Over a week ago I had another go at projecting the image of the sun using my Celestron NexStar 102SLT. I used much the same set up as before, projecting the image onto white paper. Here is the image taken at 10:02 UT (using a Samsung Galaxy A21s phone camera):-
North is at the top and East is to the left. I have added the Active Region numbers as they have been officially allocated. I would have preferred to have determined the East-West drift line myself but there wasn't time before cloud obscured the view. Instead I compared my image to that of the Solar Dynamics Observatory taken a few hours later at 13:45 UT (Courtesy of NASA/SDO and the AIA, EVE, and HMI science teams):-
To get how much my image had to be rotated by compared to the SDO image I examined both images in photoshop. By measuring the angle between the vertical and a line drawn between the same two sunspots on both images I was able to determine the difference between them. I came up with differences of 44.1, 44.6, 44.7 and 44.9 degrees, giving an average of 44.6 which is how much my image needed to be rotated by to get North at the top.
My image compares quite well with the SDO image. There was one active region to the far right of the SDO image that didn't show up in my image because it contained sunspots that were so small.
I have been wanting to try and quantify the activity of the sun and one way to do this is to calculate the Relative Sunspot Number or R number. The method for this is described in this BAA guide. It can be seen on both images that sunspots appear in groups (or active regions). If the number of groups seen is G and the total number of sunspots in all groups is S then R is 10*G+S.
A couple of things to be aware of is that a group is counted as separate from another group if its centre is more than 10 degrees away from the other group's center. However, it isn't that easy to quantify how much 10 degrees is on a 2D image of a 3D sphere! We can get an idea of how much it is by drawing on the sun's image a diameter from East to West and then a radius that at an angle of is 10 degrees to this line. The arc along the sun's edge subtended by this is then a guide to how far 10 degrees is on the surface.
Another point is that sunspots have a dark umbra and a lighter penumbra and sometimes several spots are linked by a contiguous penumbra. In counting sunspots it is the darker umbrae that should be counted rather than a "spot" linked by penumbrae.
Given all this I think from my image AR3110 has 4 spots, AR3107 has 6 spots, AR3105 has 5 spots and AR3108 has 2 spots. That is 4 groups and 4+6+5+2=17 spots. So R=10*4+17=57.
I did find this confusing because clearly a better image like the SDO one has much more detail and so many more spots and a few more groups can be counted. However, I have learned that R is a relative number that reflects the observer's estimate. It is therefore important for observers to use the same methods/equipment) each time.
All text and images © Duncan Hale-Sutton 2022 except for the SDO image.
