Evening star., 1951-06-01
Sky Has Its Seasons,
Altitude Studies Show
Tests Muy Open Method
To Trace Guided Missiles
By Thomas R. Henry
There are seasons in the sky.


At great heights-more than so miles
above the earth's surface-a strange
and inexplicable difference between sum-
mer and winter has been found by Harv-
ard and Massachusetts, Tech scientists
working under an Office of Naval Re-
search contract.
The density of the atmosphere at
such attitudes appears to be twice as
great in summer as in winter At all
seasons, also, it turns out to be twice
as great over New England as over New
Mexico. This has just been reported to
the Navy by DR.. Luigi Jacchia, Harvard
astronomer.
The finding has considerable practical
significance in determining the prob
able paths of guided missiles at great al
titudes It has resulted from extremely
accurate determinations Of the speeds
and paths of shooting stars as they
enter the atmosphere from the vacuum
of interplanetary space. Against this is
checked the observed behavior Of w
rockets shot to great heights.
Meteor's Trail Photographed.
When German rockets began landing
in England in the early summer Of 1914
experts began trying to determine their
launching sites on. the continent by OBJ
servations of their trajectories. This
would be relatively simple problem for
artillery shells, moving through an at
mosphere of known density. It proved
practically impossible, however, for the
rockets, the major part of whose flying
time was at heights where density was
unknown At the best there were glori-
fed guesses The most interesting data.
: fvAV
few meteor observations made by DR.. r.
L. Whipple of Harvard.
These were crude, compared to the
possibilities today. More precise obSer-
vations were necessary, in view Of the
growing possibility that the high strato-
sphere well might be future battlefield.
One of the facilities used was photog-
raphy to follow meteorS trail. ~
Millions of meteors enter the atmos-
phere every "a hours. Only the brightest
of them, however, are seen at night, and
only the brightest of these could be
photographed. A revolutionary new
type camera was developed at Harvard
with which it now is possible to photo
graph any meteor which can be seem
at the best only small fraction Of those
in the sky.
These shooting stars are rock frag-
ments. They are anywhere from q
Tooth of an inch to few inches In
diameter. They are moving so fast that
the friction of the very thin air through
which they move heats them red hot.
Seldom, however, is the actual, glowing
rock particle seen. The light OF the
shooting star comes from the trail Of
vanorized air molecules which it leaves
in it's wake.
Measured Accurately.
This now has been photographed ana
the trail measured with extreme accuracy
hundreds of times. It makes possible
a fairly accurate determination OF its
trajectory-that is, provided all the 1ac-
tors were known. Actually, much con-
cerning the state Of the upper atmos-
phere can only be guessed at.
Many determinations were made over
New England. The next step came with
the rockets shot to great heights over
New Mexico. Their paths could be oo-
served continuously. These also made
possible a fair estimate OF air dens1tles.
Curiously enough, there were great
discrepancies between meteor and rocket
data. When the shooting obser-
vations were transferred to New Mexico.
however, the two became nearly iden-
tical.
The discrepancies, it was found, had
been due to latitude and season. The
accurate aiming of q rocket In war
fare henceforth must take into consider
alton the place and time Of its launch
ing. The same will have important bear
ings on any future space travel.
On the whole, DR.. Jacchia says, meteor
astronomy seems on the verge OF very
striking developments, especially with
the improvement of the cameras which
will make possible photographing Of
much smaller shooting stars.