Rotation of the Sun
A sequence of 12 solar photographs enables the student to derive a fairly accurate rotational period of the Sun by measuring the positions of sunspots, and to gain some knowledge about the stability of sunspot groups. The exercise explains how to convert apparent sunspot motion to a circular arc on the surface of the Sun. Once the synodic rotation period (that seen from the moving Earth) is determined, instructions are given for converting it to the sidereal period, the rate with respect to the "fixed" stars. Solar rotation can also be measured from the Doppler shifts of spectral lines, and this exercise includes high-dispersion spectra of the Sun's east and west equatorial limbs for this purpose. The rotational velocity is then calculated from the Doppler-shift relations and known line wavelengths. Also discussed are differential rotation and the rotation of other starts.
LE014