Transcript Tycho and Kepler

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Tycho Brahe (1546-1601)
(right around Galileo)
Born Tyge Ottesen
Brahe (I can’t say
that correctly)
Last of the great
astronomers who
came before the
invention of the
telescope
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What’s the deal with Tycho’s nose?
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Tycho’s life
Born as a twin, but his brother died
His uncle took him away from his family
when he was two, but his real parents
were apparently all right with this.
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Tycho’s life
Observed the total
solar eclipse of 1560
So impressed by the
fact that it had been
predicted that he
developed an interest
in Astronomy
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Tycho’s life
Became discouraged
when he couldn’t find
any 2 astronomical
charts that agreed
with each other
Set out to complete
his own astronomical
chart of the night sky
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Tycho’s Observatory on the Island of Hven
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Tycho and the supernova of 1572
A new star appears in the
constellation Cassiopeia
and temporarily becomes
as bright as the planet
Venus
- why is a new star
important?
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Tycho’s contributions
Observed the supernova of 1572
Showed that the comet of 1577 was
beyond the earth’s atmosphere
Accumulated years of careful observations
of the apparent motions of the planets in
the sky
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Tycho’s Contributions
Made incredibly
detailed catalogues of
the sky over a period
of 25+ years!
Was able to detect
stellar parallax!
All without the aid of
a telescope!!!
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Tycho’s own model of the solar
system was not a success
The sun goes
around the earth
and the other
planets go around
The sun
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Tycho = a strange duck
At one point, he was so famous and rich,
he was said to own 1% of the entire wealth
of Denmark (in 1580)
Held large parties at his castle, and had a
dwarf named Jepp sit under the table
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Tycho = a strange moose
Had a tame moose as
a pet
The moose died after
drinking too much
beer at a party and
falling down the stairs
(the story doesn’t say
why the moose was
indoors at all, perhaps
he knew some good
moose jokes)
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Tycho’s Death
“Bladder myth” generally believed to be
false
Sick for 11 days after the banquet
Perhaps………..
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High levels of Mercury were found in samples of
Tycho’s hair recently
Tycho himself was an alchemist
– He would have known Mercury can kill
After Tycho’s death, Kepler took Tycho’s 25 years
of observations and used the data to calculate his
3 laws of planetary motion!
– Tycho was very secretive with his notes!
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R.I.P. Rockstar Astronomer
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But Tycho’s observations paved the
way for Johannes Kepler (15711630, born in Stuttgart, Germany)
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Kepler: a mystic and a
mathematician
Uses Tycho’s
observations to try to
come up with a better
model of the motions
of the planets
Mysterium
cosmographicum (The
Sacred Mystery of the
Cosmos)
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Kepler: the man, the myth, the
legend
Dabbled in the
philosophical side of
Astronomy and
Astrology
– At that time, they were
one in the same
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Kepler’s music of the spheres
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But Kepler was not wacko
He came up with 3 laws of
planetary motion
First Law
– Planets move in
elliptical orbits with the
sun at one focus of the
ellipse
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What is an Ellipse?
An ellipse looks like an elongated circle
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What are Kepler’s three laws of planetary motion?
Kepler’s First Law: The orbit of each planet around
the Sun is an ellipse with the Sun at one focus.
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No more uniform circular motion
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Kepler’s Second Law: As a planet moves around its
orbit, it sweeps out equal areas in equal times.
 means that a planet travels faster when it is nearer to the Sun and
slower when it is farther from the Sun.
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Kepler’s third law
a is the
semi-major axis
of the ellipse
For a circle a is
equal to the radius
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Kepler’s third law
P = period
a = semi-major axis (radius for a circle)
P2 = constant X a3
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For a planet orbiting the sun
Measure P in earth years
Measure a in AU (astronomical units,
Earth to sun, as we’ll see)
– Then
P2 = a3
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Examples
Earth a =1 gives P = 1
Jupiter a = 5.2 AU
P2 = a3 = (5.2)3 = 141
P is the square root of 141, which is about 11.9
So it takes Jupiter about 11.9 years to go once
around the sun
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Kepler
By getting rid of uniform motion in circles
Kepler got rid of epicycles
His laws of planetary motion predicted the
motions of the planets more accurately
than either Ptolemy or Copernicus
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R.I.P. F.D.G.
Gustavus Adolphus’ (King of Sweden) army
destroyed the churchyard where Kepler was
buried, so we don’t actually know where his grave
is anymore. Sad day.
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