astronomy
Physics 1052 (General Astronomy)
Experiment #1 — North Circumpolar Motion of Stars
The purpose of this experiment is to develop a bette r understanding of the daily motion of stars near the
North Celestial Pole by observing the motion of a particular set of stars (the seven stars of the Big
Dipper, part of the larger constellation Ursa Major) over the course of few hours. Polaris, also known as
the North Star or Pole Star, is very cl ose to the North Celestial Pole (within 1
o
), so it does not appear to
move in the sky as the other stars do. It will serve as your reference poi nt, much as it did for sailors and
backpackers (before Global Positi oning Systems were available).
In this experiment, you will make three observations of northern sky, one every 90 minutes, so pick a
night when you have plenty of time. You don’t need to stare at the stars be tween observations, but you
will probably want to get started as soon as possible after sunset. For each observation, you will sketch
the position of the Big Dipper relative to Polaris, the zenith, and the horizon. The sketches must be
sufficiently accurate so that you can distinguish the difference between linear and circular motion over
such a short observation time. If you have the time, wait two hours betw een the observations for greater
effect. You must record time of each observa tion within 5 to 10 minutes of accuracy.
Begin by finding a location with clear view of the northern horizon. If city lights obscure your view of
the Dipper, you will need to find another suitable location. Face north and use your star—finder to
locate Big Dipper. The two stars in the cup of the Big Dipper point towards the Polaris. Polaris is located
in the tail of Ursa Minor (the little Dipper); it is the last star in the handle . Some stars in the Ursa Minors
are hardly visible with naked eye. Don’t worry about them. We only need Polaris.
Use Polaris as the fixed star on your sketch. Please see the figures on the next page before you make any
sketch. Orientation of your figure should look some thing like the top figure, though larger than this.
Note that the Zenith, not Polaris, is the point in the sky directly overhea d. Polaris should be directly in
front of you — a line drawn from the Polaris to the point of the horizon you are facing should be
perpendicular to the horizon. If your view is more like the bottom dr awing, you are looking too far to the
east; rotate left until your view is like the top drawi ng. Sketch the position of the seven stars of the Big
Dipper relative to Polaris and record the time. Wait 90 or 120 minutes and observe how the locations of
these stars have changed relative to the Polaris and to the horizon. Record the new location on the same
sketch , as well as the time. Repeat this procedure one more time for the same time interval as above.
Make sure that the positions indicated on the sketch are clearly identified with the times you record.
Your report should include one sketch with the location of the stars of the Big Dipper shown at three
different times. It should also include your interpretation of the observed motion of these stars. Are they
moving in a straight line or circling a particular point? Or, do they follow some other motion? Also
describe how this motion relates to the motion of th e earth; are the stars actu ally moving, or does their
apparent motion result from the daily or seasonal motion of the Earth?
Zenith
Polaris
We s t No r t h Ea s t
Horizon
Correct orientation!!
Zenith
Polaris
North East
Horizon
Incorrect orientation!!
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