nighttime observing introduction - dartmouth … observing introduction ... make sure you click the...

Astronomy 1 Lab Manual Observing 1

Nighttime Observing

IntroductionThis 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. Youand the rest of the lab class will then carry the telescopes outside and set them up somewhere oncampus 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 variousquestions at a later time when you are writing up the lab, and don’t need to answer them while youare doing the observations.

You’ll need FIVE things to do the observing part of this lab: a notebook, a pen, a timer or watchwith a second hand, this lab description, and a 4.5inch telescope checked out from Wilder Room200. You must READ THIS WHOLE HANDOUT before you go out and try observing. It willsave you time and frustration.

Setting Up the TelescopeSet 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.


Using the two knobs near the base of the eyepiece holder/tube assembly, focus the image of theMoon. The small side telescope is meant as a finder telescope. It aids in the pointing of the maintelescope. You need to adjust the small telescope’s pointing so as to match it to that of the maintelescope. Without moving the telescope, look through the finder telescope and adjust the smallscrews on the finder telescope so the part of the Moon centered in the main telescope appears nearthe center of the small telescope.

VenusPoint the telescope at Venus. Venus is the brightest object in the sky, located low in the westernsky. Using the data-sheets contained at the end of this lab manual, make two drawings (one foreach 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 thePtolemaic model of the solar system? Explain.

Q2: Based upon your observed phase of Venus, sketch the relative locations of the Earth, Venusand the Sun. Shade in the hemispheres on Venus and the Earth which are in darkness.

MoonExamine the Moon’s surface and make an accurate drawing of what you observe. Identifyseveral features (including all of the visible maria) listed on the Moon maps included in the labwrite-up, and label your drawing. The telescope inverts and flips the image around so you shouldcarefully 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 slowlydrifts out of view to the west; that is due to the Earth’s counterclockwise rotation which is alsobeing magnified by the telescope.

Gently move the telescope and find Mare Crisium (Sea of Crises). It is located along the edgefarthest from the Sunrise (terminator side). Measure its apparent angular size by watching howlong 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 arcminutes in 1 degree, and 60 arc seconds in an arc minute, so there are 3600 arc seconds in 1 degree.


Next, find one or two small craters you can see through the telescope but which are not named onthe Moon maps in this lab write-up. Mark their locations on your maps; you’ll need to figure outwhich 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 theapparent diameter of Mare Crisium in arcseconds and the average Earth-Moon distance (384,000km). See the discussion of the small angle formula at the end of this lab for details on how to dothis 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 the Moon’s 3500 km diameter measuresout to be 10.7 cm on the photo. This means the conversion of centimeters (cm) on the photo to kmis about 330 km per cm.

Q4: For the one or two small-ish craters you marked on the Moon maps, determine their names bygoing 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 toMoon’s 3500 km diameter.

Q5: It turns out that while most other maria on the moon were formed by lava, Mare Crisium isbelieved to have been formed by an impact – either a comet (density around 1000 kg/m3) or anasteroid (density around 4000 kg/m3). The equation for the kinetic energy released due to such acollision is:

E =12

mv

2

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 weassume 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 MareCrisium if they both hit the Moon at a velocity of 10 km/sec. How would their sizes have to changeif their velocity dropped to 1 km/sec? (Recall density = mass/volume which for a sphere meansmass = density ⇥4/3pr

3; also note that in MKS units, use meters not km.)

Jupiter and its MoonsPoint the telescope at Jupiter. Jupiter is the brightest object in the night sky, other than the theMoon and Venus. It will be located fairly high in the western sky. Once you have Jupiter centered


in your field of view, switch to the 10mm eyepiece (smaller lens) so that you have the highestmagnification. 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 sheetscontained at the end of this lab manual, make a careful drawing of Jupiter and its moons. Notethat the big circle in the data sheet represents the entire field of view that you see when you lookthrough the telescope. Jupiter should be drawn as a small circle within this large circle, and youneed 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 yourdrawing.

Q7: Jupiter has an angular size of 39 arc-seconds. Based upon the angular size of Jupiter and itsappearance (i.e.. how small of a feature can you discern on the surface of Jupiter), estimate theangular 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 observingthe 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 yourobserving, use this applet to label your diagram, with the names of each moon and Jupiteritself 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, avalid e-mail address and password to register. When using the Jupiter Moon applet, you need toset it to the Inverted view (for a Dobsonian telescope) and set the date and time to the correctvalues for when you took your observations. The time entered is in UT format. To convert fromEastern Daylight Time to UT format, add four hours. For example, 9:30pm on April 22 is 01:30UT on April 23. Make sure you click the box Recalculate using entered date and time after youenter the time of your observations.


Writing up the LabMake sure you bring this lab manual to the observing session, as you will use the data-sheets torecord your observations. You will work with a partner, and only need to have one set of drawingsfor the two of you. When you complete the lab, have the TA sign your data sheet. If you arehanding in individual lab reports, photocopy the data-sheets – one person will hand in the originaldata-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 lockedbox (labeled Astro 1 Lab Reports) next to the student mailboxes, which are located to the left ofthe main stairs when you enter Wilder Lab.

Aside from the data sheet, you should use complete sentences throughout your lab report. Whenyou have equations to solve, show all of your work (not just the answer), and make sure that youinclude¡ 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 thisLab Manual. Make sure you explain your answers and describe how you came to yourconclusions.

4. Summary: A brief summary of what you have learned in the lab. This should include somediscussion of the techniques you used (e.g. using a telescope) and should not just be a repeatof the answers to the lab questions.


Pre-Lab Questions

You must answer the following questions before you come to lab. Hand in your answers to theTA at the start of the lab.

The answer to all these questions can be found in your textbook and the lab itself. The pre-labquestions will account for 15% of your lab grade. By reading the lab and thinking about it inadvance of performing the lab, you will find that the lab makes much more sense when you startcollecting 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?


Observers:

Date:

Time:

Sky Condition:

Object Name:

Eye Piece:

TA Signature:

Firs

t Qua

rter:

Vie

wed

thro

ugh

a te

lesc

ope

7

Last

qua

rter m

oon:

Vie

wer

thro

ugh

a te

lesc

ope

8

True North

nighttime observing introduction - dartmouth … observing introduction ... make sure you click the...

Documents