Day Lab Part 2: Determine the Rotational Speed of the Sun

This is the most intensive part of the lab procedure.  In doing this, we are measuring the rotational speed of the sun using the doppler effect.

In a nutshell, the Doppler Effect states that waves emitted from an object moving towards an observer will have a higher frequency and correspondingly lower wavelength.  Inversely, waves emitted from an object moving away from an observer will have a lower frequency and correspondingly higher wavelength.  This is the classic "siren drive by" effect where the pitch lowers after the police car passes you.

Since the sun is rotating about some axis, by measuring the light spectrum from the sun emitted by the different edges of it, different edges will have different doppler shifts based on weather they are moving towards or away from the Earth.

Since we do not know which way the sun is rotating, we took measurements at all of the X's below.  The diagram also features a ridiculously exaggerated doppler effect that assumes blindly that the sun rotates from left to right.

Using a CCD camera and a spectrograph, we were able to take pictures of the visible spectrum of sunlight.  We used two sodium emission lines from he sun as our chosen wavelengths in which we would measure doppler shifts, and used an unchanging water line from the atmosphere as a reference point to measure said shifts.

We then used a computational Excel spreadsheet to analyze the spectral data to find the peak values for the sodium lines of each measurement spectrum.  From there, we found the lines that had the greatest spectral difference: the left and right, which makes the above diagram accurate. We found that the two lines were about 3.9 pixels apart.  After running conversion factors to translate this into actual wavelengths, we determined that there was a net doppler shift of 0.07 angstroms in wavelength between the two measured points.  applying further conversions, we found that these points differ in velocity by 1.75 km/s.

I worked with the Tuesday 1:00PM lab group to perform this experiment.