|
Astro 1
Section 1
Professor Brandt
Wednesday
January 20th
1999
Announcements:
Lecture notes:
Class 5.
Constellations
12000 years from now
Vega will be the North Star
Vega is a very bright star near the horizon at twilight.
It's in the constellation Lyra (Lyre)
Also
Cygnus = swan
it's right above Lyra
First good black hole was discovered in this constellation
cygx-1
Main topic for today's lecture is to discuss "Why the Earth orbits
the Sun"
The answer to this question took many years to discover.
The Greeks started to answer this around 350 BC
and there wasn't really
as satisfactory answer until about 1666
-
took over 2000 years to solve this problem!
-
people didn't know for most of human history.
One way to get insight into this problem is to do a short experiment.
Like a ball swung on a string
if the string is cut
the ball flies off.
The same thing must hold for the orbit of
the Earth. There needs to be a force to keep the Earth from
flying off into space.
Gravity - the identification and understanding of this
force took more than 2000 years
There was no clear
logical progression if ideas. Many false
starts and sidetracks with wrong ideas happened. We will focus on
some of the important people in this search.
Copernicus (1473-1543) in about 1507
he first
had the idea that the sun
not the earth might be the center of the solar
system. This was important because up to that time
people had thought
that earth was the center of the universe - guided by ancient Greek
ideas of Aristotle and Ptolemy
-
This was heretical since Church had adopted Earth centered universe.
-
Didn't publish his ideas widely until 1543 when he realized he was dying
-
De Revolutionibus Orbium Copelestium - advanced heliocentric universe wasn't
exactly right since he used perfectly circular orbits.
-
widely read and admired
but didn't change the minds of most astronomers
Kepler (1571-1630)
-
from a poor family. Father was a mercenary and criminal. mother
was accused of being a witch
-
Mathematically brilliant
but a little weird
-
believed numbers were magical
-
writings are combination of astronomy
astrology
mathematics
and superstition
-
In 1600
he went to Prague to work for Tycho Brahe
-
had access to precise astronomy measurements
and used this in context
of Copernican heliocentric model to solve motion of planets
-
Realized planets move in ellipses
not perfect circles
about the sun.
-
Planets move in nearly circular ellipses about sun
with sun at 1 focus
(Kepler's first law)
-
Could very precisely predict positions of planets
NOTE seasons are not caused by highly elliptical orbits
perihelion = closest approach to sun
aphelion = furthest from sun
Kepler also found 2 other laws of planetary motion.
2nd Law - a line from planet to sun sweeps out equal areas in
equal time intervals
3rd Law - a planet's orbital period squared is proportional
to its average distance from the sun cubed
P2 = a3
P in years
a in AU
Note
a is semimajor axis
not average distance
Consider Mars
a=1.524 AU
p=1.881 years
(1.881)2 = (1.524)3
3.538=3.538
-
Kepler's laws describe how the planets orbit the sun but not why
-
For some reason
Kepler stopped short of making one of the greatest discoveries
in history - gravity
-
perhaps too mixed up in magical ides to see it
Newton (1642-1727)
-
English scientist - one of the greatest of all time
-
Famous for calculus
optics
mechanics
astrology and alchemy
-
realized some force must pull on Earth to keep it in its orbit
-
Law of Universal Gravitation
-
Derived Kepler's laws from it
|
