Physics
127
Descriptive Astronomy
SUGGESTED OBSERVING PROJECTS
During the present semester each student will be required to complete and submit a report on one observing project. It is expected that most such projects will involve observations and interpretation of naked-eye phenomena. A list of possible projects is provided as part of this outline. Students who desire to do projects of their own design rather than the suggested projects are encouraged to do so. Prior approval of the instructor should be obtained for any individually designed project. Use of simple observing instruments such as binoculars, personal telescopes, and/or cameras is permitted, but certainly not required, either for the suggested projects or for individually designed projects.
Generally the projects are simple enough that they easily can and should be completed individually. However for certain projects, where it is clearly advantageous, collaboration will be allowed in the collection of data. An example of such a project would be the observation of meteors, since different observers could watch different parts of the sky at the same time. All project reports should be prepared individually! If you do collaborate with others in data collection, all collaborators should be named in your report.
For many of the projects (including some of the best) it is necessary to begin working now (early in the semester) since observations are required over an extended period of time. Procrastination will significantly limit the number of projects from which you may choose. Successful completion of any project will require a certain amount of clear weather. The combined effects of procrastination and bad weather could be a project score of zero!
The Report
Reports should be brief but should include all essential data, results, conclusions, and should show or explain how the results and conclusions were inferred from the data. Depending on the nature of the project, information in the report might include times, dates and places of observations; sky conditions (including the state and effects of weather, moonlight, and light pollution), data tables, charts, sketches, graphs, descriptions of what was observed, calculations, and statements and explanations of results and conclusions. It is appropriate to report what is personally learned from the project, including personal impressions. Extensive background material and restatements of project instructions are neither expected nor desired.
It is expected that besides containing the above information the report will conform to the standards of good writing. It will be graded in accordance with those standards. Thus errors in grammar, punctuation, and spelling, as well as lack of clarity and awkward structure, will be penalized. The report may be either handwritten or mechanically produced, but, if handwritten, must be neat enough to be read without difficulty in order to avoid a penalty.
Honesty is valued above impressive data in grading reports. If there is evidence that data have been copied or fabricated (and it is usually more difficult to fabricate convincing data than to obtain it honestly) an appropriately low score, probably zero, will be assessed. If your data are marginal or inadequate because of bad weather or otherwise hostile observing circumstances, state so in your report and then do the best you can with the data you have. Honest scientists must often do likewise.
Suggested Observing Projects
(*Additional instructions or materials are available for projects with asterisks. If a hyperlink is indicated, (double asterisk or **), click on it; you may obtain such a copy from the web. Otherwise, or if you are unable to print a legible copy) you may obtain a hard copy from your instructor.)
**1. Make an
accurate sketch of the western horizon as seen from some
convenient location from which you can observe the setting sun on
successive
occasions. (Any open location to which you have easy access at the appropriate
time of day is suitable.) Mark the due west point on
your sketch. Watch the sun
set at least once or twice each week of the semester and mark both the location and the
time of each sunset on
your sketch. Over the period
of the semester both the north-south motion of the sun and the variation in the
time of sunset will be readily
apparent. You should also be able to note changes
in both the rate of north-south motion of the sun and the rate of change in the
time of sunset.
In
order to obtain good results you must have a fairly accurate sketch of the
western horizon as it appears from your observation point, you
must confine your
observations to the same point (a displacement of even a few hundred feet will
noticeably change the silhouette of the apparent
horizon and introduce
significant errors into your results), and you must make regular observations
throughout the term. Your instructor has
available a sketch of the western
horizon as it appears from the hilltop above the stairs to the Richards PE
Building. You may have a copy upon
request or you can click the
link above and print your own. Do not use this sketch if you
plan to observe from your apartment or any
location other than the one for which
it has been prepared! Another nice option is to take your observations
photographically. This allows
you to observe from any
location (but remember to make all of your observations throughout the
semester from that same location).
Project #1 is best when data are collected over the entire semester.
**2. Count the stars visible to the naked eye over the entire sphere of the
sky. This is not done by actually counting all the stars, but by limiting the
number counted by looking through a small tube which restricts the field of
view. (The tube from a roll of toilet paper serves this purpose very
well.)
After counting in this manner the stars visible in a number of small, but
representative, areas of the sky, the number visible over the entire
sky can be
accurately estimated using methods described in a handout available from the
instructor.
This project is more fun and more informative if it is done in the city first
and then repeated in the country or mountains, away from artificial
lights.
Other interesting variations include making similar counts using binoculars or a
small telescope and/or investigating the effects of moonlight
on the number of
stars visible.
3. Observe the moon each clear day for a period of one month. Record the time
(standard time) of each observation and plot each observed
position, relative to
the star background, on a copy of Constellation Chart SC 1 (available at the
bookstore). The resulting plot will show the
position of the moon's orbit
relative to the star background and to the ecliptic (the earth's orbital plane).
Warning: Some of the observations
must be made in the early morning or
daylight hours!
4. Make a sketch of the phase of the moon each clear day for at least one
month. Arrange the sketches in chronological order (on a calendar is
okay) to
show the changing of the phase with time. Include with each sketch the time of
observation and the direction of the moon at that time.
(Two items of data are
required to specify the lunar position: the azimuth, e.g., E, SSE, or
WNW, and the altitude, an angle between 0°
and
90°.)
Warning: Some of the observations must be made in the early morning or daylight
hours!
5. Pick a prominent star that either rises or sets at a convenient time in
the evening. Watch this star for a period of at least ten days and record the
time at which it either rises, sets, or reaches a certain position in the sky
(if you do the third variation you will need to establish a fixed line
consisting of two points, e.g., a point on a tree limb and a piece of
masking tape on a window, or perhaps a utility pole and a bush top, which
can be
used to define the fixed sky position accurately). From your observations
determine how much earlier the star rises, sets, or reaches the
fixed position
each night. If you have an accurate watch and extend your observations over
several nights you should be able to determine this
result to within an accuracy
of a few seconds.
6. Time the rising or setting of the moon on several successive days, and
from these data determine how much later the moon rises, or sets, each
day as
compared to the previous day. Observe over a sufficiently long time interval
that you can determine whether the daily variation in the time
of moonrise, or
moonset, is itself a constant or a variable quantity. (Because the local
apparent western horizon is much smoother than the
apparent eastern horizon and
because the moon will move appreciably in a north-south direction on most days
your results will more closely
conform to the variations in the actual times of
these phenomena if you observe moonsets rather than moonrises. )
Warning:
These day to day
time variations are large enough that over a time baseline long
enough to yield good results there will be a total variation in the time
of the
phenomenon you observe of many hours. Be prepared to make observations at any
time of day or night!
7. Measure the difference in latitude between two locations. This can be
done with fair accuracy using only a protractor and a plumb bob, or a
carpenter's level. Place a piece of cardboard against a post or other support
and, sighting along one edge, align it with Polaris. Then, with the
card
remaining fixed in this alignment, use the plumb bob or level to mark a vertical
or horizontal line on the card. The angle between the
horizontal line and the
line to Polaris is equal (approximately) to the latitude of the location. The
angle between the vertical line and the line to
Polaris is equal to 90°
minus the latitude.
Make such a determination of the latitudes of two locations separated by at
least 100 miles, preferably more, in north-south distance (east-
west separation
is of no consequence). If you are careful, the difference in latitude between
the two locations should be readily apparent.
A dimmed (especially a reddened) flashlight beam is helpful in this
experiment to illuminate the sighting edge while aligning it with Polaris,
and
also to see to mark the plumb line. An adequate plumb bob can be made by
suspending any small heavy object by means of a thread.
Since Polaris is actually over 44' away from the north celestial pole, the
latitude measured by this technique may be in error by nearly a
degree. However
if the two latitude measurements are made at the same time of night, within a
few days of each other, both will be subject to
about the same error and
therefore the error in the latitude difference will be much smaller than the
errors in the individual latitudes.
**7a.A
variation on project 7 is to measure the latitude of some (any) location
If you do this you will need to correct for Polaris' being 44'.15 from
the north celestial pole. Methods for
doing so are outlined on another webpage.
8. Sketch from your own observations the appearance of the east,
south, west, and north parts of the sky on a clear evening. Outline and label
the
constellations that you can see on your sketch. Do this at the beginning of
the semester and then repeat the procedure at the same hour on an
evening near
the end of the semester. Or, repeat the sketch during the hour just before dawn
any time during the semester. A variation of this
project is to pick two or more prominent constellations
and to sketch their positions (with respect to compass directions and altitude
above the
horizon) at two hour intervals throughout one night. The
constellations around the north celestial pole (Ursa Minor, Ursa Major and
Cassiopeia)
serve quite will, but more southerly constellations can also be
used.
**9. Learn to tell time by the stars. It is possible with a little practice to
tell time to within ten or fifteen minutes from the positions of the
constellations
Cassiopeia and Ursa Major or from other constellations. Detailed
instructions are available from the instructor. A proper report of this project
should include some examples of the use of the method based on your own
observations of the sky. Compare your estimated times with the
times on your
watch just to see how well you can make the method work.
**10.Learn to tell time by the moon. The phase of the moon, together
with its position in the sky, gives an indication of the direction of the sun.
The
position of the sun is the basis of our ordinary time. Therefore knowing the
sun's rough position enables one to roughly calculate the clock time.
Further
instructions on this method are available from the instructor. As with project
#9 a proper report of this project should include examples
of the use of the
method based on your observations of the moon.
**11.Make a working model of a sundial. Cardboard or heavy poster board is
suitable construction materials. Detailed instructions are available from
the
instructor. Your report should include examples of your use of your
sundial to see how accurate it is. Your sundial should be submitted
along
with your report.
*12. Determine the faintest star (i.e., the limiting magnitude) that
you can see with your unaided eye. A star chart for this purpose will be
furnished to
those who wish to try. It will be
necessary to get well away
from city lights (many miles) and to avoid moonlight to do this project
properly.
However an interesting variation of this project is to perform the
experiment in the adverse city environment and to compare the results with the
results obtained under optimum dark sky conditions.
13. Go out on any clear night and count all the meteors (i.e., falling
stars) that you observe during a one-hour time interval. It is easiest to see
meteors
in a very dark sky so, to obtain good results, it is essential that you
get into open country away from urban light pollution and that you choose a
night with little or no moonlight. Besides counting meteors note in your
observational record the direction of travel of each meteor observed, i.e.,
E, SW, NNE, etc. If three or four people cooperate in observing so that all
sectors of the sky can be watched simultaneously, the results will be
statistically more significant. One nice way to record both the position and
direction of travel of each meteor is to record each meteor track on a
star map.
Look for and note such meteor characteristics as length of track, relative
brightness, color (noticeable only in very bright meteors),
relative speed,
persistent luminescent trails, and divergence of tracks from a common point
(evidence of a meteor shower).
An interesting variation of this project is to observe for an hour both a few
hours before midnight and again a few hours after midnight and
then to compare
numbers for the two intervals. Even more enlightening are the results obtained
if several one-hour intervals are counted
throughout a single night and then the
number of meteors per hour is plotted versus time in a graph.
A bonus from this project is the opportunity to become more familiar with
constellations and stars while watching for meteors.
**14.Do project #13 on a night when a meteor shower is in progress, giving
special attention to distinguishing between those meteors which belong to
the
shower and those which are sporadic. A list of some of the more prominent and
reliable meteor showers giving dates of occurrence as well
as other pertinent
data is available from the instructor for those who wish to do this project.
15. Observe the visible planets at least weekly throughout the semester and
plot their positions with respect to the background stars on a
constellation chart. (You can
print such a chart from the Starry Knight software included with your
textbook.) What can you infer about
the
motions of the planets, i.e., directions, relative speeds, general
sky positions, etc., from your data?
16. Most people have noticed that a rising or setting full moon, especially
if the apparent horizon is low, looks larger than the moon of the same
evening
when it is seen high above the horizon. Use the crude experimental means
described in Project #17 to compare the moon's angular size
when it is very low
in the sky with its angular size when it is high in the sky. Thus determine
whether the described effect is atmospheric (which
would be the case if the
angular size does change) or psychological (which would be the case
if there is no change in angular size). How solid is
your conclusion?
17. Compare the moon's apparent angular size at apogee (most-distant orbital
point) with that at perigee (nearest orbital point). This can be done
when the
moon is near one of these points by holding a compass or other measuring device
at arms’ length and adjusting it so that it subtends the
same angle as the
moon's diameter (measured pole-to-pole, so it is phase independent). The result
is then recorded by using the compass to
draw a circle.1 Make such an
observation at either apogee or perigee and record your result then repeat the
exercise when the moon is at the
opposite orbital point. Be sure that your
measured diameter represents the full distance
from cusp to cusp. The ratio of your two
circles will
have the same value as the ratio of the moon's angular diameters at
these two points. Use your data and the direct proportion,
Perigee angular
diameter = Apogee distance ,
Apogee angular diameter
Perigee distance
to estimate the ratio of the moon's apogee distance to its perigee distance.
The dates and approximate times when the moon is at apogee and perigee during the present semester are as follows:
| Orbital Extreme | 2008 Date | Time2 | Phase |
Optimum Time to Observe/Comments |
| Apogee | Jan 3 | 1:07 am | Waning Crescent | Early morning of Jan 1-3 |
| Perigee | Jan 19 | 1:40 am | Waxing Gibbous | Afternoon or early evening of Jan 17-20 |
| Apogee | Jan 30 | 9:27 pm | Waning Crescent | Early morning of Jan 29-31 |
| Perigee | Feb 13 | 6:09 pm | First Quarter | Afternoon or early evening of Feb12-14 |
| Apogee | Feb 27 | 6:28 pm | Waning Gibbous | Early morning of Feb 27-29 |
| Perigee | Mar 10 | 3:40 pm | Waxing Crescent | Early evening of Mar 10-12 |
| Apogee | Mar 26 | 2:14 am | Waning Gibbous | Morning of Mar 25-28 |
| Perigee | Apr 7 | 1:30 pm | Waxing Crescent | Early evening of Apr 8-9 |
| Apogee | Apr 23 | 3:35 am | Waning Gibbous | Morning of Apr 22-24 |
1Alternative simple
methods may also be used to measure the relative lunar angular size. One can use
a pinhole device (a shoe box with a white interior and a pinhole
in one side or
end should work well) to form comparative images of the moon (be sure that the
"screen" is perpendicular to and equidistant from the pinhole in both
cases).
The lunar image can then be traced to record its size. One can also compare
images of the moon on photographs made at the times of apogee and perigee as
long as both photographs were made with the same camera using the same lens.
2Given times are local clock times, i.e., Mountain Standard Times (MST) for January-March dates and Mountain Daylight Times (MDT) for April dates.
For best results observe the moon as close to these times as possible. If an
impossible phase or bad weather makes you miss the optimum
times by a day or
two, you should still obtain decent results. The variation in the moon's angular diameter is actually fairly great though
generally
unnoticed. Perhaps it is most dramatically illustrated by
superimposing two circles or juxtaposing
two semicircles representing the
relative apparent size of the moon at these two
orbital points as in the figure below. Represent your results with such a
figure
drawn accurately to
scale your measurements.
Actual images of the full moon photographed near perigee and near
apogee can be seen in comparison at
|
|
