NIGHT-SKY RADIATIONS 
the more recent night-sky spectral identifications); (2) 
seemed energetically satisfactory, but not completely in 
accord with the Frank-Condon probabilities; (8) is 
speculative and has not been put in quantitative terms. 
The relatively small amount of sodium in the upper 
atmosphere necessary to account for the 5890, 5896 
emissions may come either from the surface of the earth, 
as the sea, or from interplanetary space. Both views 
have been suggested and the correct one is not known. 
There have been no explanations of the remarkable 
winter enhancement of the sodium emissions observed 
in temperate latitudes. It would be of interest to observe 
the emissions in low latitudes. Whether the sodium 
emissions are merely a minor geophysical phenomenon 
or have wider astronomical interest is not yet clear. 
Zodiacal Light 
The zodiacal light is a well-known luminous phe- 
nomenon of the night sky which adds appreciably to the 
brightness of certain regions of the sky near the ecliptic. 
At present the facts available are not adequate to allow 
us to say whether the zodiacal light arises in the ter- 
restrial atmosphere or at a greater distance, and there- 
fore space will not be taken here to describe it. There 
are two theories to explain the zodiacal light. An older 
theory [24] attributes it to sunlight scattered from a 
lens-shaped disk of dust particles in the plane of the 
ecliptic extending well beyond the orbit of the earth, 
a more recent theory [16] attributes it to sunlight ab- 
sorbed and re-emitted by a band of atmospheric ions 
surrounding the earth in the outer fringes of the upper 
atmosphere. Each theory has points in its favor but 
further investigations of the zodiacal light with modern 
spectrographic and photocell techniques and better 
determinations of its parallax are necessary to reach a 
correct explanation. 
Aurora 
We have attempted to exclude the aurora from the 
foregoing survey of the night-sky light knowing that 
this cannot be done, and in the end should not be done. 
It is probable that a complete understanding of the 
night sky cannot be reached without at the same time 
having a complete understanding of the aurora. Both, 
it is believed, derive their primal energy from the sun. 
The aurora emissions are mainly from the same familiar 
gases of our atmosphere, oxygen and nitrogen, as are 
those of the night sky, but with different spectral 
distributions, with enormously greater intensities and 
in quite different geometrical patterns. The auroral 
energy is believed to be received in concentrated form 
from explosions or outbursts on the sun, in contrast 
to the relatively gentle and steady solar-energy stream 
which supports the normal night-sky light. The kinship 
between the aurora and the night-sky light may be 
analogous to that between a hurricane and a gentle 
rain. 
Suggested Experiments 
At the present time enough is known about the light 
of the night sky to indicate that it is an important and 
345 
interesting phenomenon of the upper atmosphere. Some 
further experiments may be suggested which may in- 
crease understanding of the nature and origin of the 
light. However, the experiments are not easy and re- 
quire the best of modern equipment. This is true, of 
course, of most experimental sciences, for in any field 
the easy experiments are done so quickly by the pioneers 
that later experimenters almost always find themselves 
in the stage where further progress is difficult. 
Improved spectra of high dispersion of the night-sky 
light should be obtained. For such a purpose powerful 
spectrographs and the facilities of a first-class ob- 
servatory are required. The spectra would provide 
needed additional information about the types of emit- 
ting atoms and molecules, and perhaps might answer the 
important question whether any of the night-sky lght 
is sunlight scattered by material sufficiently distant 
from the earth to be in sunshine. Spectra should also 
be taken of the zodiacal light in order to discover 
whether it is a luminous apparition which originates in 
the outer reaches of the terrestrial atmosphere or in 
some more distant place. Spectra of the aurorae should 
be included. 
The distribution of energy in the spectrum of the 
night-sky light should be redetermined with improved 
equipment; this has been done only by means of spectro- 
graphs of very low dispersion. Detailed measurements 
of the intensities of the lines and bands of the spectrum 
should yield information about the excitation phe- 
nomena of the radiations and the physical state of the 
regions of the atmosphere in which they occur. 
Surveys should be made over the entire sky in the 
several wave-length bands in order to determine the 
nature and behavior of the variations of the luminosity. 
Sensitive photocells and filters which pass narrow bands 
of wave lengths could be used for this purpose. Such 
surveys should be carried out at several stations for a 
period of time to bring out geographical and temporal 
variations and to establish relations, if any, with other 
phenomena, such as season of the year, solar activity, 
and ionospheric variations. From the variation of the 
intensity with zenith angle the data might yield better 
determinations of the altitude of the luminosity. The 
strong infrared radiations should be included in the 
survey. 
The survey of the night-sky brightness in several 
wave lengths with carefully standardized and calibrated 
photometric apparatus should be extended to high 
latitudes and regions near the magnetic pole in order to 
determine the geographical distribution of the lumi- 
nosity. Such observations might throw further light on 
the differences between the night-sky light and aurorae. 
Measurements with photocells should be made from 
rockets fired at night to heights of 180 km and above 
in order to discover whether the rocket enters or tray- 
erses the regions of the nocturnal luminosity. 
REFERENCES 
1. Asapin, P., Vassy, A., et Vassy, i, “Altitude de l’émis- 
sion lumineuse du ciel nocturne.”’ Ann. Géophys., 1: 189- 
223 (1944). 
