The morning call., 1890-06-08
AN AID TO
NAVIGATION.
A Photographic Chart of the
Heavens Being Prepared.
Interesting Paper by the Professor Of Astron.
omy, Williams College, Mass, on
Meridian Observations.

are in any manner interested In scientific theory
and speculation, Is now In progress Of compile.
tlon and the results to bo obta ined Will be
awaited with interest. Truman Henry Stafford,
professor Of astronomy, Williams College, Mas
sachusetts, gives very satisfactory statement OF
the work, and what will be accomplished in q
paper written for the May number Of the Side
real Messenger, In which he says:
The Photographic chart Of the heavens, which
Is to be made by about twenty observatOries,
will be accompanied by set Of plates of short
exposure for catalogue purposes. The plates
will be two degrees square, SO that each one will
contain on the average about twenty Of the stars
determined by the great zones now In progress,
and this number will be more than sufficient In
many cases. But there will be some plates for
which additional stars will be needed around the
edges. Consequently 1t may be necessary to OBJ
serve a catalogue OF about 10000 stars in addi-
tion TO the 100000 OF the great zones (if, as is
probable, they are continued to the South Pole),
and to observe these 90 900 stars accurately meri
dian circles OF the largest size must be employed.
When the Photographic chart is completed 1t will
be necessary 10 redetermine the stars of the
great zones. especially in the northern hem
isphere. The necessary accuracy will, perhaps,
be greater than that Of the zones themselves:
The mean epoch of the latter is 1876. and that Of
the Photographic chart will be about 1895.
II nee, ii the great zones are repeated about
series, The positions for 1893 viii be- adyanIa.
geously maintained by Interpolation.
This repetition OF the great zones about 1905
that Is TO say, between 1900 and 9l0 will be,
also, most readily done by photography. To ac-
complish this will require about 10000 plates
q degrees sauare) for the whole heavens. With
Photographic telescope OF 13 inches aperture
only one minutes exposure TO each plate, or with
one of inches aperture, certainly no more than
three minutes exposure will be required. AL
lowing as much for time employed In setting the
telescope. we shall have six minutes observing
time for an average Of 10 stars. Or for the 5000
plates of either hemisphere 500 hours photo
graphic WORK, which can be readily accomplished
in two years by Two obseryatores.
After this has been done it remains TO measure
the plate co0rdiDates at leisure and convert them
into right ascension and decination. In the
complete survey OF the heavens down 10 the
eleventh and fourteenth magnitudes the plates
will be regularly arranged Near the equator
each plate will begin eight minutes further on the
right ascension than the preceding one; or at
least some equally systematic order will be EN-.
ployed. But In making short exposures for
ninth magnitude stars only, 1t will be better to
Increase The number at plates by about 80 to 40
per Cent, say 10 1000 for each hemisphere, so as
TO bring the brighter stars near The preceding
and following edges. and thus use them for zero
points. 1t has been found by trial from the
DurchmusIerung that in almost all parts OF tile
heavens the stars down to the eighth magnitude
inclusive (S.4 OF S.5 OF the ID. NJ scae), are suft-
ciOnt in number and sufficiently well arranged to
make this possible, provided they are very aceu-
rately determined
The conclusion from this Is that meridian OBJ
servations of stars fainter than the eighth mag
nitude need only De made UOr the next few years.
for special problems ike proper motion ana for
and that those down to the eighth magnitude
only (S.4 or SO d. NJ will be The proper work
for meridian circles IN future. OF such stars
there are considerably more than $4,000 in the
Northern Hemisphere, ana they should De OBJ.
served each four times before 1910. This can
easily be done by three meridian circles.
IL would seem, then, that up to 1910 there is
no very great need of so great masses OF meridian
observations as would have been required with
out photography. This latter art will relieve
meridian observers OF the most irksome part OF
their duties, the repetition over and over again
of the same rather lnonotonous observations.
The only real problems OF this kind for he future
will be to keep the catalogues OF stars TO the
eighth magnitude IN proper order so as TO nave
accurate places always available, and TO observe
fainter stars on the meridian only when they are
especially interesting.
The comparison stars for comets and asteroids
Will often be most readily determined by meas
urDg them from very few Photographic plates
along the course of the body, unless, indeed,
eighth magnitude stars are available, and much
time Will be saved In the work Of completely
observing all the stars employed for any particu-
lar comet.
In other words, as the Photographic process
of cataOgulng stars becomes more and more
systemtaiZed, many Of the difficulties of meridian
work VIII disappear. The zero points OF the
plates which viii be furnished by direct observa-
tion iwlll be more regularly distributed in time;
Will not so often conflict with each other that the
observer must wait years before lie can finish his
zone; and we shall have fewer star catalogues
thirty or forty years In progress. But, on the
other hand, the accuracy obtained on the small
scale by photograph, an accuracy SO great as to
compete even with heliometer work, will require
corresponding precision in the use of fixed in
strumentS-
There are about 100 sets OF frst-class meridian
Instruments In active service; number of these
are In need Of extensive repair and reconstruc-
tion 10 make them adequate to the most accurate
work.
The subject OF systematic differences between
results needs especial study at this time; 1t Is
hardly worth while 10 60 on repeating observa-
lions which have been lately made elsewhere
without such study. And the prob ble error
of the single observation should not now exceed
n second n
coordinate; nor should any iirst-raie observer
be cont ent for example, with series of declina.
tions In which discrepancies Of 2" from the mean
OF or 10 are al all common.
With regard TO the plans OF future meridian
work, tile stars OF the sixth magnitude and
brighter need now TO be observed only for zero
points and for comparisons; otherwise they need
discussion OF all available materials rather tban
reobservaton. The Greenwich ten-year cata-
logue, just out. contains a great many of them,
and the lacking ones are now ON the Greenwich
working list. The stars OF the seventh magni.:
tude are very frequently used for variety oil
purposes, both with small Instruments aud
large; a tbree-nch transit Is large enough for
the accurate observation OF star OF the ma gnl.
tude IA and two-inch for one 60. ln
fact the best stars for 212 Inch tranSt-the
size most commonly used 10 longitude and lati-
tude work-are precisely those brighter sixth
and higher seventh mSgnitudeS whose exact
places are hardest TO find in the catalogues
Any meridian observer who wishes 10 avoid
throwing away his labor can be very sure TO do
a good thing by observing seventll-magntude
Stars In some definite region with great accur-
acy; Tour observations 10 each OBJECT are usu-
ally sufficient These stars are now Of some I'M
pOrtance because they are In many cases well
determined at an early date. Bradley has few
Of them. but dAgelet, Fedorenko, Piazzi, La
land or Gioombridge great many. Those OF
the eighth magnitude will especially be needed
after 1900.
1t will be seen that photography will practic-
ally limit the great mass of future meridian OBJ
servatiUns-after the plans now In progress are
executed Ito stars of the eighth magnitude and
brighter that observers should in future contne
themselves TO making thoroughly good obserya-
ti0ns, strictly dlfTeientlal In character, unless
they working intellIgently upon the funda-
mental catalogues; that in all probability good
many of the older instruments Will be quietly
hung up and disused; but that the best and most
modern circles like those OF Repsold) Will long
aid in The solution OF important problems.
III giving this paper to the public the language
Is almost Identical with that employed by Pro-
fessOr Stafford. In no Instance has single idea
been changed. He isvery anxious 10 ascertain
the views OF younger observers and solicits Cor
respondence with reference 10 the matter.