Assignment 2 (1)
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School
University of Ottawa *
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Course
2390
Subject
Astronomy
Date
Jan 9, 2024
Type
Pages
3
Uploaded by BarristerLapwing168
Chapter 3
Question 4
How did the Ptolemaic model explain retrograde motion? Page 51-53
The Ptolemaic model was a geocentric model that places earth at the center of the solar system and all
other planets, stars and moons revolving around it. The retrograde motion is when the direction of
movement of a planet appears to be opposite to the other bodies in its system. The Ptolemaic model
explained the retrograde motion by a combination of orbital movement. By combining the motion of the
deferent (a planet's primary orbit around Earth) and the epicycle (smaller circular path of the planet) a
retrograde motion can be simulated. Figure below explains when retrograde can be seen in accordance
with Ptolemy’s model:
“Ghose and Milosevic-Zdjelar,2020” page 53
Question 9
Review Galileo’s telescopic discoveries and explain why they supported the heliocentric model and
contradicted the geocentric model. Page 60-61
Galileo's telescopic discoveries provided empirical evidence that directly supported the heliocentric
model that was proposed by Copernicus
-
Jupiter's moons: Galileo discovered four large moons orbiting Jupiter. That contradicted the
geocentric idea that all celestial objects orbit the earth.
-
Phases of venus: Galileo observed through his telescope that Venus had phases like the moon.
However in the geocentric model, Venus should always appear as a crescent. This phenomenon
can only be explained if Venus orbited the sun.
Question 10
Explain why you might describe the orbital motion of the Moon with the statement, “ The Moon is
falling.” page 64.
The moon is “free falling” towards Earth due to gravitational attraction between the two bodies.
Instead of crashing into Earth it falls around it in an elliptical path missing it because of its horizontal
velocity. Basically the Moon is continuously falling towards earth but also moving tangentially. This
balance of forces is what keeps the Moon in orbit.
Learning to Look 2
Why can the object shown below be bolted in place and used 24hrs a day without adjustment?
Many communication satellites are placed in geostationary orbit, this is an orbit over the equator with no
eccentricity and inclination. When a satellite is placed in that orbit, the position of the satellite does not
move relative to the ground. Thus when a satellite dish is set up, it can be aligned precisely to receive
signals from a geostationary satellite.
(reference:
https://earthobservatory.nasa.gov/features/OrbitsCatalog/page2.php#:~:text=A%20satellite%20
in%20a%20circular,place%20on%20the%20Earth's%20surface
. )
Chapter 4
Question 1
Why would you not plot sound waves in the electromagnetic spectrum? Page 74
I would not plot sound waves in the electromagnetic spectrum because sound waves and electromagnetic
waves are completely different types of waves. Sound waves are mechanical waves that require a physical
medium like air or water to propagate whereas the radiation in the electromagnetic spectrum (includes:
visible light, radio waves, infrared, ultraviolet, x-rays and gamma rays) do not require a medium to travel
through. Electromagnetic waves can travel in an empty space and sound waves cannot.
Question 4
Why do nocturnal animals usually have large pupils in their eyes? How is that related to the way
astronomical telescopes work? Page 78
Nocturnal animals usually have large pupils to increase the amount of light that enters the eyes. This helps
them see better in the dark. Astronomical telescopes, like the pupils of nocturnal animals, have to gather
as much light as possible to observe the faint astronomical objects in the sky and the ability of a telescope
to collect light is proportional to the area of the primary mirror.
Question 5
Why do optical astronomers often put their telescopes at the top of mountains, while radio astronomers
sometimes put their telescopes in deep valleys? Page 80
Optical astronomers put their telescopes at the top of mountains to limit light pollution, this can
interfere with the observation of very faint celestial objects. It is also favorable to place the telescope at a
high altitude because the air is thinner( more transparent) and the skies are less cloudy, the clear skies
produce clearer imaging. The high position of the telescope also reduces the amount of atmosphere the
light passes through, which makes for better observation results.
Radio astronomers observe radio waves which, unlike light, are not significantly affected by the
atmosphere (they can pass through clouds etc.), so they sometimes put their telescopes in deep valleys in
order to avoid human radio interference; the isolated remote location has less terrestrial radio emission.
Learning to look 1
Two images below show a star before and after an adaptive optics system attached to the telescope was
switched on. What causes the distortion in the first image and how do adaptive optics correct the image?
Page 78-79
Possible causes for image distortion:
-
Diffraction fringe; light has a tendency to spread
-
Turbulence in Earth's atmosphere (you can see better in a telescope when the night sky conditions
are clearer)
Adaptive optics is a computer system built in the telescope that is able to adjust the optics according to
level of distortion.
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