The Daily state journal. [volume] (Alexandria, Va.) 1868-1874, 1873-12-23
stronemv-PrOfessor Newcomb's sew
euttl Lecture

The subject of Professor Newcomb's
seventh lecture before the Columbian
University was announced to be the appli-
cation OF the telescope to astronomical
measurements. Few have an idea, when
they read OF the immensely of the celestial
spaces, how delicate are the measurements
on which all our knowledge OF this subject
rests. Several striking illustrations were
given, commencing with the
DISCOVERY OF NEPTUNE.
II is well known that the existence of
this planet was first made known through
its influence in changing the motions Of
Uranus. The latter planet was found not
to follow accurately the course laid down
for it in the tables, which took account of.
the attraction Of all the other known
planets, and its deviations remained an
enigma for more than twenty years.
AT length Adams, in England, and LIE
Verrier, in France, found that the devia-
tons could be accurately accounted for
by the attraction OF an unknown planet
moving in certain orbit, and they an
nounced the deviation in which it was to
be found.
Ihe telescope was pointed in the re-
qnred direction and there was the planet,
within degree of the predicted place.
Now, if during the whole time that astron-
omers were specializing on these deviations
a star had been moving in the heavens in
tho position Of Uranus given by the
tables, and if the real Uranus had been
visible to the naked eye, the two would, to
the unalded vision, have formed but SIN
gle star.
Again, the distances of all the fixed
stars depends on the measurement of an
angle OF parallax hardly exceeding sec
ond OF ali arc, and generally much less.l.
70 show how small this angle is, if a
hundred stars were placed in a row, the
distances between consecutive ones being
second. the whole row would, to the
naked eye, be but single round star. If
the row consisted Of two hundred, a toler.
ably good eye would just be able to see a
Slight elongation.
THE SOLAR PARALLAX
is less than nine seconds, so that the angle
would have to be increased ten or twenty
times to be clearly perceptible to the
naked eye. Yet it is upon knowledge of
this small angle that the computed dis
tanees OF all the bodies of the solar system
beyond the moon depends.
The method by which such extreme
precision in angular measurement could |
be attained by means of the telescope was |
then explained. The advantage arises not
merely from the magnifying power of
the telescope, but from its property Of |
forming an image of the object to be !
measured in The focus of the object glass.
A cross of the finest SPIDERS web was set
IN the same focus, and when the cross and
the image are brought into coincidence the
axis of the telescope points accurately at
the object. If the telescope had no mag
nitying. power at all, this pointing could
be made with far greater precision than a
rod of any soit-a gun barrel, for
instance-could be pointed at an object.
One of the great difficulties of astronomi-
cal measurements arises from the contin-
ual diurnal motion of all the heavenly
bodies, or, to speak more exactly, the
diurnal revolution of the earth carries
instrument and observer with it. In con-
sequence, fixed telescope will not point
at the same object two moments in sue
session. In the transit instrument, how
ever. this movement is actually utilized as
q means Of measurement by the aid of I'll
astronomical clock. The time at which
an object passes the meridian gives one of
the two data required to fix its position in
the heavens, the other being given by its
as measured at the moment of
crossing. s
Inc lecturer concluded with descrip-
ton OF measures made with the equatorial
telescope, which is mounted in such way
as to follow n IL, 6;,,rno .,,ot:a., =