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Astronomy 1 Lab Manual Observing 1
Nighttime Observing
Introduction
This lab involves using a portable 4.5inch Orion SkyQuest Dobsonian telescope to make observa-
tions of Jupiter, Venus and the Moon. As a bonus, you can also observe Saturn. The telescope
(pictured below) comes with two eyepieces. The 25mm eyepiece provides a lower magnification,
and hence a wider field of view than the 10mm eyepiece.
Figure 1: The 4.5inch Orion SkyQuest telescope and a close up view of its two eyepieces.
The telescopes are signed-out from the Astro 1 lab room in Wilder. On the day and time assigned
(weather permitting), go to Room 200 Wilder. Hand in your answers to the pre-lab questions.
The Teaching Assistants will help you and your lab partner sign-out one of these telescopes. You
and the rest of the lab class will then carry the telescopes outside and set them up somewhere on
campus where you will have a fairly unobstructed view of the night sky.
Do all the required observations as they are listed in this manual. You can answer the various
questions at a later time when you are writing up the lab, and don’t need to answer them while you
are doing the observations.
You’ll need FIVE things to do the observing part of this lab: a notebook, a pen, a timer or watch
with a second hand, this lab description, and a 4.5inch telescope checked out from Wilder Room
200. You must READ THIS WHOLE HANDOUT before you go out and try observing. It will
save you time and frustration.
Setting Up the Telescope
Set up your telescope and put the eyepiece with the larger lens (giving the lower magnification)
into the eyepiece holder sleeve near the top end of the telescope. Point the telescope at the Moon.
Astronomy 1 Lab Manual Observing 2
Using the two knobs near the base of the eyepiece holder/tube assembly, focus the image of the
Moon. The small side telescope is meant as a finder telescope. It aids in the pointing of the main
telescope. You need to adjust the small telescope’s pointing so as to match it to that of the main
telescope. Without moving the telescope, look through the finder telescope and adjust the small
screws on the finder telescope so the part of the Moon centered in the main telescope appears near
the center of the small telescope.
Venus
Point the telescope at Venus. Venus is the brightest object in the sky, located low in the western
sky. Using the data-sheets contained at the end of this lab manual, make two drawings (one for
each eyepiece) of Venus. Accurately draw the relative size of Venus compared to your field of view
(shown by the circle on the data-sheet).
Have the TA sign all of your data-sheets before you complete the lab.
Q1: Based upon your drawing, what is the current phase of Venus? Is such a phase possible in the
Ptolemaic model of the solar system? Explain.
Q2: Based upon your observed phase of Venus, sketch the relative locations of the Earth, Venus
and the Sun. Shade in the hemispheres on Venus and the Earth which are in darkness.
Moon
Examine the Moon’s surface and make an accurate drawing of what you observe. Identify
several features (including all of the visible maria) listed on the Moon maps included in the lab
write-up, and label your drawing. The telescope inverts and flips the image around so you should
carefully use Moon maps which do not show a telescopic-like view.
Using the 10mm (high magnification) lens look at the Moon. You will notice that the Moon slowly
drifts out of view to the west; that is due to the Earth’s counterclockwise rotation which is also
being magnified by the telescope.
Gently move the telescope and find Mare Crisium (Sea of Crises). It is located along the edge
farthest from the Sunrise (terminator side). Measure its apparent angular size by watching how
long it takes to move across the telescope’s field-of-view using the conversion: 1 second of time =
15 arc seconds1. Record your measurement.
Find one of the following craters: Posidonius, Langrenus, Aristotles, Hipparchus, or Copernius.
By timing how long it takes the edge of the field-of-view to move across a feature (or a distance),
determine how far away (in arc seconds) any one of these craters is located from the sunrise line.
Record your measurement.
1An arc second is the unit of measurement used by astronomers to measure very small angles. There are 60 arc
minutes in 1 degree, and 60 arc seconds in an arc minute, so there are 3600 arc seconds in 1 degree.
Astronomy 1 Lab Manual Observing 3
Next, find one or two small craters you can see through the telescope but which are not named on
the Moon maps in this lab write-up. Mark their locations on your maps; you’ll need to figure out
which craters they are later so mark their positions as best as you can.
Q3: Using your measurement of the angular size of Mare Crisium, calculate its true linear dimen-
sion (= diameter) using the small angle formula, assuming it is largely round, and knowing the
apparent diameter of Mare Crisium in arcseconds and the average Earth-Moon distance (384,000
km). See the discussion of the small angle formula at the end of this lab for details on how to do
this calculation.
Compare the value you calculated with one estimated from the size of the Mare Crisium as mea-
sured off the lunar photos in this lab. For example, suppose theMoon’s 3500 km diameter measures
out to be 10.7 cm on the photo. This means the conversion of centimeters (cm) on the photo to km
is about 330 km per cm.
Q4: For the one or two small-ish craters you marked on the Moon maps, determine their names by
going to various websites which show lunar maps with crater names. Two of these are:
http://www.lunasociety.org/atlas/index.shtml
http://www.oarval.org/MoonMapen.html
Estimate the diameter of one of these small craters you observed based upon its size relative to
Moon’s 3500 km diameter.
Q5: It turns out that while most other maria on the moon were formed by lava, Mare Crisium is
believed to have been formed by an impact – either a comet (density around 1000 kg/m3) or an
asteroid (density around 4000 kg/m3). The equation for the kinetic energy released due to such a
collision is:
E =
1
2
mv2
where m is the mass of the impactor and v is the velocity of the impactor.
The units of energy (in MKS) is the joule (one kilogram of TNT will release 4⇥106 joules). If we
assume that one needs about 1020 joules of energy to form a impact crater or mare 500 km across,
calculate the size of a spherical comet and the size of an spherical asteroid needed to form Mare
Crisium if they both hit the Moon at a velocity of 10 km/sec. How would their sizes have to change
if their velocity dropped to 1 km/sec? (Recall density = mass/volume which for a sphere means
mass = density ⇥4/3pr3; also note that in MKS units, use meters not km.)
Jupiter and its Moons
Point the telescope at Jupiter. Jupiter is the brightest object in the night sky, other than the the
Moon and Venus. It will be located fairly high in the western sky. Once you have Jupiter centered
Astronomy 1 Lab Manual Observing 4
in your field of view, switch to the 10mm eyepiece (smaller lens) so that you have the highest
magnification. Re-focus the telescope to obtain the sharpest possible view of Jupiter.
Jupiter has four large Moons (first seen by Galileo 1610): Io, Europa, Ganymede and Callisto.
Usually, 3 or 4 of these moons are easily visible through your telescope. Using the data sheets
contained at the end of this lab manual,make a careful drawing of Jupiter and its moons. Note
that the big circle in the data sheet represents the entire field of view that you see when you look
through the telescope. Jupiter should be drawn as a small circle within this large circle, and you
need to accurately draw the relative locations of Jupiter’s moons within the big circle.
Q6: Can you see any features on Jupiter’s surface? If so, describe them and indicate them on your
drawing.
Q7: Jupiter has an angular size of 39 arc-seconds. Based upon the angular size of Jupiter and its
appearance (i.e.. how small of a feature can you discern on the surface of Jupiter), estimate the
angular resolution of your telescope. How does this compare to angular resolution of your eye?
The magazine Sky and Telescope has a number of Java applets which are useful when observing
the night sky: http://www.skyandtelescope.com/observing/interactive-sky-watching-tools/ The ap-
plet Find Jupiter’s Moons shows the location of Jupiter’s moons. After you have completed your
observing, use this applet to label your diagram, with the names of each moon and Jupiter
itself prior to handing in your lab report.
To use this applet, you will need to register for an account – you just need to enter a username, a
valid e-mail address and password to register. When using the Jupiter Moon applet, you need to
set it to the Inverted view (for a Dobsonian telescope) and set the date and time to the correct
values for when you took your observations. The time entered is in UT format. To convert from
Eastern Daylight Time to UT format, add four hours. For example, 9:30pm on April 22 is 01:30
UT on April 23. Make sure you click the box Recalculate using entered date and time after you
enter the time of your observations.
Astronomy 1 Lab Manual Observing 5
Writing up the Lab
Make sure you bring this lab manual to the observing session, as you will use the data-sheets to
record your observations. You will work with a partner, and only need to have one set of drawings
for the two of you. When you complete the lab, have the TA sign your data sheet. If you are
handing in individual lab reports, photocopy the data-sheets – one person will hand in the original
data-sheets as part of their lab report, while the other person will hand in the photocopies.
Lab reports are due 7 days after you complete the lab. Lab reports are to be put into the locked
box (labeled Astro 1 Lab Reports) next to the student mailboxes, which are located to the left of
the main stairs when you enter Wilder Lab.
Aside from the data sheet, you should use complete sentences throughout your lab report. When
you have equations to solve, show all of your work (not just the answer), and make sure that you
include¡ units if needed. A complete lab report will consist of the following:
1. Purpose: A brief statement describing the purpose of the lab.
2. Data: The data sheets you completed in the lab, and signed by the TA.
3. Analysis: Answer all of the questions which are contained in the different sections of this
Lab Manual. Make sure you explain your answers and describe how you came to your
conclusions.
4. Summary: A brief summary of what you have learned in the lab. This should include some
discussion of the techniques you used (e.g. using a telescope) and should not just be a repeat
of the answers to the lab questions.
Astronomy 1 Lab Manual Observing 6
Pre-Lab Questions
You must answer the following questions before you come to lab. Hand in your answers to the
TA at the start of the lab.
The answer to all these questions can be found in your textbook and the lab itself. The pre-lab
questions will account for 15% of your lab grade. By reading the lab and thinking about it in
advance of performing the lab, you will find that the lab makes much more sense when you start
collecting data.
1. In what direction would you point your telescope to see Jupiter?
2. What do you need to bring with you to the lab room before you pick up the telescope?
3. Jupiter currently has an angular size of 39 arcseconds, and is located 5.05 AU from the Earth.
What is the diameter of Jupiter in km?
Astronomy 1 Lab Manual Observing 7
Observers:
Date:
Time:
Sky Condition:
Object Name:
Eye Piece:
TA Signature:
Astronomy 1 Lab Manual Observing 8
Observers:
Date:
Time:
Sky Condition:
Object Name:
Eye Piece:
TA Signature:
Astronomy 1 Lab Manual Observing 9
Observers:
Date:
Time:
Sky Condition:
Object Name:
Eye Piece:
TA Signature:
Astronomy 1 Lab Manual Observing 10
Observers:
Date:
Time:
Sky Condition:
Object Name:
Eye Piece:
TA Signature:
Astronomy 1 Lab Manual Observing 11
Observers:
Date:
Time:
Sky Condition:
Object Name:
Eye Piece:
TA Signature:
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