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ASTRONOMY: G. E. HALE 
THE DIRECTION OF ROTATION OF SUN-SPOT VORTICES 
By George E. Hale 
MOUNT WILSON SOLAR OBSERVATORY. CARNEGIE INSTITUTION OF WASHINGTON 
Read before the Academy. December 8. 1914. Received, April 15. 1915 
Whirling storms in the earth's atmosphere, from the extensive and 
moderate cyclone to the small and destructive tornado, follow a well- 
known law of rotation: right-handed or clockwise in the southern hemi- 
sphere and counter-clockwise in the northern. These directions are 
easily explained by the increasing eastward velocity of the air from pole 
to equator. As sun-spots are vortex phenomena, analogous in many 
respects to tornadoes, it is interesting to inquire as to the law of their 
rotation, since this may throw light on their nature and origin. 
It is a well-known fact that when a normal Zeeman triplet is observed 
along the lines of force of a magnetic field, the central component is 
absent, and the two side components are circularly polarized in opposite 
directions. A quarter- wave plate and Nicol prism mounted over the 
slit of the spectroscope permit either component to be cut off at will. 
Furthermore, if the polarizing apparatus be adjusted so as to extinguish 
one component, reversal of the current through the coils of the magnet 
will cause this component to reappear, while the other will be extin- 
guished. The method thus offers a simple means of determining the 
polarity of a magnetic field, which can still be applied when the angle 
between the line of sight and the lines of force is as great as 60° or 70°. 
In this case, however, the central line of the triplet is present, and the 
elliptically polarized light of the side components can be only partially 
extinguished. 
Every sun-spot exhibits a magnetic field, whose polarity is determined 
by the direction of rotation of the electrons in the spot vortex. To 
learn the polarity, and hence the direction of rotation, it is therefore 
only necessary to observe whether the red or the violet component of a 
spot triplet is transmitted by the polarizing apparatus. 
In my earlier work attention was concentrated on a few of the largest 
spots, in order to secure unquestionable evidence as to the existence of 
the Zeeman effect. The possibility of finding a law of rotation appeared 
to be remote, as spots of opposite polarity were observed in the same 
hemisphere. Subsequently, when the characteristics of bipolar spot 
groups had been discovered, the search for a rotation law was renewed, 
with the results given in this paper. 
The typical sun-spot group consists of two spots of opposite magnetic 
polarity, lying on a line which usually makes only a small angle with the 
