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Astronomy 100
Tuesday, Thursday 2:30 - 3:45 pm
Tom Burbine
[email protected]
• I was very impressed by the zodiac assignments
people turned in
Test Next Tuesday
What you should do:
Read the book
Go over the lecture notes
I will talk about it a lot more on Thursday since I
am making it up tonight
• If you try to take the test without studying and if
you haven’t been to class, I am pretty sure you
will flunk it
If I have talked about it in class,
I consider it important
Homework #4
• Its on OWL.
• The first homework assignment has 10 questions. For
each question, you will get two attempts to answer it
• After your last attempt, the correct answer will be shown.
• If you get all 10 questions correct, you will get one
homework point.
• If you get less than 10 questions correct, I will divide the
number of questions you get correct by 10 to determine
the fraction of a homework point that you will receive.
• This is due by Thursday evening (2/10) at 11:59 pm
PRS question #1
• What class is this?
A) Psychology 100
B) Astronomy 100
C) Geology 100
D) Physics 100
• When you have a model of how something works,
you should be able to predict what will happen
• If observations do not fit the model, either the
observations or the model is wrong
• The ancient astronomers wanted to predict the
positions of planets in the sky
What did the ancients think
• That the Earth was the center of the universe
• That the celestial sphere was rotating around the
• However, there was two observations that caused
problems with this idea
– apparent retrograde motion
– Inability to detect stellar parallax
Greek model
Apparent Retrograde Motion = “backward” motion
Retrograde Motion
Stellar Parallax
• Stellar Parallax – The apparent shift in the
position of a nearby star (relative to distant
objects) that occurs as we view the star from
different positions in the Earth’s orbit of the Sun
each year
The distance the star moves is greatly exaggerated in this figure.
Stellar parallax can only be seen by a telescope.
Ancient astronomers could not detect
stellar parallax
• If Earth orbited the Sun, ancient astronomers believed
that they would see differences in angular separation of
stars as the Earth rotated around the Sun
• Since they saw no changes in angular separation of the
stars, they assumed the Earth was the center of the
• They could not fathom that stars are so far away that
stellar parallax is undetectable by the human eye
Ptolemy’s (100-170 AD)
Model of the Universe
Nicholas Copernicus (1473-1543)
• Copernicus came up with a model that the Earth
revolves around the Sun
• Similar to what Aristarchus (310 – 230 BC)
thought 2000 years before
• However, Copernicus’ models did not match
observations since he wanted everything to
arouind in perfect circles
Tycho Brahe (1546-1601)
• Tycho Brahe was the greatest naked eye observer
of all time
• He lived before the invention of the telescope
• His observations of the alignment of Jupiter and
Saturn occurred two days later than when
predicted by Copernicus
• Tycho came up with a model where the planets
orbit the Sun but the Sun orbits Earth
Johannes Kepler (1571-1630)
• Tried to match circular orbits to Tycho’s data
• Couldn’t do it
• Because Tycho’s observations were so good,
Kepler had to come up with a new model
Kepler was trying to match an orbit to
Tycho’s observations of Mars
• “If I believed that we could ignore these eight
minutes of arc, I would have patched up my
hypothesis accordingly. But, since it was not
permissible to ignore, those 8 minutes pointed to
the road to a complete reformation in astronomy.”
• Kepler came up with his 3 laws of planetary
• The orbit of each planet about the Sun is an
ellipse with the Sun at one focus (there is nothing
at the other focus)
Differences between ellipses and circles
Show Video
• Perihelion – planet closest to the Sun
• Aphelion – planet farthest from the sun
• Semimajor axis – the average of a planet’s
perihelion and aphelion distances
• As a planet moves around its orbit, it sweeps out
equal areas in equal times.
• This means that the planet travels faster when it is
nearer the Sun and slower when it is farther from
the Sun
Show Video
Show Video
• More distant planets orbit the Sun at slower
average speeds, obeying the precise mathematical
p2 = a3
where p is a planet’s orbital period in years and
a is the average distance from the Sun in
astronomical units.
• The period for the Earth to go around the Sun is
1 year
• The distance of the Earth to the Sun is
1 Astronomical Unit
How long does it take Jupiter to go
around the Sun
• If Jupiter is 5.2 Astronomical Units from the Sun,
how long does it take Jupiter to go orbit the Sun
• p2 = a3 = 5.23 = 140.6
• p = √140.6 = 11.9 years
PRS question #2
• Mercury is 0.4 Astronomical Units from the Sun.
• How long does it take Mercury to orbit the sun
A) 1 year
B) 3 months
C) 9 months
D) 5 years
The calculation
• p2 = a3 = 0.43 = 0.064
• p = √0.064 = 0.25 years
You can calculate a planet’s orbital speed
• Since you know a planet’s orbital distance
• And you know its orbital time
• You can calculate a planet’s average orbital speed
Arguments against the Sun being the
center of the solar system
• 1) If the Earth was moving, objects such as birds
and clouds would be left behind as the Earth
• 2) The heavens must be perfect and unchanging.
Noncircular orbits do not fit this model
• 3) Stellar parallax would be observable
Galileo Galilei (1564-1642)
• He was able to figure out answers to these
• 1) Things in motion tend to remain in motion.
• 2) He used a telescope to see sunspots on the Sun
and features on the Moon.
• 3) Galileo found that stars were more numerous
and more distant than imagined
He also
• He discovered the moons of Jupiter and saw that
they were orbiting Jupiter
• Proving that bodies could orbit other bodies
besides the Earth
Galileo also found that
Venus orbited the Sun
• Galileo was brought to a church inquisition in
Rome and made to recant his views
• He did it to save his life
• Supposedly said Epper si muove “And yet it
moves” under his breath while getting up from his
So why did observations that the
Earth was not the center of the
universe fall out of favor
So why did observations that the Earth
was not the center of the universe fall out
of favor
• Because models based on this did not accurately
predict observations