Sunspot plotting and angular velocity

[ExoP]

Hi!

I was wondering how you plot the sunspots on a diagram/template with degrees on it. In other words, I want to plot a sunspot and calculate the position of it in degrees. I have no idea how to do it but I think it includes the assumption that the rotation axis of the Sun is perpendicular to us. By knowing that we need to compute the apparent longitude of the spot on the Sun. I think that this can be done by measuring the position of the spot..and finally compare the photos taken of the Sun for maybe one week and then get the angular velocity by difference in the logitude position. How do I calculate the longitude position? :S

Thanks!

 

[mfb]

I think it includes the assumption that the rotation axis of the Sun is perpendicular to us.

That is a very good approximation, but repeated observations how sunspots move and radial velocity measurements are even better. The axis of rotation is known, you can look it up somewhere. Afterwards, you just have to measure the position of the spot relative to this axis. This is just geometry.

 

[ExoP]

How is the geometry, that is basically the problem I'm having? I need both the longitude and latitude.

 

[mfb]

Take a sphere, make a grid on it, use trigonometry. Sorry, I don't see the issue.

 

[ExoP]

I was thinking of using a protractor, but it may exist a better way to estimate their position

 

[mfb]

You'll need the apparent position (on the "disk" of the sun, as it appears to us) first - I guess a camera (and then a computer to analyze the picture) will give good results.

 

[ExoP]

How can I do it by hand? I was thinking of using a transparent image of a disc with the longitude and latitude degrees on, and then plot the spots on it. Afterwards, I want to determine the location by looking at those spots. That may be possible right? :)

 

[gneill]

Exop, if you do a web search on "sunspot coordinate determination" you'll find some useful links.

One that caught my eye takes you to "Peter Meadow's Astronomy Sunspot Location" page. It might be worthwhile browsing it (this is not a personal endorsement, but the site does cover what I think you're looking for and may give you some leads). I believe that Meadows has been published in Sky & Telescope regarding his sunspot plotting software.