The Rotation of the Sun

W hen Galileo turned his tele­ scope on the sun in 1610, he discovered that there were dark spots on the solar disk. Observing the motion of the spots across the disk, he deduced that the sun rotated once in about 27 days. One of his contempo­ raries, Christoph Scheiner, undertook a systematic program of observing the spots. From these observations Scheiner found that the rotation period of the spots at higher solar latitudes was slight­ ly longer than that of the spots at lower latitudes. In the middle of the 19th cen­ tury R. C. Carrington, a wealthy English brewer and amateur astronomer, deter­ mined with good precision the amount of the variation of the sun's rotation rate with latitude. Scheiner's observations suggested that the sun does not rotate as a simple solid body. Since we now know that the sun is a gas throughout, it does not surprise us that it does not rotate as a solid. One might have expected, however, that the spots closer to the sun's equator would move slower than the spots at the higher latitudes, just as the outer planets of the solar system move slower in their orbits than the inner ones. The fact that the equatorial regions rotate faster than the rest of the sun implies that an excess of angular momentum-the momentum of the sun's rotation-must be transferred from the higher solar latitudes to the equatorial regions. This fact and its im­ plications yield information about the structure of the sun's interior and the nature of its activity. There are two basic methods for pre­ cisely determining the rotation of the sun at a partkular latitude. The first is to time the passage of some tracer (such as a sunspot) across the solar disk. The sec­ ond is to photograph the spectrum of the sun; the spectral lines of the solar at­ mosphere will be Doppler-shifted be-