Name: Date: Observing the Sun Physics 107 Lab In this activity, you will use a solar telescope called a Sunspotter to observe the motion of the Sun. From watching its progress across the screen, you will be able to measure its diameter. If we are fortunate enough to have a sunspot or two today, you will also be able to sketch the spots and estimate the sizes of sunspots. Part I - Measuring the Diameter of the Sun: The first step in observing the Sun is aligning your Sunspotter properly. On the front of your Sunspotter, above the lens, a small wooden stick protrudes from the device. Turn the device so that this stick faces the window. Then aim this stick directly at the Sun by adjusting the position of the device until the stick casts no shadow. Question: If the stick casts no shadow, what is the angle between the Sun s light and the stick? Within the Sunspotter, there are two target-like circles on either side of the mirror. Aim the pinpoints of light so that they strike these targets as close to the center as possible. (Note: After roughly aiming with the stick, you should already be quite close.) Once you do this, you should see a projection of the Sun appear on the white screen. Now that you can see the Sun, place a piece of blank white paper over the screen, and tape it carefully into place. Draw a small dot at the center of the projection of the Sun, then wait a minute or so. Be careful not to move the Sunspotter. Then take another look. The projection should have moved. Draw another small dot at the center of the projection (you may wish to label this number 2). Now you know roughly which way the Sun is moving on the screen. Adjust the Sunspotter so that the Sun is still visible on the screen, but toward one side so that it has as much room as possible to move. Once you have chosen your starting point, trace the outline of the Sun s projection. Label this Position 1. Use an app or stopwatch to note the time at which you made this drawing and also include it with your label. (You should be able to measure within seconds). Now wait. The projection will slowly move, hopefully in the direction you expect. As soon as the entire projection is just outside your outline, check your watch again and note the time. You should also trace the new outline on your new sheet of paper and label it Position 2. Include the time you recorded in your label. How much time passed between the two drawings?
2 The number you just wrote down is the amount of time it took for the Sun to move by its angular diameter the size it appears to be in our sky Question: What is the angular diameter of the Sun in degrees? Show your reasoning and calculation below. (Hint: There are 360 in a circle, and you needed to record the time between the two drawings ). One Astronomical Unit is 92,955,888 miles. We can use trigonometry to find the actual diameter of the Sun based on your calculation of its angular size in degrees. (measured in degrees) D r 180 D r Above and to the left is the small angle approximation equation. Whenever an angle in a triangle is small enough (meaning that the triangle is very thin), this equation can be used. Above and to the right is a schematic of our triangle. D represents the real diameter of the Sun. 1 Astronomical Unit is the distance to the Sun, or r in the diagram. The angle,, is the angular size of the Sun.
3 Question: What is the diameter of the Sun in miles? Show your calculation below. The Sun s diameter is about 865,000 miles. Question: How close does your answer come? Calculate the percent error using the following equation (you may round your calculation from above to 3 digits): % error = experimental value accepted value X 100% accepted value Question: Do you think this percent error is good or bad, based on the time spent and the materials you have to work with? Why? List at least two sources of error in your measurement. (Rounding and mathematical errors are not sources of error ).
4 Question: The diameter of the Earth is about 8,000 miles. How many Earths could fit across the middle of the Sun? Part II Observing Sunspots and The Sunspot Cycle: Place a new blank sheet of paper on the screen, or use the back of the previous sheet. Aim your Sunspotter once again at the Sun. Do you see any darker patches, or any variation in the brightness of the projection? If you see anything, trace it. Also trace the outline of the Sun. Color in any darker patches to match how much darker they appear. Compare the width of the largest sunspot you see to the diameter of the Sun. Approximate the width in miles: Question: The diameter of the Earth is about 8,000 miles. How wide is this sunspot compared to the Earth? It turns out that sunspots are more than what they seem to the unaided eye. They mark places of high magnetic activity on the surface of the Sun they are the points of contact of extremely strong magnetic field lines. The magnetic forces there make convection less efficient, and therefore make the surface of the Sun look less bright to us at that spot. But there s much more to the story than that: the average brightness of the Sun actually increases as the number of sunspots increases (it turns out the outline of the spot gets much brighter than average). For that reason, it is hypothesized that fewer sunspots means a cooler Sun, and therefore also a cooler Earth. On the next page is a graph showing the number of observed sunspots each month from 1750 until the present (from http://solarscience.msfc.nasa.gov/sunspotcycle.shtml). Question: How many sunspots do you currently see on the Sun? Based on the graph on the next page, would you say that the number of sunspots you saw is normal for this time? Why or why not?
Question: There is a cyclic pattern to the number of sunspots observed on the Sun. What would you say the period of this cycle is, and why? How long do you think you would have to wait to see a significant difference in the number of sunspots? 5