Astronomy 170B1
M. Rieke & M. Halford
Due at the beginning of class on January 22. You should enter the answers on a Scantron form
to be provided in class. This homework is worth 15 points.
Astronomers use various ways to understand large masses and distances, such as ratios and
scientific notation. For example, if you are told that a jet airplane is 40 meters long, that might
not register as well as being told that it is eight times as long as a car. The first part of this
homework explores ratios and scientific notation.
Let’s try the same logic on the sizes of the earth and moon:
The diameter of moon is 3,476 km
The diameter of the earth is 12,756 km
These numbers are too big to register in terms of everyday experience, but maybe we can make
a scale model in our minds (like the one of the solar system in the first lecture). If we divide the
diameter of the earth by that of the moon, we get
Earth diameter 12756 km
3.70 4
Moon diameter 3476 km
Here, the two squiggly lines (≈) mean “is approximately
equal.” That is, the earth is about four times wider than
the moon, or for our model, we should use something
four times smaller for the moon than for the earth.
1. Which would you pick for such a model?
a.) a basketball and a soccer ball
b.) a basketball and a baseball (hardball)
c.) a basketball and a ping pong ball
d.) a basketball and a pea
e.) a basketball and a grain of salt
2. The distance from the earth to the moon is about 384,400km (this is the radius of the orbit of
the moon, which is half of its diameter). This is about 30 times the diameter of the earth, so your
model moon would be how far from your model earth?
a.) the width of the lecture hall
b.) the height of your chair
c.) the height of the Administration building d.) the length of the campus
The diameter of the sun is 1,392,000km. We then have
Sun diameter
1392000km
109 100 1 102
Earth diameter
12756km
3. Therefore, for your model to include the sun, you would need a ball about as big as
a.) a large SUV is long
b.) a two story house is high
c.) a football field is wide
d.) the UA campus from end to end
e.) the distance to Phoenix
4.) Now, let’s compare the diameter of the sun with the size of the moon’s orbit around the
earth, which is 769,000km in diameter. The ratio of the sun’s diameter to the diameter of this
orbit is about 2:
Sun diameter
1392000km
2
Moon ‘ s orbit diameter 769000km
Does this mean that
a.) two suns placed side to side would fit within the orbit of the moon, or that
b.) two orbits of the moon would fit inside the sun?
Volumes of spheres can be compared by comparing the cubes of the diameters. Therefore, the
volume of the sun compared to the volume of the earth is
3
Volume of Sun 1392000km
3
2
2
2
100 110 110 110
Volume of Earth 12756km
5. In scientific notation, this ratio is
b.) 1 x 103
c.) 1 x 106
a.) 1 x 105
d.) 1 x 108
6. In scientific notation, what is the ratio of the volume of the sun to that of the moon?
a.)16 x 105
b.) 8 x 104
c.) 64 x 106 d.) 4 x 108
Looking at Distant Objects
Imagine that you have just received four pictures of four different children who live near four of
the closest stars to the Sun. Each picture shows a child on his or her 12th birthday. The
pictures were each broadcast by radio directly to you on the day of the child’s birthday. Note the
abbreviation “ly” is used below to represent a light-year.
Peter lives on a planet orbiting Barnard’s Star, which is 6.0 ly from the Sun.
Celeste lives on a planet orbiting Sirius, which is 8.6 ly from the Sun.
Savannah lives on a planet orbiting Alpha Centauri, which is 4.3 ly from the Sun.
Ron lives on a planet orbiting Procyon, which is 11.4 ly from the Sun.
7.) What is a light-year?. Is it an interval of time, a measure of length an indication of speed, or
something else?
a.) Interval of time
b.) Measure of length c.) indication of speed d.) something else
8.) Which child lives closest to the Sun?
a.) Peter
b.) Celeste
c.) Savannah
d.) Ron
9.) What was the greatest amount of time that it took for any one of the pictures to travel from the
child to you?
a.) 11.4 years
b.) more than 11.4 years
d.) it is not possible to tell
c.) less than 11.4 years
If each child was 12 years old when he or she sent his or her picture to you, how old were the
children when you received their picture?
10.) Peter
a.) 12 years
b.) 6 years
d.) it is not possible to tell
c.) 18 years
11. Celeste
a.) 12 years
b.) 8.6 years
d.) it is not possible to tell
c.) 20.6 years
12.) Suppose the children were issued intergalactic standard radio transmitters that have the
same intrinsic output strength. Which one will have the strongest signal as you received it?
a.) Peter
b.) Celeste
c.) Savannah
d.) Ron
13.) Compare the signal from Celeste with that from Savannah. Which will be weaker?
a.) Celeste
b.) Savannah
c.) they will be the same
14.) The telescope image at the right was taken of
the Andromeda Galaxy, which is located 2.5 million
ly away from us. Is this an image showing how the
Andromeda Galaxy really is right now, how it was in
the past or how it will be in the future?
a.) right now
b.) 2.5 million years in the future
c.) 2.5 million years in the past
Imagine that you are observing the light from a distant star that was located in a galaxy
100 million ly away from you. By analysis of the starlight received, you are able to tell
that the image we see is of a 10 million year old star. You are also able to predict that
the star will have a total lifetime of 50 million years, at which point it will end in a
catastrophic supernova.
15.) How old does the star appear to us here on Earth?
a.) 100 million yrs b.) 10 million yrs c.) 60 million yrs d.) 50 million yrs e.) 40 million yrs
16.) How long will it be before we receive the light from the supernova event?
a.) 100 million yrs b.) 10 million yrs c.) 60 million yrs d.) 50 million yrs e.) 40 million yrs
17.) How many years ago did the supernova occur?
a.) 100 million yrs b.) 10 million yrs c.) 60 million yrs d.) 50 million yrs e.) 40 million yrs
18.) Where on earth can you in principle see the largest number of different stars over the
course of a year?
a.) the north pole b.) the equator c.) the number of stars should not depend on where you are
19.) March 20 will be an equinox. This means
a.) the amount of sunlight reaching the northern and southern hemispheres will be the same on
that day
b.) the monsoon season will finish
c.) Alaska will start having midnight sun
20.) When it is winter in Tucson, it is
a.) winter all over the world
b.) summer in Australia (southern hemisphere)
c.) winter in Australia
d.) spring in Australia
e.) fall in Australia
21. How long does it take the Moon to rotate on its axis once?
a.) the Moon does not rotate
b.) 24 hours
c.) 365 days
d.) about 29 days
22.) Our modern calendar
a.) has been used for so long no one remembers where it came from
b.) is largely the creation of the famous emperor of Rome, Julius Caesar
c.) Is still not well tuned to the mismatch of the day to the year and will need adjustment soon
d.) Was brought down unchanged from the Babylonian astronomers
e.) Is based entirely on recent observations and concepts.
23.) a "blue moon" is
a.) when the moon looks blue against the sky in the daytime
b.) a new moon seen in the daytime, so the part of the disk of the moon not directly illuminated
by the sun looks blue
c.) the second full moon within the same month
d.) the moon seen on a bitter cold winter night
24.) the "man in the moon" is
a.) an arrangement of lunar features that can look like a human face
b.) the body of an astronaut that was left there when there was an accident that killed him
c.) just a face artists make up to make the moon look more approachable
d.) a concept left from the Greek gods
25.) Scientists use facts
a.) to support or to refute theories
b.) only to support theories
c.) only to refute theories
26.) Summer is hotter than winter because
a.) the Earth is closer to the Sun
b.) the Sun’s output varies periodically on a time scale of a year
c.) the tilt of the Earth’s axis results in sunlight hitting the ground more directly in summer
d.) of a the periodicity in the Earth’s climate
27.) Ancient peoples built large structures to measure positions of astronomical objects to help
them
a.) show their superiority over other tribes
b.) predict the seasons to assist agriculture
c.) keep everyone employed
d.) show off their architectural skills
28.) If tonight is a full moon, what will the moon’s phase be in two weeks?
a.) full
b.) first quarter
c.) last quarter
d.) new
29.) What fraction of the Earth’s surface can view a lunar eclipse?
a.) half
b.) a narrow strip
c.) all of it
d.) one quarter
30.) A solar eclipse occurs at which moon phase?
a.) any
b.) new
c.) first quarter
d.) full
e.) last quarter
