National Republican., 1873-12-20
ASTRONOMY.


PROF. NEwCoMBs SEVENTH LECTURE
The snbicc, Of Prof.. Newcombs seventh lecture
before tho Columbian Untvcrsijy was announced
TO be the application OF the telescope TO nstronom
lcal measurements. Few have an IJoa, when
they read otthc immensely OF tho celestial spaces,
@ now nelieate are tho measurement on which az
with tho
DISCOVERY or NEPTUNE.
II Is well known that the existence Of This
In changing the motions Of Uranus. The latter
planet was found not TO follow accurately tho
course laid down for u in the tables, which took
account OF the attraction OF all tho other known
planets, and Its deyintions remained an enigan
Tor more than twenty years. AT length Adams
Y ~ ndns1snG and hs 'srrisr In france found tha,
by the attraction Of an unknown planet MOVING
r. certain orbit, and they announced the devia'
tion In which u was to he lound.
The telescope was pointed In tho required direc.
q the predicted place. Now, ifdurlns the whole
deyiations q star had been moving in The heaven,
IN the position o. Uranus given by tho tables,
ann !, the rcai Uranus had been visible to the
naked eye, the two would, to the unaided vision,
have formed hut single star.
Again, the distances OF all the hxed stars dc.
pends on the measurement Of an angle OF parallnx
hardly exceeding a second OF an are, and grncr
ally much lees. To show how small this angle H.
u hundred stars were placed In q row, the dis.
ANcrOUd
the whole row would, to the naked eye, be but 4
single round star. IL the row consisted OF two
hundred, tolerably good eye would Just be able
TO see slight elonGstion.
TnE SOLAR PAuALLAx
e less than nine seconds, so that the angle would
have TO bc increased ten or twenty times TO bc
clearly pcrccptible to the naked eve. Yet II Is
upon knowledBo Of this small angle that the
computed distances OF all the bodies of tho solar
system beyond the moon depends
Tho method by which such extreme precision In
angular Measurement could be attained by means
Of the telescope was then explained The advan-
taye arises not merely from the inagnlfying power
ofthe telescope hut irom Its property OF forming an
image Of the Object to be mensred In the focus Of
the object glass. A cross OF the hncst spiders
web was set In tho same focus, and when the cross
and the inrgc arc brought Into coincidence the
aSy OF the telescope points accurately at tho OBJ
ject. IL the telescope had no maynffylng power
at alt this pointing could be made with Tar greater
precision than rod Of any sort-a gun barrel, for
instance-could be pointed at an object. One OF
nent arises from the continual diurnai motion OF
all the heavenly bodics,or, to speak more exactly
the diurnal revolution Of the earth carries In.
strumept and observer with Il. In consequence,
hxed telescope Will not point at the same object
two moments In succession. In the transit inslru.
mcnt, however, this movement II actually utilized
as means Of measurement by tho MD Of the as
Economical clock. The time Mt which an object
passc. the meridian gives ono Of the two data re.
aurcd to nx lts position IA he hcavens, the other
being given by Us altitude, us measured at the
moment Of crossing.
Tho lecturer concluded with a description Of
measures made with the equatorial telescope,
which is m6nHtcd IN such way as to follow star
in Its diurnal motion.