Method
You need to do this series of observations every week or so
throughout the semester; a minimum of three times per month would
be good.
You will need a compass of the kind used by backpackers and orienteers,
and your astrolabe. These compasses have a built-in magnetic compass
and good scale of angles, and a serviceable one can be obtained
at most outdoors shops, such as Whole Earth Provisions, REI, and
Wilderness Whitewater, all of which are convenient to the University
area. A fancy one is not needed; a serviceable can be found for
about $10. You will need to read the instruction leaflet that
comes with your compass carefully. Practice with it until you
can use it proficiently.
Make a sketch of the Western horizon, noting significant landmarks
such as trees, buildings, mountains, or TV transmission antennas.
You can use this sketch each time you make an observation. Each
day that you make an observation, as sunset takes place, note
carefully the exact point on the horizon where the Sun disappears.
Mark it on your sketch. With your compass, take the bearing (from
North) of that point and record it. You should be able to estimate
the Sun's bearing to the nearest degree. Also, record the time
and date that you made the observation, both on the accompanying
table and on your sketch.
Correct all your compass readings for Austin's magnetic deviation,
which is about 7 degrees East; This means that the compass points
about 7 degrees East of where it should point. Thus, you have
to add 7 degrees to the compass bearing (measured from
North towards East) to get the true bearing.
Interpreting your results
After you have made all your observations, for each observation
determine the number of days since you started making observations,
and record that in the table. Take a piece of graph paper and
on it plot the bearings from your table, corrected for Austin's
magnetic deviation, against the number of days since you started
observing. Then answer the following questions.
1. In what direction did the Sunset point move during the course
of the observations?
2. About what date did the Sun set due West? Did this agree with
what your thought should have happened? If not, how do you explain
the discrepancy?
3. About what date did the Sunset point move most rapidly? At
this date, about how many degrees per day did the Sunset point
move?
Now use your astrolabe to predict (calculate), for the date of
each observation, the azimuth (that is, bearing, measured towards
from North towards the East) that the Sun should have set. Be
sure to correct each observation for the Equaton of Time, the
longitude of Austin, and the Daylight Savings Time correction
(if needed). Show your calculations on the table. Plot the predicted
azimuths on your graph. If everything were perfect, the predictions
would agree exactly with the measurements; but that would be highly
unlikely to happen. There will certainly be an error for each
observation. This error is what we call the Observed - Callculated
(O-C) and it is gotten by subtracting the azimuth you got from
the astrolabe from the azimuth you observed. Calculate the O-C
for each observation and record it in the table. Now answer the
following questons:
4. What was the average error of each observation, compared to
what you calculated?
5. Analyze the errors. What are the possible causes of the errors?
Which ones are most important? About how big do you think each
source of error is? Considering the two or three largest sources
of error, are you able to account adequately for the discrepancies
between what you observed and what you calculated?
6. Do you notice any systematic effects in your O-C's; that is,
are they consistently high (positive) or consistently low (negative)?
If so, can you think of any plausible reasons for this?
For full credit, be sure to turn in all graphs, calculations,
and other information that you used in doing this assignment.
Table for recording results
| Date | Days since start | Compass Bearing | Corrected Bearing (O) | Bearing (C) | Error (O-C) |
| ___/___/___ | ___________ | __________ | ___________ | ___________ | _________ |
| ___/___/___ | ___________ | __________ | ___________ | ___________ | _________ |
| ___/___/___ | ___________ | __________ | ___________ | ___________ | _________ |
| ___/___/___ | ___________ | __________ | ___________ | ___________ | _________ |
| ___/___/___ | ___________ | __________ | ___________ | ___________ | _________ |
| ___/___/___ | ___________ | __________ | ___________ | ___________ | _________ |
| ___/___/___ | ___________ | __________ | ___________ | ___________ | _________ |
| ___/___/___ | ___________ | __________ | ___________ | ___________ | _________ |
| ___/___/___ | ___________ | __________ | ___________ | ___________ | _________ |
| ___/___/___ | ___________ | __________ | ___________ | ___________ | _________ |
| ___/___/___ | ___________ | __________ | ___________ | ___________ | _________ |
| ___/___/___ | ___________ | __________ | ___________ | ___________ | _________ |
| ___/___/___ | ___________ | __________ | ___________ | ___________ | _________ |
| ___/___/___ | ___________ | __________ | ___________ | ___________ | _________ |
| ___/___/___ | ___________ | __________ | ___________ | ___________ | _________ |