Smyrna times., 1935-07-04
LIGHT SOURCES
IN NIGHT SKIES
Completing Huge 81 Inch
Telescope To Make
a Study
SOME OF THE PROBLEMS

servatory, joint project OF the UNI
yersity of Texas and the University
of Chicago, on Mt. Locke, Texas,
is not yet completed, an investiga-
ton is already under way near the
new observatory to determine the
sources of light in the night sky.
Using an old 12-inch refracting
telescope which was presented some
years ago to the Yerkes observa-
tory of the University of Chicago
by DR.. G. E. Hale, assistant Pro-
fessor @. I. Elvey of the Yerkes
staff and DR.. f. E Roach of the
Mcdonald observatory have been
recording the light of the night sky
with a recording photo-electric pho
tometer designed by Doctor Elvey.
The light of the night sky Is
made up Of direct starlight; star
light which is scattered both by
the earth's atmosphere and the mat
ter in interstellar space; zodiacal
light which is concentrated in the
ecliptic but which may extend over
the entire sky; and of the light
originating within the atmosphere
of the earth; the permanent aurora.
This aurora is not the polar aurora,
or northern lights, although 1t Is
related to it.
Many Study Problem.
Many investigators have studied
the problem Of the composition of
light, and a general investigation
by the Dutch astronomer, Van Rhijn
has shown the following percent
age composition: Direct starlight,
Is per cent; starlight scattered by
the earth's atmosphere. per cent;
zodiacal light, 43 per cent; aurora,
Is per cent, and scattered earth
light, 10 per cent.
1t is well known to astronomers
that there are large fluctuations in
some Of the sources of light, par
ticularly of the zodiacal light and
the aurora. Lord Rayleigh found
that the light Of the night sky un-
dergoes large variations, with a
range Of eight-fold between the
smallest and largest.
The variations are sometimes fair
1y rapid, Doctor Elvey having found
In his work at Yerkes as much as
a 25 per cent change in brightness
of the sky within period Of two
hours. There also are daily varia-
tions In the intensity of the aurOral
light in the night sky, which In
some parts of the world reaches
maximum about 2 a. M, and in
other parts is at its maximum just
after twilight has ended.
These large variations have made
detailed analysis of the light diff
cult, because during the several
hours required to cover the sky,
there are many changes in the light.
To make possible observations in
a brief space of time Doctor Elvey
devised the recording photo-electric
photometer. 1t consists of a very
sensitive galvanometer, which meas
ures the output of the amplifier.
The recording is achieved by shin
ing a lamp on the galvanometer
mirror, which reflects the light to
a slit behind which is a moving
sheet of bromide paper.
DefIections Recorded,
The reflections of the galvanome-
ter are then recorded on the bro
mide paper, which is directly con-
nected to the photometer so that 1t
moves past the slit as the instru-
ment is rotated around the verti-
cal axis. With this instrument a
complete record of the brightness
of the sky on a circle parallel with
the horizon can be made in approx
mately five minutes, and the entire
sky can be observed in about an
hour. A check Of the variations in
light during this time can be made
by observing a given region of the
sky several times.
Doctor Roach already has OBJ
tained over a hundred tracings with
the instrument, from which he and
Professor Elvey have determined
the axis of the zodiacal light, the
cone of light that can be seen ris-
ing from the horizon after sunset
and before sunrise.
Contrary to general belief, the
axis does not coincide with the
plane Of the earth-the ecllptic.
Observations show the morning 20
diacal light Is displaced to the
north Of the ecliptic by as much as
five degrees. A series of evening
observations Indicate that in part
of the ecliptic the zodiacal light
was near the ecliptic. apparently
crossing 1t.
The astronomers expect that
detailed analysis Of the tracings
will give the relative amount and
the distribution of the light from
various sources contributing to the
total light from the night sky.