Evaluation of solar rotation by spectroscopy


The principle of this experiment is based on the Doppler-Fizeau effect. The Sun being in rotation, the gas at the Eastern limb moves towards us while the gas at the Western limb moves away from us. By taking a spectrum with the Western limb and another with the Eastern limb, one must be able to observe a shifting of wavelengths if the dispersion of the spectroscope is sufficient.

The spectra are obtained by successively positioning the slit of the spectro at the limbs East and West


In rounded values, the sun rotates in 25 days at the equator, and a point of its equator courses 4.4 million km during this time. Speed to be measured is thus about 2 km/s. This speed must induce a Doppler shifting about 589*2/300000 = 0.0039 nm for the yellow line of Sodium. That thus represents a variation of 0.0078 nm between the spectra East (shifted towards blue) and West (shifted towards red).

The test that I carried out give values completely comparable and thus confirms the correct operation of my spectro.

The measured value of the shifting between the lines obtained on the edges East and West of the Sun is indeed of approximately 0,0075 nm. Notice the lines due to the Earth's atmosphere (indicated by " T") which are not affected by this shifting.


Here is another attempt at the same wavelengths, with a better focusing