Omaha daily bee. (Omaha [Neb.]) 187?-1922, 1910-06-13

Tonight one OF the four moons Of Jupiter
will be In transit across Abe face Of that
big planet and anothers will pass behind Il
in eclipse. II was through observations
OF the movements OF the moons of Jupiter
that the theory OF the velocity OF light
was evolved--a discovery Of prime i'm
portance in astronomy. Observations OF
their movements were being made in or
der TO assist seamen to reckon their posi-
lions, and It was found that the eclipses
occurred twenty-five minutes earlier when
the earth was nearest to Jupiter than when
IL was at it's most distant point 1t was
concluded that IL required twenty-five
minutes for light 10 cross the orbit Of the
earth. 1t was later found that the motion
OF the earth and other things had to be
taken into the reckoning, and then the
actual velocity of light was determined.
After the solution was worked out in
directly II was concluded that II must be
worked out directly. and many ingenious
machines were brought out to do so. Per-
hRps the most accurate and conclusive
tests OF light velocity ever made were
those Of the late Simon Newcomb and A.
A. Michallson. acting on behalf OF the
naval observatory at Washington. To
measure the velocity OF something which
flies so fast that It will race around the
earth seven aud a half times in a single
second might seem an impossible task;
but these two representatives OF the naval
observatory set themselves resolutely TO IL.
At the base OF the Washington monument
they set up big mirror. In the grounds
at Fort Myer. two and a quarter miles
distant across the Potomac river in Vir-
ginis, they stationed a revolving mirror,
with speed Of 80 revolutions to the
second. By revolving this mirror with
great rapidity they were able to note how
far II would turn in the time required for
a ray OF light to flash across the inter-
vening space between Fort Myer and the
Washington monument and back again.
They found il would cover the distance
in the forty-thousandth part OF a second,
and thus they were able to settle positively
the question of the velocity of light. Their
instruments were so delicateiy adjusted
that they would measure the millionth
part OF a second as easily as a carpenter
measured a board.
The naval observatory is one OF the
sights OF Washington that the tourist sell
dom sees, and, in fact, its work is of such
a character that to make a show place of
1t would interfere with its usefulness. 1t
is here that America gets its time. The
day was when sun time was accurate
enough for any man. but with the advent
OF the railroad and the telegraph, standard
time became a necessity, and the complex
conditions of modern civilization requires
a time accuracy as near to the absolute
as may be. In the single matter of naviga-
ton, an error in time might lead the
commander of a fleet TO miscalculate his
position, and a war might be decided
the disadvantage Of a fleet which
did not have the correct time.
If there is a necessity for correct time
the naval observatory is the result of that
necessity. Its great master clock, kept in
an isolated vault, in a sealed case, the
temperature of which is not allowed to
vary more than the hundredth part of a
degree. is one OF the wonders OF the world
of delicate scientific instruments. 1t Is
SO accurate that Il never varies more than
three-tenths of one second, and st times
has run for weeks without getting out Of
the way more than the hundredth part of
a second. In the sealed case in which II
is kept there is e little thermostat and an
electric light. A change of the hundredth
part of degree of temperature will affect
that delicate thermostat as much as a
CnAnBe OF forty degrees Will affect a
human being. When it gets the two
part of a degree colder than
normal the thermostat shivers and turns
on the little light, which serves it as an
automatic stove. When IL gets the two
hundredth part of a degree hotter than
normal the thermostat begins to mop its
brow and turns off its electric stove. Not
only must the temperature, but also the
barometric pressure, be kept constant.
Yet with all the wonderful accuracy of
this clock a careful check must be kept
upon Its performance, in the doing of which
one finds precision almost startling in
It's care. In little square house on the
naval observatory grounds there is an in
trument which verdfies the time two or
three times week, and Is after all, the
source of the nations time. II is mounted
on two great pillars Of granite, and looks
like cross between a cannon and a teie-
scope. It is on the exact meridian of Wash
lngton, and can be pointed only north and
south. To make Us readings worth while
1t must be adjusted with delIcacy almost
1t cannot rest with but a
small percentage of Its weight on its pivots
The entire weight except for a few pounds,
is supported by sling arrangement, only
enough OF its weight being allowed on the
real bearings to give 1t steadiness and to