Addition to the “Lab 11 Sunspots lab: Additional Instructions”
Sunspot Lab introduction
In this lab you will work with a number of images of the Sun taken on successive days which show the motion of a sunspot across the solar disk. This motion is the result of the rotation of the Sun. The sunspot is stationary on the surface of the Sun.
You will determine the longitude of a single sunspot in each image, and then determine the change in longitude from image to image. You will also determine the time between these images. You will divide the change in longitude by the change in time between the successive images, obtaining the rate of change of the sunspot’s longitude. All these rates of change of longitude will then be averaged. Finally, the period of rotation of the Sun will be determined by dividing 360 degrees/rotation by the average rotation rate which has units of degrees/day. The units of the period of rotation are days/rotation.
I have found an easy way to obtain the coordinates of the sunspots. Instead of printing out the images of the solar disks and the grid pattern, an image of a transparency of the grid pattern can be placed over an image of a solar disk and you can easily measure the coordinates of the sunspot. Files containing images of the solar disks and additional instructions have been placed in the drop box under Attached Files. The times and dates of the solar disk observations are included in the filename. 20070101 1709 UTC in the filename refers to 2007, January 1 17:09 hours Universal Time Coordinates or 5:09 p.m. in Greenwich, England. Report on the sunspot #933.
Insert file of solar disk into an Excel spreadsheet. I have tried to do this in Word, but I was unable to move the transparency image. Open the word document containing the transparent grid. The filename is “transparent grid degree-5.” Select the image by using your cursor and clicking on it with your mouse, and then copy it either by using the dual key-stroke, CONTROL-C, or selecting COPY from the EDIT menu. Paste the grid image into the spreadsheet using CONTROL-V or use the PASTE command from the EDIT menu... Move the grid onto the solar disk image. You can fine tune the movement of the grid by using the arrow keys of your computer. You can easily measure the coordinates of the sunspot on the gridlines.
You could insert the image of the “transparent grid +5” into the Excel sheet and then rotate it 180 degrees. You will have to select the image by clicking on it with your mouse. It will be outlined and a green circle will appear above it. Select this green circle by clicking on it with your mouse and keeping the mouse button depressed, move the mouse in an arc, which will rotate the image. Release the mouse button once you have the image rotated. You want the line “Solar Coordinates B = -5” to appear in the lower left corner.
In this configuration, ignore the sign of the grid numbers. Treat them like a regular grid where zero is in the center of the grid, upward is positive values of y, downward is negative values. To the left of the zero point are the negative values of x and to the right are the positive values of x.
Remember, when subtracting a negative value from a positive value, you end up adding the two numbers since the negative of a negative is positive. For example: 5-(-3) = 5+3 = 8.
Lab 11 Sunspots lab: Additional Instructions
The sunspot data is available at the Soho web site: the web home of the Solar and Heliospheric Observatory.
The link: will contain a list of images of the solar disk taken daily. Some will contain sunspots. These images are repeated in the same list further down, but with larger file sizes that are larger images with greater resolution. The smaller files will be sufficient for our use.
Print out the images for Jan 1, 2007 – Jan. 11, 2007. Then direct your Internet browser to the site:
and choose the January-Negative B selection and print it.
You will want to use the grid labelled Solar coordinates B = -5. This will require you to rotate the page 180 degrees around such that “B = -5” can be read on the lower edge of the sheet.
The reason for using this grid is that in January of the year, the Earth is slightly below the plane of the equator of the Sun in its orbit about the Sun. We are looking up at the Sun, such that we have a slight view of its southern pole. Thus its equator will not appear as a straight line drawn horizontally across the disk of the Sun, but will have a slight upward bow, as will the rest of the lines of latitude.
The positions of the sunspots on the solar disk images are to be indicated on the grid so that the longitude and latitude of the sunspot can be recorded.
I found that the solar disk images and the grid were almost the same size. If you find that they have different diameters, you will have to make a copy of your grid and either reduce or enlarge accordingly. (See the instructions given in the lab directions.) I took a pair of scissors and cut out the circular grid. On each solar image, measure the center of the top edge and bottom edge of the frame, and drew a vertical line through the image, extending a little above and below it. Place the grid directly atop the solar image such that the vertical line coincides with the central vertical line of the grid. I held these two pieces of paper together on top of a sheet of plastic (the top of a cd jewel case) that I held up to a lamp. I was easily able to see the sunspot image through the grid sheet, and marked its position on the grid with a pencil. You are to repeat this for each of the solar disk images as you follow sunspot #933 (as it is labelled on the solar images) across the face of the Sun, always using the same grid.
If an opaque projector is available, the grid sheet could be taped to a wall and each printout of the solar disk could be projected onto it. The distance between the sheet and the projector could be adjusted until they overlap each other. Likewise, if your computer can output its screen to a projector (many college classrooms have such projectors, and then the solar disk could be projected onto the grid sheet. Either adjust the distance of the projector to the screen, or adjust the degree of magnification until the image completely overlaps the grid. In this case, you will not need to print out the images of the Sun.
Notice that the grid is divided into sections of 15 degrees width. You will have to estimate the longitude of the sunspot location. If it appears one third of the way between the 15-degree line and the 30-degree line, then the longitude will be 20 degrees.
These images have been taken about 24 hours apart. The time of the image is stated in the listing of the images on the web page: is in the 24 hour notation sometimes referred to as Universal Time (UT), military time or Zulu time. The time appears in the column labeled Last Modified. When printing these images, a time code will appear at the lower left hand corner of the sheet. It indicates the date and time of the printing, not the date and time of the image.
The longitude is measured from zero in the middle of the grid. It has negative values left of zero (the central line), and positive values to the right of zero. You will record the absolute value of the difference of each successive longitude value in the column of your report form that is labelled “Longitude Change (degrees).”
You will divide the longitude change by the time elapsed, which is usually one day, and record the rotation rate in degrees/day. Find the average of these rotational rates. Dividing 360 degrees, which is a full rotation, by the average rate of rotation will yield the rotational period of the Sun. Record these values in the report form.
Thus, if a sunspot has moved from a position of longitude –60 degrees to –50 degrees from one day to the following day, and both images were taken at 17:02 (which is 5:02 p.m.), then the angular velocity of the sunspot is (-60 degrees-(-50 degrees))/1 day = -10 degrees/day. Since we are only interested in the absolute value of the difference in the angle, then the velocity can be stated at 10 degrees/day.
Also estimate the latitude of the sunspot’s location and record it. You need only write this once since its latitude does not change with time.
Compare your result for the Sun’s rotational period with an official published value.
Indicate the difference between the size of your grid and the size of the solar disk. Calculate the percentage difference.
Notes:
When filling in the report form, the “time elapse” is the time from one image to another. Typically, this is one day.
Also:
The longitude change is the absolute value of the difference of two successive longitude readings (the longitude of two different but successive days), thus this change is always reported as a positive value.
The last column is the rotation rate (deg/day). It is the ratio of the longitude change per time elapsed (day). Below this column, average the rotation rates, and then finally, use this average to determine the period of the Sun’s rotation in units of days by dividing 360 degrees by the average rotation rate (degrees/day).
At the web site:
the solar disk images can be accessed. The name of the file contains the date the image was taken. The second last column indicates the time of day the image was taken. If you use a mirror site, then the time of day may be reported in the local time of the web site.
[IMG] sunspots_512_20070101.jpg 01-Jan-2007 17:09 24K
[IMG] sunspots_512_20070102.jpg 02-Jan-2007 17:09 24K
[IMG] sunspots_512_20070103.jpg 03-Jan-2007 17:09 24K
[IMG] sunspots_512_20070104.jpg 04-Jan-2007 17:09 24K
[IMG] sunspots_512_20070105.jpg 05-Jan-2007 17:09 24K
[IMG] sunspots_512_20070106.jpg 06-Jan-2007 17:09 24K
[IMG] sunspots_512_20070107.jpg 07-Jan-2007 17:09 24K
[IMG] sunspots_512_20070108.jpg 08-Jan-2007 17:09 24K
[IMG] sunspots_512_20070109.jpg 09-Jan-2007 17:09 24K
[IMG] sunspots_512_20070110.jpg 10-Jan-2007 17:09 24K
[IMG] sunspots_512_20070111.jpg 11-Jan-2007 17:14 24K