Astronomy Exam

Page 1

AS 105 EXAM 1, FALL 2020

Directions: There are 8 short-answer questions, for a total of 100 points. All responses must be in
your own words. (Turnitin has plagiarism-detection software, and I will assign a 0 if I find you
have answers with significant overlap from other students, answers on Chegg, or a Google search.)

Please download this Word/text document, edit it with your answers (TYPE YOUR ANSWERS
DIRECTLY INTO THIS DOCUMENT), and the print a PDF and upload a PDF/Doc of your
responses by the end of class (12:15pm) on Tue. Oct. 6 to Turnitin on Blackboard. Please type in
your responses (you are free to embed any figures you hand-draw and take a photo of), but please
do not include figures you find online or from a textbook. You should be able to answer all
questions from your studies and without reference material, but are welcome to refer to your
notes or lecture material.

Before you begin, you must write your name below. By writing your name below, you agree that
you will not share with anyone else this exam or any of your answers, and you attest that all
answers given are completely in your own words:

NAME:

S1 [10 points]. A few times in class we discussed a scale model where we shrank the Sun down 10
billion times its actual size, to the size of a grapefruit at the Air & Space Museum in Washington, D.C.
In this scale model, Earth is the size of a pen tip orbiting roughly 15 meters from the grapefruit. (a)
Where is the nearest star to the Sun in this scale model? (b) Based on this scale model, do you think it is
common for stars in our Galaxy to collide? Why or why not?

S2 [15 points]. We have sent probes to explore the worlds in our solar system, especially the relatively
nearby planet Mars. (a) Describe at least two lines of evidence we discussed in lecture that suggest Mars
at one point had a deep ocean and considerable flowing liquid water at its surface. (b) Describe the
process by which most scientists think is the likely the reason Mars lost its atmosphere and with it most
of its water? (c) Why would you suspect that Earth still has a thick atmosphere whereas Mars does not?
(It may be useful to compare the physical size of Earth and Mars.)

S3 [15 points]. Almost all planets in our Solar System orbit the Sun in the same plane, orbit the Sun in
the same direction, and rotate in the same direction as their orbital motion. These observations strongly
suggest that the planets in our Solar System all formed at the same time and in the same way, and
condensed from the leftover material from the Sun’s formation. (a) How long ago did all of this material
condense to form the Sun and planets? (b) How do we know how long ago this formation happened?
Please describe how the aging method works, and what material scientists use the method on.

Page 2

S4 [10 points]. The planet Earth is tilted by 23.5 degrees from the plane of our orbit around the Sun.
Likewise, the planet Mars is titled by roughly 25 degrees relative to a very similar plane of its own orbit
around the Sun. (a) Based on this information, do you think there are seasons (e.g. summer, winter, etc.)
on Mars? (b) Imagine future humans create outposts near the water supplies frozen in the glaciers at
both the north and south pole of Mars. Would you expect the temperature at the north pole station to be
similar to or different than the south pole station on the same day? Why? (c) Would you expect to be
able to see the star Polaris from the north pole of Mars?

S5 [15 points]. In Homework 1 you used Stellarium to observe the planet Venus undergoing apparent
retrograde motion. (a) Describe the motion of Venus you saw, especially compared to the background
stars. (b) The ancient Greeks had a very difficult time explaining why the planets “wandered” in the sky.
How did they explain the motion of the planets? Feel free to draw a diagram. (c) How do we correctly
explain the apparent retrograde motion of the planets? Again, feel free to draw a diagram.

S6 [15 points]. Copernicus laid the groundwork to establish the model that correctly describes the
motion of planets in our solar system, with the Sun at the center. (a) What was the main reason why
Copernicus’s model was not widely accepted by contemporaries in the 1540s? (b) Describe at least three
things that Galileo observed through a telescope more than 60 years later that solidified the Copernican
revolution? In each case, make sure to describe how these observations either supported the heliocentric
model or raised serious doubts about models posed by Ptolemy and other ancient Greek astronomers.

S7 [10 points]. Perhaps you have seen the video clip where QVC host Shawn Killinger and fashion
designer Isaac Mizrahi argue about the Moon (https://www.youtube.com/watch?v=aQKgpm1SJmQ).
Let’s clear up the multitude of misconceptions in this exchange. (a) “Isn’t the Moon a star?” What
differentiates the Moon from a star, from the perspective of the generation of light? (b) “The Moon is a
planet, darling.” What differentiates the Moon from a planet? (c) “We don’t know what the Sun is.”
How is the Sun fundamentally different from the Moon or a planet? (d) “What else is it if it’s not a
planet?” What is the Moon if it’s not a planet? (Don’t overthink this question.) (e) “But things live on it,
that means it’s a planet.” Have we ever discovered life on the Moon?

S8 [10 points]. Newton provided an insightful formulation for how the force of gravity operates:
Fg = GM1M2 / d2. In words, this relationship says the force of gravity is proportional to the mass of each
object, and falls off with the distance between objects squared. (a) Say you are in a room and your cell
phone 1 meter away. Even if it is insignificant, is there a force attracting you to the phone? Why or why
not? (b) How much would the force of gravity change if you were 5 meters away instead of 1 meter? (c)
Imagine you and the phone have been shot into space and are very far away from any other mass. Would
you orbit the cell phone, would the phone orbit you, or would you both orbit something else? If the
latter, what would you be orbiting? Assume here that you have much more mass than the phone.