H. H. Turner (1913; cf. Sampson, 1914) has worked out an hypothesis which is stimulating, even if not yet acceptable. He supposes that the Leonid swarm of meteors, revolving once in about 33 years in a very eccentric orbit, is at the basis of the sunspot recurrence. These meteors were observed in countless swarms, filling the sky for a few nights in November 1799, and again in 1833 and 1866. In 1899 they were expected, but failed to appear in large numbers, having probably been swerved to one side through the attraction of some planet. Turner finds that they have passed near Saturn several times in the last 2,000 years. At some of these encounters a quantity of meteors may have been detached and losing their own velocity may have fallen nearly straight toward the sun, grazing its outer surface in their circuit at a velocity of 400 miles per second, then swinging out to aphelion near their place of encounter, and completing their revolution in about 11 years. Successive returns of the main Leonid swarm, approaches of Saturn, and perhaps even the influence of other planets would be sufficient to perturb this meteoric swarm and cause the variations in period observed. On their terrific flight close to the sun many would be caught in the sun's outer atmosphere, thus in some way causing sunspots.
This hypothesis attempts to explain the period and its irregularities, including the double and triple period. I refer to it at some length because the investigation of trees gives evidence not only of climatic variations in the sunspot period, but of double and triple sunspot periods and possibly of still larger fluctuations. Turner's hypothesis warrants further discussion to explain why the spots appear in sub-tropical latitudes but not at the solar equator. In the planetesimal hypothesis of Chamberlin and Moulton, the rotation of the sun on its axis is attributed to the material falling back upon it after receiving a slight orbital motion from the visiting star. The authors state that the process may still be going on. This view is sustained by arguments based on the zodiacal light and on meteors, both of which seem best explained as planetesimal matter not yet returned to the solar mass. Matter as yet unabsorbed would very likely consist of particles which had been given just enough orbital motion to escape the surface of the sun on their periodic return. The particles for the most part would then have extremely eccentric orbits and pass close to the sun's surface at tremendous velocity. They would be moving largely in the plane of the solar system and consequently would pass close to the sun's equator. If finally caught in the sun's atmosphere, friction would reduce their motion, turning a large part of it into heat and a part into forward movement of the sun's atmosphere. Thus the planetesimal hypothesis explains the equatorial acceleration. A large meteoric group, as suggested by Turner, is therefore consistent with the hypothesis. The undefined zone between the accelerated equator and the
