Year: 1876




We may well hope fora clear sky all over the country
this evening for otherwise Ehall lose the oppor-
tunty, which may not again lifetime. of
witnessing the phenomenon under its most advan-
tageous circumstances of the occultaon of the princi
pal stars Of the Pleiades The above diagram repre-
sents the phenomenon at Oh 26h. Sos, whom 1t is
predicted that the bright star will disappear behind
the dark edge Of the The dark line diply SEEN
passing from the moons centre toward the opposite
OF the diagram represents the direction OF the
moons motion, and the stars in its path will success
sively disappear with suddenness which will be all
the startling II the eye fixed upon the and
the bright portion OF the kept OF the field
In order that the may not be too bright Is is
desirable that It should not be too the full On
the other hand. 10 order that the occuations
may be visible over large extent Of country It is
necessary that II should not be too the
new. Occurring in February have the happy mean
between these two extremes and the hour OF
day nicely adjusted that while tho Pacific
coast hour earlier would have caused the daylight
sadly to interfere, the Atlantic hour later
would have brought the moon the horizon that
the later contacts would have been lost. Let first
give some idea of the which induce astrono-
mers to look upon the occultatons Of the Pleiades
under such circumstances this evening with peculiar
interest


In navigation is necessary that the position Of the
ship should be known both IN longitude and In latitude
The latitude is easily obtained from the meridian at
tude Of any known star, because the direction Of the
Of the earth is fixed but the longitude obtained
with difficulty, the meridian OF Greenwich from which
the longitude is reckoned, well the meridian Of
tho observer, being in motion at the of about
l. 900 miles per hour. IL is easy to determine the
position of any meridian among the stars, which the
local time. But the difficulty is to determine any
othar mar dian what ie ha loca ima
The most accurate OF determining the longitude
of distant stations is by telegraph signals IT. Oh 0.
P. NJ, local time in New York a signal by the cable
indicates that is Oh Som. p N. at Greenwich know
that longitude is Al Som. west of Greenwich when,
as sea- there of direct signals, chron-
ometer is relied upon to give Greenwich time. If the
chronometer with accuracy, the sea captain
arriving New York. would find that his chronometer
Indicated Oh. 56m when the City Hall clock Indicated
5h. 0m- But no chronometer with perfect accu-
racy, and century ago they far less rea-
BILE than AS a check upon the chronometer the
changing place of the among the stars
has been generally resorted to. The motion
OF the moon being thirteen degrees per day
or more than half n degree hour.
captains observe 'sunar distances y the apparent dis
tance OF the from the larger fixed stars along its
path and obtain from the nautical almanac the Green
wch time at which the moon is predicted TO be at the
observed distance II is true that the moon would not
appear exactly the place among the the
time, as seen from two different stations, but
this difference from parallax, we'll as the smaller
difference from the refracton OF our atmosphere Is
allowed for the calculation. These corrections
duce the observed position Of the moon TO Its position
supposed TO be seen from the earths centre" We
should naturally suppose that from the carths
the could not be at all but this
preston, which has been made the subject Of much
criticism refers TO its position 1t would be from
any point On the line between the moon and the earths
centre


This method is not sufficiently accurate for the needs
OF astronomy and 1t Is common 10 employ the method
OF moon cuminatons, which gives the moons place
nearly as IL can DO measured in the telescope The
OF the moons passing the meridian is accurately
observed, and also that OF neghborng star From


the latter the local time is known verified and from
the former the local time al Greenwich may DO OBJ


tanned by computing at what Greenwich time the
would be in the observed position among the stars
great advantage Of this method that both the
and the observed. in the meridian, neither
refracton nor paraax affect their position in right as
censon and consequently no correction has 10 be ap
Piled reduce the observations 10 the esrths centre


Il is obvious that at the instant when star is oc-
Quilted by the its distance IRON the moons CEN
tire Is exactly one-half OF the moons diameter As a
bright disappears behind the dark limb OF tho
with suddenness IL is 10 obtain the distance
OF the from the star with much accuracywith
other instrument than sextant with which to OBJ
the local time, small spygass with which to
observe the occutatioD-as II be obtained with
the transit meridian circle Of the observatory by tho
method OF moon culmnations OSCILLATIONS OF stars
by the moon have, therefore, long been
means OF determining the longitude even OF fixed OBJ
servatores,


We're the perfect sphere perfectly regular
spheroid, probably occultatons would have retained
their pre-eminence, notwithstanding the additional
complication OF the computations. But when q
happens be obscured by lunar mountain before
tme when i happens TO descend into lunar valley
at the edge of the disk and remains visible too long in
either there is naccuracy the result In the


former there may be an uncertainty whether some
terrestrial cause may not have caused the too early
disappearance; but in the latter " the time Is
certainly known, can only refer the OF the
visibility Of the star 10 depression In the disk OF the


Ifthe moon always turned precisely the same face
toward US we might determine ON what paris Of the
moons limb the mountains OF the valleys 10 be
found, and make allowance for them, although II would
be slow process and require very many observations:
but the difficulty is immensely Increased almost beyond
the limits OF possb bility, when take n
sideration the bration Of The moon that the
tain which to-day occuts a star may tomorrow have
been carried either beyond the edge Of the disk
brought far toward not 10 project at all Fer
the motion Of the not like that OF the ball
representing in the orrery rigdly fastened 10 wire
compelled keep the face toward the earth
but the moon Is freely poised In space and appears TO
swing back and forth like the rolling Of ship This Is


apparent motion with respect 10 the earth arising
from varying orbita velocity, combined with per-


ecty uniform velocity Of rotation. When the moon is
near us and moving most rapidly, the western side
comes more and more into view and in the opposite
part It's orbit while moving slowly eastern side
comes more and more INTO view. Before corrections
be applied for the lunar mountains valleys their
exact positions and dimensions be ascertained,
and thon It must be also ascertaned precisely what
stage OF lbration they are upon the moons limb SO
hopeless has It seemed 10 undertake this that am not
aware that II has yet been attempted.


The chances of error much diminished when sew
era contacts are observed within a few hours and
especially when the moon passes centrally such
group Of stars as the Pleades The hastenine Of one
contact By mountain will be counerbaanced by the
delay Of another by depression that the average
result will be very nearly the II there'were
neither mountains nor depressions There are other
advantages scarcely less important Astronomy has
not YET reached such position that predictions be
as accurate as observations The may be at the


OF the oceultation itte further OF west than
vas predicted; u may DO little further north or
south y may be little further from US or nearer TO
u.s. varying its parallax and apparent semi
diameter and its motion In longitude IN lat-