Where Does the Sun Set?
Warning: DO NOT LOOK DIRECTLY AT THE SUN.
Goals of the Lab
To observe and measure how much the position of the Sun at sunset moves over a period of a few months, before and after the equinox.
To see how the motions of the Earth-Sun system over the year may be connected with the observed position of the Sun, and with the seasons.
Requirements: paper, a pencil, a watch or clock, and an observing location with a view of the Western horizon.
Part I: Observations
Pick one location from which you have a good view of the western horizon. Note exactly where you stand to make your observations. You should put your feet in exactly the same spot each time you make your observations.
Note 1: if you are a morning person, you can pick a spot where you can observe the eastern horizon, and do this lab with the sunrise instead. Be sure to clearly note this.
Note 2: You may choose to do this lab from outside of the observing facility (i.e., the top of the parking garage), from which 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. Note that for most of the semester, sunset occurs before 7pm!
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 or to an exact copy thereof. To make a suitable sketch, you must keep a constant horizontal scale. Measure the angles between landmarks just as you would measure angles between stars on the sky (see How to Measure Angular Distances). 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. (Finding your landmarks on the Vanderbilt Campus Map may help you figure out which point on the horizon is due West.)
Draw the disk of the sun on the horizon at the position where it is as it sets. Label this position with the calendar date, and the local time of the sunset.
Repeat your observation once a week from exactly the same position. Mark the sunset location each time on the same sketch.
Part II: Analysis
Although you won't be able to complete this section before you have all of your observations, you can certainly get started thinking about it and working on it after you have four or five observations.
- Make a plot of the position or 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 0° for North and increasing toward the East (East=90°, South=180°, West=270°.)
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° north of west)? If not, what is the range of directions and what is the pattern of change?
c) On what day(s) did the Sun set directly west, not north of or south of west?
d) If you see a change in the azimuth of the setting Sun with time, is the change constant, or is the change faster at certain times than others? When is it faster?
e) Do you notice a correlation between the azimuth of the sunset and the time of the sunset? (Note: in the Fall semester, after the change to standard time, the Sun is effectively setting "one hour too early" after the time change. In the Spring semester, after the change to daylight savings time, the Sun is effectively setting "one hour too late" after the time change. You should take this into account when answering this question.)
f) What do you expect to happen to this sunset angle during the periods beginning September 21 and ending December 21 (Fall), beginning December 21 and ending March 21 (Winter), beginning March 21and ending June 21 (Spring), and beginning June 21 and ending September 21 (Summer)? How does the sunset angle correlate with the seasons?
g) How do you think the azimuth of the sunset will correlate with how high the Sun is in the sky at noon, if at all?
h) Suggest the reason(s) as to why the direction of sunset does or does not change? Can this reason also explain the change in the times of the sunset?