Evening star. [volume] (Washington, D.C.) 1854-1972, 1894-11-09
The Planet Will Cross the Face of
the Sun Tomorrow.
GREAT PREPARATIONS FOR THE EVENT
Its Importance in Testing the
Earth's DiumaI Rotation.
WHAT PROF. HARDNESS SAYS

or the United states, ana In w aSnlnS Ton
the instants of the beginning and ending of
the transit, expressed in standard time, are
1855 am and APP pm.
The transit of Mercury Is regarded as an
astronomical event of some considerable
importance, and elaborate preparations have
been made at the new naval observatory to
observe the transit and record its features
for future study and comparison. Pho
Biography will, of course, be utilized to the
fullest extent in this work, and great deal
can be accomplished in these days OF i'm
proved methods and equipment that were
impossible few years ago. On the grounds
of the observatory, and just to the south of
the main building a photoheliograph has
lust been constructed, and workmen were
busy all today putting the finishing touches
the rather complicated structure, work
on which has been delayed somewhat by
the In clement weather of the past few
days.
The Last of the Century.
Mercury's transits across the sun are not
of infrequent occurrence but the one to-
morrow will be the last one OF the present
century, and the astronomers are naturally
hoping that the conditions will be favorable
for complete observation. Its accurate
observation is of great Importance, partly
for the sake of improving the clements or
Mercury, and partly because 1t is one or
the few phenomena available Tor testing the
uniformity of the earthS diurnal rotation.
For this reason the officials at the obSerV-
atory some time ago issued pamphlet to
astronomers all over the country ASKING
them to make special efforts on this oc-
casion and to notify the observatory peo-
pie here of the results OF their work. In
StructionS as to the necessary equipment
and methods to be followed in recoiulnG
observations were embodied in this circular,
IL bot IN hn1n m
nished by amateurs in different parts of the
United States may be OF some considerable
service. The chief requisite for the obserV-
er is a suitably mounted telescope. having
a clear aperture Of three inches or more,
and @ magnifying power of nDt less than
ninety diameters. Observations with in
struments of less power Will, of course, be
less valuable, but may be none the less in
tereSting to the student and amateur.
All the four contacts of Mercury with the
suns disk will be visible throughout the
United States except in Maine and the
northeastern parts Of New Hampshire and
Vermont, where the sun will set before the
third and fourth contacts, and IN Alaska,
where the first and second contacts viii oc-
cur before sunrise. In the northeastern
states the sun will be low at the end of the
transit, and in Alaska it will be low at the
beginning, but in all other parts Of the
country the conditions will be favorable for
observing the whole transit.
What Prof. Hardness Says.
When a Star reporter called at the OBJ
servatory this morning Prof. darkness, who
is in charge of The purely astronomical
work of the observatory, In distinction from
the naval almanac work. etc, was hard at
work experimenting the worlImen who were
finishing up the photohelioSrlph. He said
that he hoped to have everything in rcadi-
ness for tomorrow, but that IL would be d
hard day s work and with no room for de-
lay. notwithstanding its imposing name
the photohelioSraph is not such a tremen-
dousiy complicated arrangement after all.
Running north from the new building that
is 10 be used permanently as a Photographic
mouse Is hollow tube protected by the
little ShnSle roof, and at the end of this
tube is hXed the object glass with a focus
OF thlrty-elsht feet. The sensitive plate
which will be used to photograph the transit
across the lace OF the sun, is placed wilhin
the Photographic house, in a room that is
made totally dark. In other words, the
room Itself is the camera, and the opera tor
will be inside of it. The plate will be
mounted on a frame directly i'll front of the
mouth OF the tube where il opens into the
room, and this mouth will be closed by a
sliding shutter, which will be worked by a
the operator. Just beyond the object glass
a helostat, operated by cIockwOrk, viii
keep the reflection of the sun constantly In
line in the tube and thrown upon the sensi-
tive plate, so that photographs can be talk
EN- from the instant of the beginning or first
contact until the close. Anywhere from 100
will be placed afterward in measuring
machine in the main building, and, as each
exposure will be accurately timed, the dis
lance of the planet from the center of the
sun and from its outer edge can be cSti-
mated at any time to the nth decimal OF
an inch. The shutter of the camera Is so
arranged that exactly al the instant of the
exposure spring Is touched, which is con-
nected by an electric wire with a chrono-
graph in an adjoining building, SO that the
time of each plate VIII be registered beyond
the possibility of any appreciable error
This is, of course, only the crudest de-
SCrlptiOn of the instrument that Will be
used, and all sorts of appliances Will be
utilized as auXiliaries, in the way of trans
its and other machinery, for determining
azimuth and other necessary clemens.
Among the most important matters to be
observed will be the four contacts of the
planet with the sun, the instant when the
nrst edge of the planet touches the outer
edge of the sun and the instant when it has
gotten fairly across the outer edge, the in
Stant when the first edge of the planet
reaches the other side and when II finally
energies and parts company.
The Most Important Matter.
Perhaps the most important matter on
which this transit of Mercury throws any
light is the rotation Of the earth in its orbit.
1t has long been known that there was some
uncertainty in the earth's running. Either
the moon or the earth is wrong, but certain
1t Is that they 90 not keep time together,
and the motion of Mercury can be utilized
to show which is presumably the nearer
right. AS Prof. Hardness expressed it to
a Star reporter this morning, it is as If a
person had two clocks which were running
almost in harmony, but yet not quite, and
he had third which would warrant some
presumption as TO which Is the better time
keeper There il really very little that is
not known in this connection, for the i'm
provement in methods and instruments
have cleared up a great deal that was
formerly in dot bt, but no one supposeS that
everything will ever be known to an absolute
certainty with reference to the heavenly
bodies New and improved instruments
simply discover differences and disagree
ments that were never suspected before.
The Thing Ia he Observed.
In Prof. Hardness circular with refer
ence to the transit tomorrow is contained
the following, which is of general interest:
THE thing to be observed Is the exact in
slant when the limbs of Mercury and the
sun come into contact. For the first con-
tact all that can be done is to note the in
slant when Mercury first makes a percep-
tbla indentation of the suns limb, and as
the indentatiOr must acquire a certain
magnit Lde before it can attract attention
the instant recorded Is almost necessarily
a little late. For the second and third con-
tacts two methods of observing have been
extensively practiced. The first consists In
guessing al the instant of contact by watch