemaic model matched observations sufficiently
and no contrary evidence was produced, it was supported
for nearly 1,500 years!
• After all, if the Earth was moving, shouldn't we feel it?
• Also, the Greeks were smart enough to realize that if the
Earth was orbiting the Sun, it would produce stellar parallax
– The Greeks didn't believe it existed because they didn't have
telescopes to observe such small variations in a star's position
• On top of all this, the Dark Ages provided relatively little
advance in any sciences for Europe
Astronomy in the Dark Ages
• While Europe was
suffering through the Dark
Ages, Islamic nations were
undergoing a golden age
of astronomy
• Much of the knowledge of
the Greeks was preserved
and expanded upon during
this time
• Many of the proper names
for stars are relics of this
era
– Rigel, Betelgeuse, Vega ...
The Copernican Revolution
• At the end of the Dark Ages, a Polish
cleric name Copernicus devised a new
model of the universe where the Earth
was no longer at the center
• The heliocentric (Sun centered) model
placed the Earth out of its central
position, yet still maintained many of
the observations we see
• The beauty in his model was its
simplicity over the Ptolemaic
– Occam's Razor
• The simplest solution is the best
Nicolaus Copernicus (1473-1543)
The Copernican Model
In the Copernican model, retrograde motion is an
apparent effect caused by the Earth 'overtaking' an
outer planet in its orbit
The Copernican Revolution
• Despite the fact that the Copernican model was a better
representation of the solar system, it was not widely
accepted
• While it did provide a much simpler description compared to
Ptolemy, it did not necessarily improve the predictive power
of the model
• The religious dogma of the time insisted upon Earth being
the center of the universe
• Copernicus published his works in Latin, which was
unreadable by the common public
Galileo - The Observer
• A century after Copernicus' work,
other scientists began to make
strides toward popularizing the
heliocentric model
• Galileo was the first to use a
telescope to make detailed
observations of the sky
• Though he did not invent the
telescope, he made many working
prototypes and trained them on a
variety of celestial bodies
Galileo Galilei (1564-1642)
Galileo's Observations - I
• Galileo used his telescopes to make
observations of many heavenly
objects
• The sketch to the right shows Galileo's
observations of the moons of Jupiter
• He noticed that the position of these
four moons changed night to night, as
if they were rotating around Jupiter
• These moons now bear his name
– The Galilean moons are:
•
•
•
•
Io
Europa
Ganymede
Callisto
Galileo's Observations - II
• Galileo also noticed that
Venus was not simply a
point of light, but actually a
disk
• He watched Venus go
through complete phases,
just like the Moon
• This cycle of phases can
only be satisfied by the
heliocentric model, not the
geocentric
The phases of Venus
Galileo's Observations - III
• Galileo also pointed his
telescope toward the Sun
– NEVER DO THIS
• He discovered that the disk of
the Sun was not perfect and
was occasionally dotted with
small black spots
• By making daily sketches of
these spots, he was able to
determine that the Sun itself
was rotating
Galileo's Conclusion
• All of Galileo's observations were pointing towards
a heliocentric view of the universe
• Galileo published his observations and
conclusions in multiple works, including some
published in Italian to appeal to a wider audience
• Galileo was threatened with torture, forced to
deny his beliefs in the heliocentric model, and
sentenced to house arrest for the rest of his life
• The seeds of the Copernican Revolution had
been planted
Acceptance of the Copernican Model
• Despite the efforts of
Galileo and many others,
the acceptance of a Suncentered Solar System
was a very slow process
• In 1728, James Bradley
demonstrated the
aberration of starlight,
proving that the Earth is
moving in its orbit around
the Sun
• In 1838, Friedrich Bessel
was the first to observe
stellar parallax
Aberration of starlight