Download Ancient to Modern Astronomy

Ancient to Modern Astronomy
The Astronomers and what
they did
The Ancients
Early Babylonians Egyptians and Chinese
made important astronomical observations
The five closest planets were observed
and novas and comets were recorded
Comets were blamed for disasters and
were seen as portents of doom
The Golden Age of Astronomy –
Greece 600 BC to 150 AD
The basics of geometry and trigonometry helped
them explain the motions of planets, sun and
moon
However, they erroneously decided that
everything in the “heavens” revolved around the
earth
That is what appears to happen, but this
apparent motion is really due to the earth’s
rotation
They decided that the Earth couldn’t be rotating
because we feel no motion
Some of the famous Greeks
Aristotle
Aristarchus
Aristotle knew that the earth was a
sphere because during an eclipse
the earth’s shadow was a circle.
However Aristotle believed in an
earth centered(Geocentric) solar
system
Aristarchus was the first to propose
a sun-centered solar
system(Heliocentric), but no one
agreed with him. Aristotle’s influence
was too strong and lasted over a
thousand years.
Aristarchus also tried to calculate
sizes of the moon and sun and
distances to them. The method was
good, but his measurements were
off, so his answers were wrong.
Some more!
Eratosthenes
Eratosthenes calculated the
circumference of the earth.
(We will demonstrate his
method.) Eratosthenes Link
Hipparchus observed and
recorded over 1,000 stars for
his star catalog. These
measurements were later
recorded by Ptolemy
Hipparchus
Earth circumference math
Two cities have incoming light at two different angles
We can measure the distance between the two cties
Using basic geometry (Can geometry students understand the
method?) he found the angle difference between two cities
Then Circumference/distance = 360/angle difference
(Why 360? Because there are 360 degrees in a circle)
Sample problem: The difference in latitude
between city A and city B is 30 degrees.
The distance between cities A and B is 2000
km. What is the circumference of the planet?
Did you get 24,000 km?
Solution:
Circumference = 2000 * (360/30)
WORKSHEET
Ptolemy
Ptolemy compiled a
13 volume book
which explained the
work of Greek
Astronomers
In the dark ages to
follow, these works
were preserved by
Arab scholars
The problem
The Greeks believed that the planetary orbits
were circles and that everything went around the
earth, including the sun and planets, and the
stars. The stars resided on a crystal sphere
through which light shown.
Unfortunately for the Greeks, the solar system
was not that simple.
They had trouble reconciling this model with the
observed motion of the planets.
So, they tried changing things slightly
The problem with retrograde
motion
As planets revolve, they
sometimes appear to go
backwards in their orbit.
This is called retrograde
motion.
How can this be
explained?
This geocentric model
shows one model that
was tried
The Ptolemaic System
Ptolemy had
introduced the
concept of epicycles
As a planet orbited
the earth, it moved in
small circles
These were called
epicycles Link
Did it work?
This made it somewhat better, but it was not
simple. Ptolemy had to devise many of these
to explain the apparent motion of the planets.
What is actually going on? Look at the
picture here.
What is actually happening is that Mars appears to go backwards as we pass it
in our orbit, like an inside car passing an outside car on a racetrack.
Watch this link.
But the problem with this explanation is that you need to realize that the
solar system is heliocentric, not geocentric.
Problems with models
This illustrates the danger of models.
If you too firmly believe in your model, you
tend to make your data fit that model.
It has been said that scientists, like artists,
fall in love with their models.
You have to be willing to throw out your
model if it doesn’t fit the data.
And revise it as new data becomes
available.
What’s next
For over a thousand years, there were very few
changes
The middle ages were a time of scientific
stagnation, as well as hard times for society
It is often true that changes in science and
society feed off of each other
Many of the most revolutionary times in science
occur with revolutionary times in society and
changing political systems
Copernicus
Nicholas Copernicus, a
Polish astronomer, was
the first great astronomer
in a long time.
He knew that a
heliocentric system
worked better than a
geocentric model.
Unfortunately, he insisted
on using circular orbits,
so he still had to add
epicycles to make his
system work.
Copernican Model
His revolutionary book was
published as Copernicus
was dying.
He was afraid to publish it
because the Catholic
Church, very powerful at
the time, firmly believed in
the geocentric model, which
put man in the center of the
known universe.
His theory was a threat to church teachings and you were in trouble
with the church if you were accused of being a “Copernican.”
Religion and Science Issues
Giordano Bruno, who refused to deny the
heliocentric theory of Copernicus, was
burned at the stake in 1600
We will see what happens to Galileo
shortly
Tycho Brahe
The Danish astronomer Brahe was a
meticulous observer who made many
accurate measurements of the motions
of heavenly objects.
Telescopes were not yet invented and
he used devices called pointers to
accurately line up stars to measure
them
Tycho had some interesting personal
history
Also, he did not believe in the
heliocentric model due to what he saw
as a lack of stellar parallax.
Parallax
The concept of parallax
shows that as you view
an object from two
different positions it will
appear to shift against the
background.
This is actually what
gives us 3D vision and
why we must view things
straight on to measure
them accurately.
Brahe stated, accurately, that if the
earth were traveling around the sun, we
should be able to see this parallax shift
of closer stars against the stellar
background, at different positions in the
earth’s orbit. Since we don’t, we can’t
be revolving around the sun.
Stellar Parallax
This was very smart of Brahe.
But there was one problem with his thinking.
Let’s try the parallax demo to see why.
See the small
change with a
farther star.
And, as we now
know, the stars
are VERY far
away, so we
can only see
stellar parallax
with modern
technology.
Johannes Kepler
Kepler, an Austrian by
birth, became Brahe’s
assistant about a year
before Brahe’s death
Kepler, a brilliant
mathematician, analyzed
Brahe’s 20 years of
astronomy data and
discovered some
interesting relationships in
the data
Kepler’s laws of Planetary Motion
So, Kepler’s 2nd law shows that the
triangles above are all equal in area.
Closer to the sun the planet goes faster,
covering a larger section of its orbit, but
the sun-planet distance is shorter.
The 1st law states that
planets travel in elliptical
orbits, rather than circular
orbits. This eliminates the
need for those pesky
epicycles
The 2nd law states that a
planet travels faster
closer to the sun, so that
it covers equal triangle
shaped orbit sections
during the same amounts
of time.
The 3rd law of planetary motion
Let’s look for a minute at a solar system model running –
hyperlink
You probably noticed that the planets travel at different
speeds.
The inner planets travel more quickly than the outer
planets
Kepler’s 3rd law give a mathematical relationship for this.
This relationship is: period(of revolution)^2 = distance
(from the sun to the planet)^3
Simply put: p2=r3 (p is in years and r is in AU’s – what’s
an AU? Watch the Link.)
Kepler
Interestingly enough,
Kepler figured out
how this all worked
However, he had no
idea why it was so.
That explanation
would have to wait a
little while for the
concept of Gravity as
proposed by Isaac
Newton
Galileo Galilei
Galileo was one of the
first scientists to use
experimentation to figure
things out in science.
He came up with the
concepts behind inertia,
among other things
Kepler and Galileo were
contemporaries and
actually corresponded.
Galileo’s astronomical observations
Galileo may have been the 1st scientist to use the
newly developed telescope to make astronomical
observations. These included:
– The discovery of the four largest moons of Jupiter
– The discovery that planets are not just “points of light.”
– The discovery that the moon is not a smooth ball, as
proposed by others.
– The discovery of sunspots (which probably caused him
blindness later) and observations of them to estimate
the rotational period of the sun
Galileo’s observations
And the discovery of
the phases of Venus,
which showed that
Venus must orbit the
sun, not the earth
This was evidence
supporting the
heliocentric model of
the solar system (study
the sketch here.)
Galileo vs the Church
Many of his observations went against the
accepted geocentric model.
The heliocentric model had been banned by the
church
Galileo was told to drop the heliocentric theory
from his writing
When he didn’t, he was tried by the Inquisition
and was sentenced to house arrest until he died.
He continued to work until his death
The church exonerated Galileo in 1992, more
than 350 years after his death.
Link
Sir Isaac Newton
His law of Universal Gravitation
explained how the force decreases with
distance. This is similar to how light
spreads out from a source. This is
somewhat complicated. See if you can
understand the link here. (It’s hard.)
One of the greatest
scientists who ever
lived, although not a
“nice man.”
Newton’s 3 laws of
motion (CP Newton’s
Laws video), (Other
3rd law demo video)
Orbits controlled by
gravity and inertia
(sketch here)
Law of Gravitation
Gravity activity
Newton continued
Newton also revised Kepler’s 3rd law to include the force
of gravity. This allows us to find the mass of an object
from the orbits of its satellites. For example, we can find
the mass of Jupiter by calculating how long it takes one
of its moons to go around it. (possible lab) We can even
use a more complicated version of this law to find the
masses of galaxies
Newton also made other great contributions to science in
the areas of optics. He also invented Calculus, and the
reflecting telescope. Link
He would not have been surprised by space travel. He
understood how we could actually leave the earth with
his “cannon model”. Link
The End