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Lecture 2

# AST201 Lecture 2 Notes.docx

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University of Toronto St. George

Astronomy & Astrophysics

AST201H1

Stephen Reid

Winter

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LECTURE 2:
1) Ancient’s and Astronomy? A)Humans are curious, we all know that, but
ancient Greeks, and the elder generation people used astronomy as a way
of telling the weather. For example, in Africa they use to observe the
crescent moon, and look for the horns to tell whether it was going to rain
or not. Knowing the weather then was important just as it is today
because people needed to grow agriculture. B) They used the changing
positions of stars, moons, and suns to determine what time it was , this is
essential for agriculture – marking Seasons etc. C) Luna Calenders: Some
months have 29 days, while others have 30 days. They paid particular
attention to those Don’t forget that the sun (Sunday), moon (Monday) and
5 other days are named after 5 other planets. ---------Note that back in the
days people use to think earth was geo-centric (everything revolved
around earth) This is now proved otherwise.
2) Greeks and their beliefs: They use to think earth was geo-centric, and
that there are spheres in every planet, The outermost sphere holds the
stars, and is the most perfect circle. (Like Plato’s ideology, that the
exterior mind is perfect little did he realize that our planets move
backwards sometimes, So how can it be perfect?)
3) Claudius Ptolemy comes in. He came up with the geo centric Ptolemaic
model (different from earlier geo-centric models).
4) Ptolemaic Theory Each planet moved in a small circle(epicycle), which
moved around earth on a larger circle(deferent) .
5) Retrograde Process believed by the greeks: Sun revolved (orbits)
around earth, and all other planets revolved (orbits) around the sun. This
is the retrograde process motion, where other planets revolve around the
sun. Not many took this theory seriously.
6) Islam and Astronomy: After the greeks (Alexander the great’s library
was destroyed) Muslims, in Baghdad especially looked for constellations,
that’s why most of the stars names start with Al, meaning “the” in Arabic.
They kept their research in tact . This helped carry the knowledge till
today.
7) Copernican Theory: Copernicus came up with the sun-centric model,
which did not go very well in his days.
8) Tycho’s Theory: He believed in a sun centric model.
Kepler: Kepler was Tycho’s student, Tycho was very hurt that no one
took his theory seriously, so he begged his student (whom he saw
potential in) to look for an answer, and continue with his theory. A) When Kepler looked for answers, he found his results slightly different
from Tycho’s. His first law was that planetary orbits are not circular, they
are ellipses (oval)
9) Pencil Example: a)To draw a circle (orbit) you can attatch a pencil with a
string, keeping the string at one spot – the radius, and pushing far and
going around makes a circle, b) but to draw a Ellipse(oval) You have to
use two focal points, attach both strings from the focal point to the pencil,
than it makes a oval.
10) Foci: The two points in a ellipse(oval). In the image above you see
an x axis, and a y axis.
11) Major Axis: x-axis
12) Minor Axis: y-axis.
13) Semi Major Axis: each half of the x-axis.
14) Eccentricity: The quantity between the foci’s. The closer the 2 foci’s
are the smaller the orbital is, the farther the 2 foci’s are the larger the
orbital is. See the image above on the side . Therefore a circle is an ellipse
that has 0 eccentricity.
15) Perihelion: Lies on the X axis, and is closest to the focus point.(I.e-
sun)
16) Aphehelian: Lies on the X-axis and is furthest away from the focus
point. (i.e- sun)
17) Kepler’s Three Laws:
First Law: The sun is the focus and does not have another
focus, and the orbitals around the sun are ellipses. (Note
that a planet is not going in circles around the sun it is
going in an eclipse meaning that the distance from the sun
to the earth is constantly different and changing) Second law: A) As the planets orbit the sun, it sweeps out in
equal areas, at equal times. Meaning that, as the planet gets
closer to Perihelion, which is closer to the sun, the planets
move fast. B) When the planet moves closer to the
Aphelion, which is far from the sun, the planet moves
slower.
Third Law: More distant planets orbit the sun at a slower
speed. (similar to the when planets move closer to the
Aphehelion) He came up with a mathematical formula,
which stated that P 2= a 3.Where P = planets orbital
period in years, and a = the distance from the sun. We can
use this formula to calculate the average orbital speed that
a planet travels.
18) What did Keplor Challenge?
1) He challenged the typical views of the greeks.
The absolute perfect circle- most exterior orbital outside of earth is not a
circle. The orbital is an oval. Thus it is not perfect.
2) Aristotles Idea that the earth is not moving but everything around it is.
Similar to Copernicus. -> Aristotle believed if earth moved than the birds
and everything would fall to the ground.
3)No one detected steller paralex orbited the sun.
19) Kepler’s law was later worked more precisely, on my Isaac Newton,
but until than, Galileo c

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