Astronomy 101
The Solar System
Tuesday, Thursday
2:30-3:45 pm
Hasbrouck 20
Tom Burbine
[email protected]
Course
• Course Website:
– http://blogs.umass.edu/astron101-tburbine/
• Textbook:
– Pathways to Astronomy (2nd Edition) by Stephen Schneider
and Thomas Arny.
• You also will need a calculator.
Office Hours
• Mine
• Tuesday, Thursday - 1:15-2:15pm
• Lederle Graduate Research Tower C 632
• Neil
• Tuesday, Thursday - 11 am-noon
• Lederle Graduate Research Tower B 619-O
Homework
• We will use Spark
• https://spark.oit.umass.edu/webct/logonDisplay.d
owebct
• Homework will be due approximately twice a
week
HW #3 (due today)
• 10 questions
• In Assessment in Spark
Hw #4 (due Monday)
• Under Assessments in Spark
Exam #1
• Next Thursday
• Material we cover today will be on the exam
New Discovery
• http://www.cnn.com/2009/TECH/space/09/16/ne
w.rocky.planet/index.html
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
motion
Kepler’s
st
1
Law
• 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
Eccentricity (e)
• e = distance between the two foci/length of major axis
• e of circle is 0
• The larger e becomes, the more eccentric the orbit
Definitions
• Perihelion – planet closest to the Sun
• Aphelion – planet farthest from the sun
• Semimajor axis (a) – the average of a planet’s
perihelion and aphelion distances
Kepler’s
nd
2
law
• 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
• http://surendranath.tripod.com/Applets/Dynamics/
Kepler/Kepler1Applet.html
Kepler’s
rd
3
Law
• More distant planets orbit the Sun at slower
average speeds, obeying the precise mathematical
relationship
p2 = a 3
where p is a planet’s orbital period in years and
a is the average distance from the Sun in
astronomical units (AU).
Calculations
• The period for the Earth to go around the Sun is
1 year
• The average distance of the Earth to the Sun is
1 Astronomical Unit (AU)
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
once
• p2 = a3 = 5.23 = 140.6
• p = √140.6 = 11.9 years
Another example
• Mercury is 0.4 Astronomical Units from the Sun.
• How long does it take Mercury to orbit the sun
once?
–
–
–
–
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
• An asteroid takes 8 years to go around the Sun
• How far is the asteroid away from the Sun?
–
–
–
–
A) 1 AU
B) 3 AU
C) 4 AU
D) 8 AU
The calculation
• a3 = p2 = 82 = 64
• a = (64)1/3 = 4 AU
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
Orbits
•
•
all the planets orbit the Sun in a
counterclockwise direction (but they do not orbit
it at the same rate).
The Earth rotates counterclockwise
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
moved
• 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
arguments
• 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
Who came up with the first
understanding of how gravity and
forces really work?
Isaac Newton (1642-1727)
http://www.hulu.com/watch/63319/cosmos-the-harmony-of-the-worlds
Any Questions?