The Washington times. [volume] (Washington [D.C.]) 1902-1939, 1921-07-16
The Speed of Light


from conjunction to opposition
with Jupiter is equal, approximate
ly. to the diameter of the earth's
orbit-about 10000, 900 miles.
Now. Roener noticed that al
though the actual interval between
any two successive eclipses of any
one of Jupit2r's satellites must AL
ways be of practically the same
length, since their motions are reg
ular, yet if table is prepared pre-
dieting the times of the eclipses in
accordance with this regular inter-
val. and covering the entire period
during which the earth is passing
round from one conjunction with
Jupiter to the next (about thirteen
months), then, starting, say. at the
moment of conjunction OF the two
planets, 1. e., when they are near
est together, the actually observed
times OF the eclipses begin to fall
behind the predicted times as soon
as the earth moves farther away
from Jupiter, and this retardation
accumulates until the earth has arr
rived on the opposite side of its
orbit from Jupiter.
But from that point, where the
earth begins once more to APP
proach Jupiter, the observed times
fall ahead OF the predicted times,
and continue to do so until the bal
ance is restored on the earth's ar
rival again at the point of nearest
approach to Jupiter.
Roemer having satisfied himself
OF the existence of this phenome-
non. rightly ascribed it to the al
ternate increase and decrease OF
the distance that the light has to
Navel between the eclipsed satel-
lite of Jupiter and the eye of the
observer on the earth.
When the observed time fel, be-
hind the time predicted in the
table. and he saw that the mean
ing was that If he had been close
to the satellite he would have lost
sight of it simultaneously with
it's actual DISAPPEARANCE. in the
shadow, but being many millions OF
miles away, he had to walt to note
its disappearance until the last
wave of the light ray that issued
from il could reach him.
From his observations he found
that the retardation OF the time Of
an eclipse when the earth was far.
thest from Jupiter as compared
with the time at which the eclipse
would have been noted the earth
had been nearest, amounted 10
about 1876 minutes, or 900 seconds.
The irresistible conclusion was
that light required that amount OF
time to cross the diameter OF the
earth's orbit. The diameter OF the
orbit in miles divided by 800 would
then give the velocity of light in
miles per second. In those days
the distance of the sun was esti-
mated considerably too great, willis
the observations OF the eclipse
times were not OF the utmost ac-
curacy. so that the deduced veloc-
ity of light was too large. some es-
some even 100000 or more.
But the great fact remains that
Roemer was fundamentally right.
and truly did make the first meas
urement of 1.-The velocity OF light.
and indeed was the first to estab-
lish the fact that light requires
time to traverse space. Yet for nifty
years astronomers refused to accept
his conclusion; but they had to
come to II after additional dis
coveries demonstrated its truth.
Now, however, Roemcr's method
Ia no longer employed. The veloc-
ity OF light has been more aceu-
rately measured by experiments
conducted entirely on the earth.
There IL no room. here to describe
them, and have described them
briefly before. The now accepted
velocity of light is ls6,324 miles
per second .