The Democratic advocate. [volume] (Westminster, Md.) 1865-1972, 1874-10-31
Distance of the Sun,

could send to and fro between the earth
and the sun, and who could tell us exact
ly 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 distance of that body, just as we would
get the distance Of two cities when we
knew that n train running forty miles an
hour took seven hours to run between
them.
Such messenger is light. Let US find
how long it takes light to come from the
sun and how fast il flies, and we have the
distance required. How hopeless 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 by two entirely independent
methods The first is by the eclipses Of
Jupiter's satellites. The inner Of these
satellites revolves around Jupiter very
regularly in less than two days. He passes
through the shadow of Jupiter in every
revolution, and thus becomes eelipsed.
These eclipscs can be observed with small
telescope, and the time noted within few
seconds. The tune Of revolution and the
form of the orbit being precisely detcrmin-
ed the moments at which future eclipses
are to occur can be exactly predicted.
The first astronomer who accurately inves-
tigated this subject found that when the
earth was between tho 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 sun, they were as
much too late as they had been too early.
As the earth returned, the eclipses again
came earlier, and the irregularity continued
in 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 such irregularity in the
motions of those bodies could really exist,
he attributed the appearance to the pro-
gressive motion of light, and hence con-
cluded that this agent was about eight
minutes in passing over the space which
separated the sun from the earth. The
most exact determination Of this time was
made about seventy years ago from an
examination Of all the oclipses of Jupiters
satellites, observations of which were to be
found in the Paris Observatory. The
result was minutes 1872 seconds.
The other method Of finding the time
light requires to pass from the sun to the
earth is due to Bradley, who was Astrono-
mer Royal Of England in 1850. In max
ing observations to determine the parallax
of the fixed stars he was surprised to find
an apparent annual motion Of the stars of
about 20 seconds, which did not CORES
pond to the parallax at all. After 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
its true direction unless it was in the line
Of direction of the motion; but the motion
of the light itself would be combined with
that Of the earth. The angle being 90
seconds showed that the velocity Of the
light coming from the star was 10000
times that Of the earth in its orbit. Since
the velocity of the earth would carry it to
the sun in about 81 days, light would, by
this theory, require about ,Oss Of 81 days
to reach the sun. Calculate this time, aud
it will give 8 minutes IL seconds The
most exact observations of recent times
give 8 minutes 1S seconds. There is
therefore an outstanding discrepancy of
FIVE seconds only between the results Of
the two Methods and TINS Is accounted Tor
BY the uncertainty OF the old observations
Of Jupiters satellites.
The terrestrial velocity OF light is also
obtained by two different methods. The
mechanical d'ihculties involved IN them
may be judged BY the TACT that 1t has
been necessary TO measure space OF time
less than millionth OF second. The
most accurate method Is that OF Foueault,
IN which a beam OF light Is made to fall
on rapidly revolving mirror, from which
it is reflected to second mirror, which
throws 1t back to the revolving mirror.
If the light required no time whatever
to pass between the two mirrors, the sta
a Il. L.