Evening star. [volume] (Washington, D.C.) 1854-1972, 1891-05-07

| face should be protected BY covering glass Of
the same size as the original slip and separated
from it by bits of thin card inserted at the
ends, the whole being bound together by .
narrow strip of paper pasted around the edges
| Error of Timepiece.-Most observers will
probably have no other timepiece than Q good
POCKET watch and no means of determining its
| error except the telegraphic noon signals. To
utilize these signals the watch should be car
| ried to the telegraph Office Oh May z, aud g.
| and on each OF these days the hour, minute and
| second indicated by it when the signal is re-
ceived should be noted. Before May .,
care must be taken to see that when the sec
ono hand points to 60 the minute hand il ex-
acfiy over minute mark; and in noting
signals If the seconds are near 60 special care
| will be required to avoid recording q wrong
| minute. For example, when the time 10 min
| utes o. seconds the minute hand will be almost
exactly over minutes, and special care must
| be taken not to record minutes 50 seconds in
| stead Of minutes So seconds. Previous practice
| In calling and noting time will greatly dimin-
| lsh the risk OF such mistakes. No matter how
| the watch may run it must not be meddled
with in any way, either by moving its hands or
| touching its regulator, between May and %.
| without this precaution neither its rate nor
the degree of dependence to be placed upon il
| can be determined.
| CIRCUMSTANCES OF TEE TRANSIT IN THE UNITED
sTATEs.
| The first and second contacts Of Mercury
with the SUMS disk will be visible throughout
the United States, except to the eastward Of
line passing near New York city and the
| eastern coast of Florida. In the neighbor-
hood Of that line, ingress will occur
30 near sunset that opportunities for
making good observations can scarcely
be expected. To the westward the sun will be
higher, and on the Pacific coast nearly the
whole transi. will be visible, but Alaska is the
only part Of the United States where the third
and fourth contacts can be observed.
The Greenwich mean times of the first and
second contacts are figured out at the OBSERVE
tory as follows
LIS deg.-lst contact. 11h. 52m; I'd, 11h. SIM.
L30 deg.-1st contact 11h. 52m; I'd, 1lh. SIM.
LAO deg.-1st contact, lib, 53ln; I'd, Ilh. Ssm.
L5o deg.-lst contact. 1lh, 53m I'd, l1h, 5sm.
Recent observations of Mercury. made at this
observatory, seem to indicate that these times
are somewhat too late and it may be well to
commence looking for one contact q minute or
two before the predicted time.
10 turn the Greenwich mean times into
eastern standard time subtract five hours
For example. the latitude Of Washington is ap-
proximately 40 degrees, and by subtracting 5h
from the above times we find: First contact
oh. 52m. 39s. Second coutsct-5h. Sim. 37s
In Washington.
III we imagine line drawn from the center
OF the sun to the north pole of the heavens
and designate the point where that line cuts
the suns limb as the SUMS north point, then any
point on the suns limb may be defined by its
angle east or west OF the north point. In the
transit under consideration, the first contact
OF Mercury with the suns limb will occur at
point lis degrees east Of the north point. or
more simply, If the observer imagines the disk
OF the sun to be figured like the face of watch.
the MI being at the north point. then the frst
contact will occur at the figure VIII. That,
however. is for the direct image. II an invert-
ing telescope is used the observer must imagine
the figure vi to be at the apparent north point
and then the contact will occur at the figure
VIII, as before.
METHOD OF OBSERVING.
Atmospheric disturbances, diffraction and
other causes produce mara q. Lac. ;,a;..;-..
nae. i, .ho ;,,ar,os :, 49,. .u.'''
Ao' ~ .n"e' VI neioury anu inc sun ana
renaer " difneuit TO determine the exact in
s.ants Oh tne contacts; Among these phe-
n I 'hat khown as the black drop Is famous.
II occurs near the tunes OF second and third
CONTACTS, wnen the planet Is almost wholly within
Me sun aisk, ana rs caused by the cusps OF
sunlight becoming blunted, and thus giving
mercury pear-shaped appearance, as if a
olack crop were chnSing to IT'S side. A first
rate teiescoPe, carefully adjusted to focus and
Pr0Nioeo with sufnciently dense shade glasses,
wiii 90 much to minimize these appearances,
but nothing can entirely overcome the effects
OF bad condition OF the atmosphere.
The thing to be observed is the exact instant
when the limbs OF mercury and the sun come
into contact For the hrst contact all that can
be done Is to note the instant when mercury
nrst makes perceptible indentation in the
suns limb; and, as the indentation must
acquire certain magnitude before 1t can
attract attention, the instant recorded is almost
necessarily little late For the second con-
tact, two methods Of observing have been ex-
!!ensi.eiy prac.ced. Inc hrst consists in guess
d;s,o q. s s ,ns =. con.ac. watching the
to estimate when thew wopid hsv .au ua
true geometrical figure. The second Consists
in noting the earliest instant at which sun
light certainly flashes across the interval be.
tween the two cusps an thew close aronna .h~
planet That is Comparatively ds,nits
phenomenon and the axner;anna ~, lei a.
transit Of NAV 1870 shpw. H.. =. ~ VII - VIC
served with lar creater secnrs., t,'s,. az. ~ w
called oeomatr;cs5 cantae. Nay assI sae 20
motion Of the planet T. so slow tba, sn nn sus
ta;nt, - ,haa 80 49, la. an uncer
t sAinv VI .nree or Tour SECONDS attacnes 70 the