Where Does the Sun Set?
Summary:Observe the time and direction of sunset over a period of several weeks, near the equinox.
Warning: DO NOT LOOK DIRECTLY AT THE SUN.
Needed Supplies: Logbook, pencil, a place to view the western horizon at the time of sunset.
Start Date: Start now! This lab requires an obsevation once a week throughout the semester, so you should start the first week of the term.
Due Date: If you do this lab, final observations and answers to all the questions below are due on December 4, the final due date for your observing log. However, observations are required once a week throughout the semester. This means that you must have completed at least four observations by October 3, and at least nine observations by October 31. For these observations due on these first two dates, you should have performed procedure steps 1-3.
General Description
Choose one observing location to use for this lab for this semester. It is best to choose a location where you can see close to the horizon (e.g., a dorm or campus classroom or office building with a window facing west, ideally a top floor; the 9th floor of the Physics building works fairly well). At each observing session, stand in exactly the same spot. Determine the cardinal directions [north, south, east, west]. Using buildings, trees and other landmarks, determine in which direction (i.e., where on your horizon) the sun sets. Repeat this observation at least once per week, beginning the first week of the term and concluding by the end of the term.
Note: Alternately you can do this lab at sunrise if you are a morning person.
Procedure
Pick one location where you have a good view of the western horizon. The TAs will provide a list of suggested locations. Make a written note in your log of exactly where you stand to make your observations. You should put your feet in exactly the same spot each time you make your observations.
Draw a careful sketch of the horizon or skyline (trees, buildings, landmarks, topography) sufficient for using this sketch to mark the position at which the sun sets. Take a full page (sideways) of your observing log for your sketch. You will keep adding new sunset observations to that same sketch. To make a suitable sketch, you must keep a constant horizontal scale. This you can do by measuring the separation between landmarks on the horizon using the fist and finger rules for measuring angles or a ruler at arm's length or one of the crossbows. Mark these angles or ruler positions at the base of your sketch. Take time to do this one sketch precisely as your ability to complete this lab depends on it. Indicate the direction of WEST as best as you can on your sketch.
Note: You may choose to do this lab from outside of the observing facility, where there is an excellent view of the western horizon. In this case, you can make your sketch of the horizon using the crossbow to measure precisely the location of landmarks and make your sketch of the horizon. You would need to do this at the beginning of a lab (when you have access to the crossbows). You can then return at the same spot at the time of sunset on any day. Sunset occurs before 7pm!
Mark the sunset position on your sketh of the horizon. Label this position with the calendar date.
Repeat your observation once a week from exactly the same position. Mark the sunset location each time on the same sketch.
Once you have collected all the observations: Make a plot of the position or azimuth angle where the sun set (in degrees) vs. the date of the year. This angle is known as the azimuth. It is measured along the horizon, starting with 0o for North and increasing toward the East (East=90o, South=180o, West=270o).
Based on your sketch of sunsets and your plot of the azimuth of the setting sun, answer the following questions:
a) Does the sun always set in the same direction?
b) If so, what is the direction (e.g., 10 degrees north of west)? If not, what is the range of directions and what is the pattern of change?
c) On what day did the Sun set directly west rather than north of or south of west?
d) If you see a change, is it constant with time or is there an "accelaration" or "deceleration" of the azimuth of the setting Sun?
e) Given your answer to "b", what do you expect to happen to this sunset angle during the periods beginning September 21 and ending December 21, beginning December 21 and ending March 21, beginning March 21and ending June 21, and beginning June 21 and ending September 21?
f) Suggest the reason as to why the direction of sunset does or does not change?