Year: 1874

space In an hour. The earth, tue centre OF which
Is sixty times nearer to US than the moon, moves
over the same space in about two minutes. as may
be seen by watching the western horizon at sun
set, as IL rises and covers the comparatively sta
tionary disk or the sun.


As the different celestial bodies pass over each
other In their motions they produce the phenomena
styled transits and occultationA When the nearer
body Is apparently the larger OF the two the more
distant body Is occulted. As the sun is SO brill
liant as TO prevent our seeing any other body be-
yond 1t, in immediate contact with It, and as the
disks OF the planets are so small, and
their motions comparatively slow, occul
tations are almost exclusively OF the
stars and planets by the moon. When
the nearer body is apparently the smaller OF the
two, It produces transit OF the smaller body over
disk OF the larger. Lunar eclipses, as shown in
the HERALD OF October a. belong to . different
class OF phenomena, and the same IL true OF the
eclipses OF the satellites Of Jupiter. In solar
ecpses the light or the sun is 30 opscured that
whether the apparent size OF the moon II smaller
or greater the phenomenon Ia not called transit
or an occultation; but solar ecllpses, or even the
daily setting OF the sun, obscuring its light by the
interpositon OF the body OF the earth, may be re-
garded as phenomena OF the same class with the
transit OF Venus, class OF phenomena too common
30 attract attention.


The astronomers OF the world tomorrow, II the
sky ahal be sufficiently clear, VIII see. at frst,
near the centre OF the disk or the sun, two small
spots. Measurements at different hours will show
that they are moving slowly toward the west but
the most careful measurements made in
Siberia and in Oceanica, will fail to
detect any difference In their position
upon the SUMS disk as seen from these widely
separated stations. These spots, therefore, belong
upon the sun itself. Then there will appear, at
about nine O'Clock P. H., OF New York time,
larger spot, circular, and moving more rapidly,
also toward the west. This will be the planet
Venus In its transit. In less than five hours Venus
viii have crossed the disk and passed away from
view. The smaller spots, meanwhile, will have
moved toward the west, from the rotation Of the
sun upon its axis. only about one-fortieth part OF
the SUMS diameter. The astronomers will pack up
their instruments and return home, no wiser than
they went; but they will bring with them materials
which, when properly compared and reduced, Will
give, more accurately than 1t Is now known, the
distance Of the from the earth, and therefore,
the scale by which we may compute the distances
OF the planets and even OF the nearer fixed stars.


During the eventtul hours when the Eastern
Hemisphere will bristle with hundreds OF parallel
telescopes. and when thousands OF eager eyes will
be watching the transit, thought Will arse to
the practical use OF the phenomenon. any more
than looking at beautiful painting or In listen
Ing to strains OF delightful music we attempt to
calculate in what way it will yield money to our
purse. million dollars Is not too mucn to spend
for so important an advance In scientific knowl-
edge. But lest, by and by, when we are to have
transit OF own and shall want further appropri-
atons 1t should be said that we have spent enough
Tor mere intellectual amusement. yielding no re-
turn, let me try to explain one or the ways In
which the observation or the transit OF Venus will
Immediately repay, In money. many times the out
lay.


1t has been already shown that, essentially. we
observe the transit or Venus to obtain the distance
OF Venus, because we can obtain 1t relatively with
four times the accuracy THAI would attend OBJ
servatlons upon the paralax OF the sun directly
at four times the distance and because, by the aid
OF Kepler's law, from the distance OF Venus we
can obtain with tho same relative accuracy the
distance Of the sun. BY having this iucrum, or
stepping stone, We thus increase by four times
the accuracy Of the determination OF the SUMS dis
tancc, leaving out OF view the peculiar advantaQeA
already shown, which win enable us to determine
the paralax OF Venus with less margin Of error
than in the case OF the sun.


The same advantages which Venus possesses our
moon possesses In q much greater degree. Con.
sequenuy, the distance OF the moon from the
earth ia known. from direct observations OF It's
parallax sixty times as accurately 90 the distance
of the sun. We fnow also the average
eject Of the attraction OF the sun upon
the motion OF the moon in its orbit. This
eject varies with the position OF the moon in its
orbit. the SUMS attraction being greater at new
moon, when the moon 10 nearer to the sun than
the earth H. and less at full moon, when the moon
10 further from the sun. II the sun were, as for.
merly taught, 400 times as far from the earth as
the moon, its distance from the new moon would
be $780, and from the lull moon 401. As the attrac-
tion varies with the square Of the distance, the
attraction at now moon would he to that ar 1ull
moon the square OF 4o1 to the square Of 30s tnat
is, about one.hundredth part greater. But II the
sun is only $50 times the distance Of the moon,
more nearly corresponding to later determine
tions, Its attraction OF the new moon il about one
ninety.nth part greater than OF the full moon.
For 1t must be remembered tpat in reducing the
distance Of the sun from 060oO,000 to
o20000o0 miles astronomers have not re.
duced the moons distance by single
mile, that having been obtained from entirely
Independent observations and computations. 1t
Is evident that we cannot know with accur acy
how much the moons position is varied by the
attraction OF the sun In different parts OF Its orbit
until we know with accuracy the distance OF the
sun. Pursuing the opposite course, Hansen, from
the effect OF the attraction OF the sun upon the
moons position In different parts OF Its orbit,
came to the conclusion that the distance OF the
sun must be not far from ninety.two millions OF
miles, agreeing with that obtained from observa.
tions OF the parallax OF Mars and in other modes,
and now adopted In the american Nautical
Almanac"


1t ls upon the knowledge Of the exact position
OF the moon, predicted in the "Nautical Almanac"
three years In advance. that the navigator must
necessarily rely Tor safety from hidden rocks and
dangerous coasts. The increased safety in navi-
cation, not only with regard TO life, OF which we
think so little, but with regard to money, OF
which, as q commercial people, we think SO
much, will amply repay oil the expen.
dlture upon the transit expeditions Of
1874, and all that can be asked Tor In
1882, when we shall nave our turn Tor 1t Is q for.