Grade 9-12:
Determine the Rotation Rate of the Sun
You are going to observe and track the movement of sunspots across the Sun's visible disk. These observations will
allow you to calculate the mean solar rotation rate.
You'll need:
INTRODUCTION:
The earliest measurements of solar rotation used sunspots. By measuring the daily position of sunspots in latitude and longitude we can easily determine the rotation rate of the Sun.
PREPARATION:
The instructor will print out each data set to be used: consisting of several (15-16) images of the sun and one sample longitude-latitude grid (henceforth called the "grid"). The grid should be copied onto transparent material, one copy for each student works best but students may share grids.
The instructor shows the class two or more images of the sun from one of the data sets to demonstrate that the sunspots change position from day to day. They all move in the same direction. The motion of the sunspots is due to their being on the surface of a rotating sphere: the Sun. Each student will measure the latitude and longitude of each labeled sunspot individually and then combine results to calculate the rotation rate of the sun.
1. Each student receives a solar image, a grid, and one copy of each worksheet (students may share grids if they are in short supply). The student puts the grid over the solar image, aligning the sheets so the outer circle of the grid overlays the outer circle of the Sun.
2. Determines the latitude and the longitude of each sunspot. The latitudes and longitudes of all labeled sunspots are recorded onto worksheet #1 (see sample worksheet#1). Typically the sunspot will not fall directly on a grid line so the student must estimate the values of latitude and longitude. The date of the image and the observation day number are recorded on the worksheet. Note: Latitude ranges from +90 to -90 degrees. Latitudes that are south of the solar equator are negative whereas latitudes above the equator are positive. Longitude for the visible face of the sun ranges from 0 to 180 degrees).
3. After measuring and recording the positions of the sunspots, students will collaborate and combine all their sunspot longitudes onto their copy of worksheet #2. The observations are listed under the corresponding observation day. This will create a data table containing the East-West longitude positions of all the sunspots.
4. For each sunspot, the rotation is calculated by subtracting the initial longitude from the final longitude and dividing by the number of days between observations. This answer will be in degrees per day and is a measure of the rotation rate of the Sun.
ie. from sample worksheet#2
Spot Group "A"
the difference in longitude is 148-100 = 48 degrees
the number of days between observations = 5 observation days minus 1 = 4 or Day#5 - Day#1 = 4
Rotation rate is 48/4 = 12 degrees per day.
5. After finding the rotation rate for each sunspot, the students will calculate the
average rotation rate for the Sun by averaging rates from all the sunspots. If the class uses more than one data
set, they should compare the results between both data sets.. Are the averages bigger or smaller than the range
of rotation rates obtained for individual sunspots?
6. Make a graph by plotting the position of each sunspot for each observing day and observe the shape of the curve. Are they straight lines (ie. do sunspots rotation at a constant rate? Is this surprising? Do the sunspots rotate at the same rate? Is this surprising? For further information check out "solar differential rotation")
7. Question: From the given results, how many days does it take for the Sun to make one
complete rotation?
