The Wheeling daily intelligencer. [volume] (Wheeling, W. Va.) 1865-1903, 1874-11-23

H., 9tsTAscE OF THE sux.-supposo we
ha9 messenger whom we could send 10
anu no between the earth and the ann
ana VINO could tell US exactly now ionS x
Look MN 10 perform the journey: snnnSss
We could also find by tiial exactly how
lasl nc could travel; then multiplv bia
speed ny the time occupied In Coming
from The Gun, and we shall have The an.
lance Of that body, Just as we would pet
me distance OF two cities when We knew
that Train running forty miles An hour
LOOK seven hours TO run betweor thor,
such messenger Ia lipht To,7ns t,a
how loDg It takes light 10 come iron, ~. th.,
sIu anu now last 1t flies, and VIC have Inc
distance required. How hopeless a task
Il seems to ind eithe. the time or the vs.
locily But both have been found Inv
methods which admit Of being underRtooa
without any great amount oil scIenti6c
knowledge. The time Of the Journey has
been found by two entirely independent
methods. The first Is by The eclipses al
Jupitet's satellites. The inner Of these
satellites rev0lves around Jupiter verv
regularly In less than two days. He passes
through the shadow OF Jupiter In every
revolution, and thus becomes ecllpsed,
lhese ecIipsGs can DO observed with a
small telescope, and the time noted Within
lew seconds. The time Of revolution
enu lhe form OF tho orbit being precisely
delermined, the moments at which ftfttifa
ecllpses arc 10 occur can be exactly nre-.
dicted. The first astronomer who neeH.
rately investigated this subject found that
when the earth was between the sun and
Jupiter the eclipses were seen several
minutes sooner than they should have
occurred; as the earth moved away, thev
were seen later, and finally, when the
earth gotto the opposite side Of the sun,
they were as much too late as they had
been too carly. As the earth returned
the ecllpses again came earlier, and the
irregularity continued In the same order
year after year. The variation was found
to BIO about eight minutes in time on each
side of the mean. Persuaded that no
such irregularity In the motions Of those
bodies could really exist, he attributed
the appearance to the progressive motion
OF light, and hence concluded that thfs
agent was about eight minutes in passing
over the space which separated the sun
from the earth The most exact deter.
minatlon OF this time was made about
seventy years ago from an examination
Of oil the eclipses OF Jupiter's satellites,
observations OF which were to be found
in the Paris Observatory. The result
was minutes Is 2 seconds,
Tho other method Of finding the time
light requires TO pass from the sun 10 the
sarth is due TO Bradley, who was Astron.
omer Royal of England in 1850. In max.
ing observations 10 determine the parallax
Of the fixed stars he was surprised to find
40 apparent annual motion Of the stars Of
about 90 seconds, which did NOT cores.
pond to the psrsllax ut oil. Alter long
study on the subject, 1t occurred to him
that, owing to the rapid motion Of the
earth in its orbit, no star would be seen In
ia true direction unless it was In the line
d direction of the motion; but the motion
d the light itself would be combined with 2
hat Of the earth. The angle being 20
seconds showed that the velocity OF the
light csming from the star was 10000
lines that OF the earth in its orbit. Since
tho velocity of the earth would
carry it to the sun in about 81
Jays, light would, by this theory, require
about 10000 Of 81 days to reach
the sun. Calculate this time, and 1t will
give 3 minutes IL seconds. The most exact
observations OF recent times give minutes /
IL seconds. There is therefore an out
standing discrepancy Of five seconds OILY
between the results OF the two methods.
and this.is accounted Tor by the uncertain.
" Of the old observations oil Jupitea's
satellites. y
The terrestrial velocity OF light is also
obtained by two different methods. The
mechanical difficulties involved in them :
may be judged by the fact that it has
been necessary to measure space OF time
less than millionth of second. The
most accurate method is that oil Foucault,
IN which beam OF light is made to lali
IN rapidly revolving mirror, from which
! is reflected 10 second mirror, which
throws it back to the revolving mirror.
II the light required no time Whatever to
2ass between the two mirrors, the sta
lonary mirror would reflect ii back to-
he same point from which it came in the.
irst place, for, however fast the
'ther mirror might turn, 1t would
DC III the same position when
he light was first reflected and when IL
came back after the second refleclion,
But it the light takes any time 10 20 and
;ome, the mirror will have moved a lit
lie between the two reflections, and the
avs will be thrown back to different
Joint from that from which it came. By
living the mirror velocity Of a thous.
md turns per second the deviation OF the
econd reflection was sufficient to be
measured by the aid OF O telescope. and
he results OF the measurements indi-
'Sted that the velocity o! light was 1885,
,Uo miles per second. Multiply this BY
he AUS seconds It takes light 10 come
rom the sun TO the earth, and we find
he distance passed over to be 90, 10000
niies, result remarkably accordsnt with
he, other recent determination OF the
un's distance.-Prot. Simon Newcomb,
n RdrptrS Magazine for N0psnJsr