Richmond dispatch. [volume] (Richmond, Va.) 1884-1903, 1898-12-04


HORIZONTAL circle, and upon which the
azimuth can be read
The usefulness Of the altazimuth will
De seen at once. With the transit circle
any particular object can only be OBJ
served as II crosses the meridian. If the
weather should be cloudy, or the observer
late. the chance of observation is lost
Tor four and twenty hours. and, in the
case OF the moon. for which the altazi-
muth is specially used, it is on the me-
ndian only in broad daylight during that
part or the month which Immediately pre-
cedes and follows new moon. At such
times IL is practically impossible to OBJ
serve it with a transit circle; with the al
tazimuth it may be caught in the twilight
before sunrise or after sunset; and at other
times in the month. If lost on the meridian
in the transit circle. the altazimuth still
gives the observer a chance OF catching
1t any time before IL sets. But for this
instrument observations of the moon
would have been practically impossible
over at least one-fourth of its orbit.
There is quite another way Of determi7
ming the place OF the moon. which is some
times available. and which offers one of
the prettiest OF observations TO the as
tronomer. AS the moon travels across
the sky moving amongst the stars from
west TO east, it necessarily passes in from
Of some Of them, and hides them from u
for q time. Such a passage, or "occuta,
ton, offers two observations+the "disap-
pearance," as the moon comes. up 10 the
star and covers It; the temperance.
as it leaves IL again, and so discloses II.
Most striking is a disappearance or re-
appearance at the dark limb" In this
case the limb of the moon is absolutely
invisible, and II may be that no part Of
the moon is visible In the field of the tele-
scope. In this case the observer sees
a star shining brightly and alone in the
middle OF the field Of his telescope. He
takes the time from his faithful clock,
counting beat after beat, when suddenly
the star is gone! So sudden is The dis
appearance that the novice feels almost
as astonished as If he had received a slap
in the face and not unfrequently he loses
all count or recollection Of the clock beats.
The temperance Of the dark limb is
quite as startling; with bright star IT
is almost as If a shell had burst in his
very face and II would require no very
great Imagination to make him think that
he had heard the explosion. One moment
nothing was visible; now a great star is
shining down serenely on the watcher.
A little practice soon erables the observer
to accustom himself TO these effects, and
an old hand finds no more difficulty in
observing an occultation of any kind than
in taking a transit.
It may be asked why these different
modes OF observing the moon are still
kept up, year in and year out. do we
not know the moons orbit sufficiently
well, especially since the discovery OF
gravitation?" No, we do not. This sim
pie and beautiful law gives rise to the
most amazing complexity Of calculation
If the earth and moon were the only two
III .aftlA
be a simple one But the earth, sun, and
moon are members of triple system,
each of which is always acting on both
of the others. More. the planets, too,
have an appeciable influence, and the net
result is a problem so intricate that our
very greatest mathematicians have not
thoroughly worked ii out. Our calcuia-
tions of the moons motions need, there
fore. to be continually compared with
observation-need even TO be continually
corrected by 1t.
There is a further reason for this con-
tinual observation--not only Of the sun,
which is our great standard star, since
from IL we derive the right ascensions OF
the stars: and ii is. also. our great time
keeper-not only of the moon, but also or
the planets Their places as computed
need continually to be compared with
their places as observed, and the discord-
ances, II any. inquired into. The great
triumph which resulted 10 science from
following this course-to pure SCIENCE
since Uranus is too faint planet to be
any help TO the sailor in navigation-is
well-known. The observed movements
~ Uranus proved not 20 be in accord with
computation. and from the discordances
between calculation and observation
Adams and Leverrier were able 10 predl-
cate the existence Of hitherto unseen
planet beyond, 20 see II as Columbus saw
America from Spain It's movements were
felt by them trembling along the far-
reaching line OF their analysis, with
certainty hardly inferior to that OF ocular
demonstration
In 1915 the largest telescope availabie
at Greenwich was an equatorial or only
6 "a inches aperture, provided with smaii
and insufficient circles for determining
nositions. and housed in very small and
announcement dome. II was not creditable
10 the nation that the Royal Observatory
sholld be SO ill-provided with powerful
telescopes. and few years later AIRY Cd
tanned the sanction OF the government TOT
the erection of a larger equatorial in a
much more ample dome. This was Tor
thirty-four years the great or SOUTH
east equatorial, and the mounting sun
remains and bears the old name, though
the original teltSCOpc nus bSNh AN"N' -
elsewhere. The object glass had an apert-
of 10 "a inches, and a focal length OF -
feet The mounting was so massive ana
stable that the present Astronomer Royal
had found it quite practicable and safe
10 place upon II a telescope OF many times
the weight, Of % inches aperture, and 70
feet focal length-the largest refractor
in the British empire though surpassed
nv several OF your American instruments.
The stability OF the mounting rendered
the telescope suitable for special work
rhis was the observation OF minor
planets" on the first day OF the present
century tho first Of these little bodies
was discovered by Piazzi at Palermo
Three more were discovered at no great
interval afterwards. and there are inter-
walls OF ihirty-eight years without any al
dition TO their number. But from De-
ce,uber s, l8i5, up 10 the present time the
work OF picking up fresh individuals OF
these pocket planets has gone on without
interruption, until now more than 400 are
known. Most Of these are OF no interest
10 US. but a few come sufficiently near
10 the earth for their distance to be very