Evening star., 1901-12-14
All Are Smaller and Denser Than
Heretofore Supposed
MADE In DAY TINE
Why This Has Resulted in a Change
of Figures.
WORK OF OUR OBSERVATORY
Written for The Evening Star.

written Tor The Evening Star.
That all of the planets are surprisingly
smaller than heretofore estimated by the
most careful astronomers, and that some
have even double the density assigned
them. is remarkable discovery now being
made at the naval observatory here.
This discovery is due to the application
of an entirely new method of measuring
the diameters of our neighboring worlds in
the solar system-a method which admits
Of all observations being made in daylight.
While it is true that we can see the moon
in the heavens while Old Sol is yet up and
doing, the naked eye cannot view our sis
ter planets between dawn and dusk. The
idea of studying other worlds by daylight
seems at first blush impossible. But Uncle
Sans great twenty-six-inch telescope-one
of the largest in existence-has been fitted
with an attachment which will enable IL to
penetrate the azure of the day sky and
focus into the eyes of our national astron-
omers the day time doingS of those huge,
far-away, spinning tops which revolve
about that mighty star which we call our
sun.
These attachments are color screens
glass boxes containing a golden fluid. When
they are placed across the great lens of
the telescope the eye of the astronomer
sees the sky in a beautiful greenish yellow
hue. The planets stand out on this back
oAOiN' oithouSi lhSs appear III IltdIIy lne
same tint Yet they are perfectly sharp
and capable of measurement, with a degree
of accuracy never before dreamed of.
Now you have doubtless noticed that the
delicate, curly wires in an incandescent
electric light globe, although barely visible
when cool, appear to swell to great thick
ness when lighted. This is due to the phe-
nomenon of irradiation which causes the
image of a bright object to spread upon the
ret1na OF the eye just as it will upon a
Photographic plate. Thus the moon looks
Ian ger to us at night than it does by day
you have often noticed that The same an
plies to the planets. When seen through
the color screens of the great telescope in
daylight they appear to have shrunk.
The Old Way-why Wrong.
Always by night have the diameters Of
the planets been measured hitherto. AS a
result of these night measurements we are
told in our ASTRONOMERS that Uranus for
instance, is 29,s69 miles from pole to pole
But the daylight measurements now being
made show that the diameter of Uranus is
but 6545 miles. In other words 1t is 1924
miles smaller than astronomers have be-
lfeved up to now.
The diameters of the planets have been
measured with a reasonable degree of ac-
curacy only since the time of the great
English astronomer, Sir William Herschel
contemporary of George Washington. AS
the nineteenth century advanced different
investigations at different times were made
with various instruments by the German
astronomers Shroeter, w. Struve. H. Struve
and Bessel; by Secchi, a priest at Rome
and by the American astronomers Hail.
Barnard and Campbell. These made their
measurements at night when the planets
were nearest the earth and when they ap-
peared largest. In 1894 Prof. Campbell of
the Lick observatory began new observa-
tions of the diameter of Mars in which he
made measurements both by night and by
daylight. He obtained from these investi-
cations entirely new results, and discovered
that the planet hly in
daylight than at night. But Prof. Camp
bell thereupon discontinued the research
and took up other important problems. The
daylight method of measurement was not
applied to any of the other planets until
1899, when 1t was taken up at the naval
observatory by Prof. T ]. ]. See, the as
tronomer in charge of the great twenty-six-
inch telescope. The work was applied to
one planet after the other, and even to
many Of the satellites, without any knowl-
edge that Prof Campbell had ever worked
along the same lines. The work became es-
pecially active during the winter and sum-
mer just passed. 1t has now been applied
to all OF the planets of the solar system.
The work has not vet been concluded. 1t
will probably extend over another year. In
the cases of the fainter planets, Uranus
and Neptune, twilight measurements are
also made The old methods of measure
ment will hereafter be discarded at the OBJ
servatory.
Details of New Method.
I will tell you how Prof. See makes his
measurements. having brought a planet
into view by aid Of the color screens
The actual measuring is done by aid of a
micrometer. This is very delicate piece
Of mechanism. 1t is primarily a scale
across whose frame are stretched two
pieces of spider web. exactly parallel One
of these is fixed; the other is moved nearer
to or further from the first by the turn
ing of a screw. planet having been
brought into the field of the telescope, the
screw Is turned until the extreme edges of
the planets disk are included precisely be-
tween the two pieces of web. A delicate
scale gives the apparent diameter. The