Lecture 6

Question 1

aristotle’s beliefs?

Answer 1

• earth was stationary
• moon, sun, planets , stars revolve around the earth on crystalline spheres
• geocentric cosmology
• unknown force keeps spheres revolving
• this view could not explain changes of brightnesses, sizes of planets, apparent retrograde motion of mars

Question 2

what were ptolemy’s beliefs?

Answer 2

tried to unite aristole’s views w/ his observations

introduced epicycles to explain anomalies in planetary motion; explained apparent retrograde motion of mars

Question 3

what were copernicus’s beliefs/discoveries?

Answer 3

• noticed that ptolemy’s predictions for planetary positions were off
• realized that earth and other planets orbit around the sun
• was problematic against the church b/c they thought the earth was the centre of the universe
• this showed why mercury, venus are always near the sun
• thought that orbits were circular

Question 4

what did tycho brahe discover?

Answer 4

• made first precision astrometric measurements

Question 5

what did kepler discover?

Answer 5

• that planetary orbits are not circles, but are ellipses

- measured time for each planet to orbit the sun

Question 6

what is the orbital period of earth?

Answer 6

365.25 d

Question 7

what are kepler’s laws?

Answer 7

1) planets travel in elliptical orbits w/ sun at one focus
2) planets travel faster when closest to the sun (equal areas are swept out in equal times)
3) square of orbital period P is proportional to cube of semi-major axis

Question 8

what is the eqn that represents kepler’s third law?

Answer 8

P^2 is proportional to a^3

a = semi-major axis

Question 9

what were galileo’s discoveries?

Answer 9

• first to use a telescope
• saw craters and sunspots
• 4 moons of jupiter (not everything orbits the earth) and phases of venus
• inertia and mass
• acceleration: all objects accelerate in the same way when it falls toward earth (independent of mass)

Question 10

what were netwon’s discoveries?

Answer 10

• laws of motion

- universal law of gravitation

Question 11

what are the 3 laws of motion?

Answer 11

1) object stays at rest or at constant speed unless a force acts on it
2) a force results in an acceleration that depends on the object’s mass
3) every force has an equal and opposite rxn force

Question 12

what kind of rays are blocked by the upper atmosphere?

Answer 12

gamma
X
UV

Question 13

how is visible light affected by the earth’s atmosphere?

Answer 13

it is still observable on surface

has some atmospheric distortion

Question 14

how is infrared spectrumaffected by the earth’s atmosphere?

Answer 14

most is absorbed by atm gases

Question 15

how are radio waves affected by the earth’s atmosphere?

Answer 15

they are observable

Question 16

how are long wavelength radio waves affected by the earth’s atmosphere?

Answer 16

they are blocked

Question 17

why have a bigger telescope?

Answer 17

the amount of light collected is proportional to the area of the telescope

S is proportional to pi(D/2)^2

s = sensitivity (total number of photons collected)

D = diameter of telescope

Question 18

how much more sensitive is a 10’’ telescope than a 5’’ one?

Answer 18

4 times

Question 19

disadvantages of refractors?

Answer 19

• lens gets very big
• telescope structure bends
• chromatic aberration

Question 20

what is chromatic aberration?

Answer 20

failure of a lens to focus all colors to the same point

It is caused by dispersion; different amounts of light is refracted depending on the wavelength of it

Eg. blue light is refracted a bit more than yellow and red light

Question 21

old telescopes are ____

modern telescopes are _____

Answer 21

refractors

reflectors

Question 22

describe modern reflector telescopes.

what are pros of this?

Answer 22

uses curved mirror to collect and focus light

pros:

• has no chromatic aberration
• lighter
• can make bigger mirrors