Year: 1874

DISTANCE OF THE SuN.-Suppose We
had messenger whom we could send to
and fro between the earth and the sun,
and who could tell US exactly how long it
took him to perform 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 dis
tancc Of thnt body, Just as we would get
the distance of two cities when we know
that train running forty miles an hour
took seven hours to run between them.


Such messenger is light. Let US find
how long 1t takes light to come from the
sun and how fast it flies, and we have the
distance required. How hopeless task
it seems to find either the time or the ve-
locity But both have been found by
methods which admit OF being understood
without any great amount Of sientfc
knowledge. The time of the journey las
been found by two entirely independent
methods. The first is by the eclipses Of
Jupiter's satellites. The inner OF these
satellites revolves around Jupiter
in less than two days. lie passes through
the shadow Of Jupiter in every revolution
and thus becomes eclipsed. These eclip.
ses can be observed With a small telescope
and the time noted within few seconds.
the time of revolution and the form OF
the orbit being precisely determined, the
moments at which the future eclpses are
to occur can be exactly predicted. The
first astronomer who accurately investi-
gated 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, they were seen
later, and finally, when the earth got to
the opposite side Of the SUD. they were Us
much too late as they had been too early


AS the earth returned, the eclipscs again
came earlier, and the irregularity contin-
ued ip the same order year after year


The variation was found to be about eight
minutes in time on each side OF the mean


Persuaded that no irregularity in the mo-


tions OF those bodies could really exist he
attributed it the appearance to the pro.
gressive motion Of light, and hence con.
cluded that this agent was about eigh,
minutes in passing over the space which
separated the sun from the earth. The
most exact determination of this time wa.
made about seventy years ago from an ex


animation Of all the eclipses of Jupiter,
satellites, observations of which were t
be found in the Paris Observatory. The
result vas minutes 1872 seconds.


The other method Of finding the tim,
light requires to pass from the sun to tn,
earth is due to Bradley, who was Astron


omer Royal of England in 1850 Ii
making observations to determine tn,
paralax of the fixed stars be was surpris
ed to find an apparent annual motion o


the stars of about 20 seconds, which die


not correspond TO the parallax at all AT


ter long study on the subject, it occurre,


to him that, owing to the rapid motion o


the earth in its orbit, no star would b.
|, seen in its true direction unless it was il
the line Of direction of the motion; bu


the motion Of the light itself would b


combined with that Of the earth. The an
gle being 20 seconds showed that the We
} locity OF the light coming from the sta
was 10000 that Of the earth in its orbit


Since the velocity of the earth would car
ry it to the sun in about 81 days, ligh
would, by this theory. require abou
, 10000 OF 81 days to reach the sur
Calculate this time, and it will give


minutes II seconds The most exact OF


servation Of recent times give $ minute


| 3 seconds. There is therefore an on,
standing Of discrepancy Of five second
only between the results Of the two meth
r DDS, and this is accounted for by the uu
, certainty Of the old observations Of Jup
= ters satellites.


! The terrestrial velocity of light is ais
. obtained by two different methods. Th
, mechanical difficulties involved in theI
may be judged by the fact that it ha
; been necessary to measure space Of tim
0 less than a millionth of second. Th
t most hocurate method is that of Foucanl
in which beam of light is made to ia


on rapidly revolving mirror from whic
. it is reflected to a second mirror, whic
Q throws it back to the revolving miro
= If the light required no time whatever I


pass between the two mirrors, the station
J, airy mirror would reflect it back to th
: same point from which it came in the fir!
3 place, for, however fast the other miro
I might turn, i would be in the same pos
1 tion when the light was first reflected an
e and when it came back after the secon
4 reflection, But if the light takes an
I time to go and come, the mirror will hav


moved little between the two reflection
A aud the rays will be thrown back to di


ierent poin; from that irom which it cam,
5 BY giving the mirror velocity Of ther
/ sand rurns per second the deviation Of th
/ second reflection was sufficient to be mea
/ ured by the all of telescope, and h
D results of the measurements indicate
4 that tho vel, city OF light was 800
miles per second. Multiply this by th
J. 49S seconds it takes light to come frol
e the sun to the earth, and We find the di
b. tanee passed over to be 92, l30 900 mile


l.