POLARIZATION OF SKYLIGHT 
By ZDENEK SEKERA 
University of California, Los Angeles 
Introduction 
Since the discovery by Arago in 1809 that the light 
of the clear sky is polarized, interest in the problems 
of skylight polarization has varied greatly. At first, 
attention was concentrated more on the development 
of a suitable measuring technique to determine the 
magnitude of the polarization and its distribution over 
the sky. Arago first discovered a neutral point, named 
after him the Arago point, where the polarization dis- 
appears, about 20° above the antisolar point. The other 
neutral points were discovered in 1840 about 20° above 
the sun by Babinet and below the sun by Brewster. 
Because of the simplicity of the determination of neu- 
tral points, more attention was paid later on to their 
study and soon a complete picture of diurnal, seasonal, 
and secular variations in the position of the Arago and 
Babinet points was obtained. 
Arago’s discovery of a point of maximum polariza- 
tion 90° from the sun in the sun’s vertical was followed 
during the next two or three decades by studies of the 
variability of the maximum polarization at this point 
(Bernard, Rubenson, Crove, Cornu, etc.). 
For several reasons, interest in atmospheric polariza- 
tion culminated at the end of the last century. The 
photopolarimeter, constructed by Cornu in 1882, rep- 
resents the highest point in the development of the 
visual measurement of polarization. The accumulated 
results of polarization measurements gave rise to a 
series of attempts to explain the observed facts theo- 
retically, culminating in Lord Rayleigh’s theory of 
molecular scattering. 
The famous eruption of Krakatao (1883) showed the 
extraordinary sensitivity of skylight polarization to the 
presence of volcanic dust in the upper atmosphere. For 
several decades thereafter, the investigation of polari- 
zation was considered almost exclusively as a suitable 
tool for the study of perturbations of a similar kind. 
But since the last eruption of Katmai in 1912 no anom- 
alies of this type occurred, and the interest in problems 
of atmospheric polarization rapidly decreased. Smaller 
fluctuations in the polarization were studied and the 
measurements were extended to rather narrow spectral 
ranges. Surprisingly, a great discrepancy was found 
between the results of different authors, and the only 
possible explanation for this is a large variability of the 
dispersion of polarization (variation with the wave 
length) with the size and number of scattering par- 
ticles. As these quantities vary greatly with local con- 
ditions (weather, season, etc.), the corresponding vari- 
ations in polarization follow. But if the presence of 
scattering particles of different size is admitted, the 
79 
question arises whether it is the secondary scattering 
or the presence of larger particles (excluded by the 
assumptions of Rayleigh’s theory) which is responsible 
for the observed deviations of the atmospheric polari- 
zation from expectations based on theory. 
After Ahlgrimm’s successful attempt to compute the 
effect of secondary scattering, in which he explained 
most of the facts better than he explained the observed 
variations, Milch in 1924 presented a theory in which 
the secondary scattering was neglected and the effect 
of larger particles was considered responsible for the 
deviations from Rayleigh’s theory. The close relation- 
ship between Linke’s turbidity factor and the degree 
of polarization, predicted by Milch’s theory, was dem- 
onstrated in 1934 by Blickhan from simultaneous ob- 
servations of polarization and turbidity. But he ob- 
tained much better agreement between the observed 
and theoretical values of the maximum polarization 
when he considered the effects of both secondary scat- 
tering and the presence of larger particles. 
Even though Blickhan’s results clearly pointed the 
way for further investigations, no appreciable increase 
of interest in this direction has followed. The reason 
for a recent slight increase of interest in problems of 
skylight polarization is quite different. First, the intro- 
duction of objective methods, such as the photoelectric 
techniques in photometry, showed the possibility of 
more accurate and systematic measurements. Then, 
after the first attempts to use optical methods for the 
exploration of the upper atmosphere, attention was 
brought to the polarization of the skylight during twi- 
light and during the night. The great technical difficul- 
ties in the measurement of the extremely low intensi- 
ties of the skylight during these hours led even to the 
use of searchlight beams in the study of atmospheric 
scattering. But since, during twilight, direct illumina- 
tion from the sun is less and less intense, the secondary 
and multiple scattering become more and more impor- 
tant. And just at the present moment when there is a 
need for computation of the effect of secondary and 
multiple scattering, recent research in theoretical astro- 
physics offers help. The scattering by free electrons 
produces polarization of stellar radiation, the theoreti- 
cal study of which led Chandrasekhar to develop an 
excellent method for computing multiple scattering 
exactly, suitable for application to the problems of 
atmospheric polarization. Thus we are now in a posi- 
tion to use a highly developed modern experimental 
technique, together with excellent theoretical tools for 
solving many problems of skylight polarization in such 
a manner that very useful information can be ob- 
tained. 7 
