Homework #1
Astronomy 101 – Fall 2010
Due: September 16, 2010
1. Study the photo on page 1 in the textbook. What’s your personal reaction to this
image taken by the Hubble Space Telescope? Previously, what did you known about
the universe and its contents?
Before taking this image, even the scientists making the observations did not realize just
how vast and populated this universe is. This is the “deepest” – a term from observational
astronomy, meaning it reaches the furthest in time and distance away from us to very faint
objects – optical image in astronomy, and the evidence of the billions of galaxies in the
universe. Nearly every object (bright spot or smudge) in this image, from the large to the
small, is actually from an entire galaxy, not a single star. If you were to look at this patch of
sky with your own eyes, it would look like a dark area of sky, but every “empty” patch
actually looks something like this. One of the obstacles in astronomy can be comprehending
the incredibly large numbers and distances; as the caption says, some of these objects are so
far away that the light from them take twelve billion years to reach us.
2. Textbook problem: p. 53, #14
(Short-answer/essay) “What is the difference between a hypothesis and a theory in science?
Explain clearly.”
In essence, a hypothesis is an untested, but educated, guess about a specific situation or
the results of a specific experiment, while a theory is an overarching statement or model that
explains a collection of observations, and has been tested repeatedly. For example, before
performing his legendary kite-struck-by-lightning experiment, Ben Franklin formulated a
hypothesis about lightning actually being a form of electricity, unknown at the time, and how
that would affect the kite and attached key. More than a century later, the theory of
electromagnetism was formulated, and it still stands today as the physical and mathematical
model for how and why electricity and magnetism work in all situations.
Either a hypothesis or a theory may be changed based on new evidence, and neither can
ever be completely proven correct.
3. Textbook problem: p. 35, #14
(Short-answer/essay) “Causes of the Seasons. Suppose a friend tries to claim that it is
summer when Earth is closer to the Sun and winter when Earth is farther away. Refute this
claim in at least two different ways: (1) with an argument that invokes seasonal differences
between the Northern and Southern Hemispheres and (2) with an argument based on the time
of year at which the Earth actually is closest and farthest from the Sun. Write out your
arguments clearly in one to two paragraphs each.”
(1) We know that the seasons differ between Earth’s Northern and Southern hemispheres;
when it is winter in the Northern Hemisphere, it is summer in the Southern, and when is
summer in the Northern Hemisphere, it is winter in the Southern. If the seasons were simply
a result of the Earth’s distance from the Sun, then the entire Earth should be in summer when
it is closest to the Sun, and the entire Earth should be in winter when furthest from the sun.
This is simply not true, so your friend is wrong.
(2) Like any object orbiting another, the Earth does not revolve around the sun in a
perfect circle, but it is fairly close, so the distance between the Sun and the Earth varies only
slightly during the year-long orbit. The diagram on page 20 of the textbook gives the
numerical values for this varying distance, and thus the evidence needed for this argument.
On January 3 of each year, the Earth is closest to the Sun, at approximately 147.1 million km,
and, on July 4 of each year, the Earth is as far from the Sun as it will ever be, at 152.1 million
km away. Even if you knew nothing about the seasons in the Southern Hemisphere, you
should still see this flaw in your friend’s logic: as College Park area denizens would know,
on January 3 of this year, even though the Earth was closest to the Sun, the DC area was
buried in snow, and on July 4, the Earth was furthest from the Sun, and yet temperatures
were well over 90ºF throughout the day. Past experience – your own observations – simply
contradict your friend’s claim, which would say that we should experience summer in
January and winter in July.
4. Textbook problem: p. 22, “Think About It” Question
“Jupiter has an axis tilt of about 3º, small enough to be insignificant. Saturn has an axis tilt
of about 27º, slightly greater than that of Earth. Both planets have nearly circular orbits
around the Sun. Do you expect Jupiter to have seasons? Do you expect Saturn to have
seasons? Explain.
As we have learned from the text and lectures, the seasons on Earth are purely a result of
the tilt of the Earth axis. We have also convinced ourselves (and our friends) through the
answers to question #3 that having a nearly circular orbit means that the distance of the Earth
to the Sun has nothing to do with the seasons. These features of Earth can be generally
applied to the other planets, as well. If Jupiter has a nearly circular orbit, just like Earth does,
its distance to the Sun would not give it seasons. If it also has a small, insignificant axis tilt,
then there is no reason to expect Jupiter to have seasons. Likewise, if Saturn also has a nearly
circular orbit, its distance to the Sun does not lead to seasons. Saturn has a large axis tilt,
even larger than the Earth’s, we would certainly expect it to have seasons. Just like the Earth,
Saturn’s seasons would be opposite in its Northern and Southern Hemispheres, and the
bigger axis tilt angle would mean even more extreme differences between seasons.
5. Textbook problem: p. 17, #18 – Show your work!
(Short-answer/essay) Light-minute. Just as a light-year is the distance that light can travel in
one year, a light-minute is the distance that light can travel in one minute. What is a lightminute in kilometers?
To do this problem, we need the mathematical relationship between distance (d), speed (or
velocity, v), and time (t) for a moving object:
d = vt
The speed of light, “c,” is 3x105 kilometers/seconds.
The time in question is one minute, but let’s use 60 seconds, so that the units match up.
A light-minute is the distance traveled in 60 seconds at a speed of 3x105 kilometers/seconds,
so solve for “d” in the above equation, and we have the answer:
d = (3x105 km/s) x (60 s) = 1.8x107 km
Solutions by Ashlee Wilkins