Juniata sentinel and Republican. [volume] (Mifflintown, Juniata County, Pa.) 1873-1955, 1874-12-23
Distance Of fhe Sun.


Suppose We had messenger whom
we could send to and fro between the
earth and the sun, and who conld tell
MS exactly how long it took him to per
form the journey; suppose we could
also find by trial exactly how fast he
could travel; then multiply his speed
by the time occupied in coming from
the sun, and we shall have the DISTANCE
of that body, Just as we would get the
distance of two cities when We knew
that a train running forty miles an
hour took seven hours to run between
them.
Such a messenger is light. Let us
find how long it takes light to come
from the sun and how fast it flies, and
we have the distance required. How
hopeless q task it seems TO find either
the time or the velocity But both
have been found by methods which
admit of being understood without any
great amount of scientific knowledge.
The time of the journey has been found
10y two entirely independent methods.
The first is by the eclipses of Jupiter
satellites. The inner of these satellites
revolycs around Jupiter very regularly
in less than two davs. He passes
through the shadow of Jupiter III every
revolution, and TINS becomes eclipsed.
These eclipses can be observed with q
small telescope, and the time noted
within a few seconds. The time of re-
volution and the form of the orbit
being precisely determined, the mo
ments at which future eclipses are tC
occur can be exactly predicted. The
first astronomer who accurately inves
tigated this subiect 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
they were seen later, and finally, wheI
the earth got to the opposite side o
the sun, they were as much too late as
they had been too early. As the eartl
returned the eclipses again came earlier
and the irregularity continued In the
same order year alter year. The varla
tion was found TO be about eight
minutes in time on each side OF the
mean. Persuaded that no such irregu
larity in the motions of those bodies
could really exist, he attributed the
appearance to the progressive motion
of light, and hence concluded that this
agent was about eight minutes In pass
ING over the space which separated the
SIM from the earth. The exact deter
mination of this time was made about
seventy years ago from an examination
of all the eclipses of Jupiter's satellites,
observations of which were to be found
IN the Paris Observatory. The result
was 8 minutes 1872 seconds.
The other method of finding the time
light requires to pass from the sun tC
the earth is due to Bradley, who was
Astronomer Royal of England In 100
In making observations to determine
the parallax OF the fixed stars he was
surprised to find an apparent annua
motion of the stars of about 20 seconds
which did not correspond to the paral
lax at all. After long study on the sub
iect. it occurred to him that, owing tc
the rapid motion of the earth IN Its
orbit. no star would be seen In Its true
direction unless it was in the line o.
direction of the motion; but the mo
s..,n of the light itself would be com
s,irSd with that of the earth. The
,neIs h,ing 30 seconds showed that
the yeaSeity of the light coming from
the star was 10000 times that of the
earth ~ it, orbit. Since the velocity
of the earth you'd carry 1t to the SUI
in about 81 days, light would, by tins
theory, ri quire about 'lOoOo OF % days
10 reach the sun. Calculate this time
aud II will give minutes IL seconds
'ri,e most exact observations OF reeen
times give : minutes Is seconds. There
is therefore an outstanding discrepancy
Of five seconds only between the results
of the two methods, and this is accounted
tor bv the uncertainty of the old obser
vatja1s OF yupiter's satellites.
The terrestrial velocity of light II
otso obtained by two different methods
The mechanical difficulties involved IL
them may be judged by the TACT that
has been necessary to measure spac'
a, time less than milliontb OF second
The most accurate method IS that o
E,ucault in which a beam OF light u
msJe TO fall on a rapidly revolving
,,;rror from which it is reflected TO
seaond mirror. which throws 1t back u
the revolving mirror. II the light re
ouired no time whatever to pass DE
tween the two mirrors, the statioEar.
mirror would reflect it back to th'
same point from which it came IN tn
first place, for, however fast the otne
Mirror might turn. it would be In tn
same position when the light was mrs
reflected and when it came back ate
the second reflection. But II the hgh
takes any time to go and come TN
mirror will have moved little betweel
.he two reflections, and the rays VII
be thrown back to a different pom
from that from which it came. b.
giving the mirror a velocity of thous
and turns per second the deviation o
the second reflection was sufficient q
Ice measured by the aid of telescope
and the results of the measurement
indicated that the velocity OF hgh
was IS5. 900 miles per second. Mul
tiply this by the 49s seconds 1t take
light to come from the sun to tn
earth. and we find the distance passe
over TO be 81, 130, 600 miles, resul
remarkably accordant with the othe
recent determination of the suns di!
tapce.-Harper's Magazine.