Pittsburg dispatch. [volume] (Pittsburg [Pa.]) 1880-1923, 1891-01-11

Mr.. Common and Mr.. Isaac Roberts, Of
Liverpool, have brought the art Of photo
graphing nebule to Q marvellous state Of
advancement, as they have gained exper-
ience, particularly as to the time necessary
for making the exposure.
In the case of some Of the photographs of
nebul@ recently taken by thsse amateurs the
plates were exposed 8, and even 8 hours,
and details of structure were brought out
which no eye is able to detect in the objects
themselves, and which NO eye, probably,
ever will be able to detect. The advantage
which the sensitive plate Of the camera has
over the human retins is that the former il
capable of receiving and retaining the i'm
preston of the faintest trace of light which
falls upon it, provided 1t il given time,
whereas the eve must see ut once or not ct
all, since it becomes weary and loses its SEN
sitiveness with long gazing.
In the star Sirius we have another object
to which good deal Of attention has been
given by astronomers in recent years. Its
extraordinary brilliancy has caused 1t nat
urally to be selected for the first expert
ments aiming at an advance step in sideyeal
astronomy and the items of informa alton that
have been gathered with respect to this star
may be cited to illustrate the present state
Of our knowledge regarding these almost
infinitely remote bodies
Unit Of Star Distances.
Although not the nearest of the fxed
stars, Sirius is among the nearest. Its dis
tance from us now is COMMONLY put at about
600, 900 times that of the sun. Let rs take q
moment to consider what this means. Or
dinary units of measurement fail us in an
emergency like this; new unit has been
adopted for measuring star distances-viz,
the distance travelled by light in one second
OF time-186, 900 miles. To reach Us from
the sun light requires little over eight
minutes; it flashes across the space inter
vening between rs and the moon In about
one and quarter seconds; to reach Us from
Sirius, assuming that the distance given
above is correct, itrequires nearly ten years.
That is when we look upon Sirius tonight
we see it, not as it is now, but a. 1t was ten
years ago.
1t has been estimated that the Light Of
Sirius is equal to that of 70 suns placed
side by side. If its intrinsic brilliancy is
the same as that of the sun, It may easily be
calculated that its diameter il rather more
than eight times that of the sun. There are,
however. reasons for thinking that Sirius IL
really hotter and brighter than our lumin-
ary, in which case the estimate of it, size
must be correspondingly diminished.
The Motion Of Sirius.
Within the past few years the spectroscope
has been applied to perform the astonishing
feat of determining the rate of stars
motion in direction toward or away from
the earth. Sirius is the first star to which
the spectroscope was turned for this pur-
pose, by DR.. Higgins, vino confidently an
nounced that the star was receding from rs
at the rate of 20 miles q second. Other OBJ
servers have confirmed the conclusion of
this distinguished spectroscopist as to the
fact that the star is moving along the line of
sight, but its motion has been found to vary
both in its rate and in its direction in .
manner thus far unexplained.
Mercury will be an evening star during
the first half of the month, and during the
second halfla morning star, but too near the
sun to be visible. Venus Is now . morning
star. Its greatest brilliancy was on the 9th
of the month. at which time, seen through q
telescope, it presented the crescent phase of
the waning moon. The Comptes Rsndues
for October II, 1890, contains an interesting
illustrated note by NJ. Perrotin, on "Obser- |
vations Of the Planet Venus at the observa-
tory at Nice" made with Q view Of deter
mining the period Of the planets rotation
on its axis. N. Perrotin confirms the con-
clusion recently announced BY N. Schia-
parelli, of Milan. that Venus turns on its
axis in very nearly, if not exactly, the sams
time that it takes to perform its revolution |
n its orbit-viz., about 225 days
Eternal Day and Eternal Night
That ID Venus always keeps the same side
turned toward the sun, just - the moon
does toward the earth, and just a. Mercury,
according to NJ. Schiaparelli, does toward |
the sun. On one side Of this planet there is,
q acontinuous day and on the other |
side eternaI NIGHT Mars In still struo0lino
to keep pace with the earth; but, although |
now long way in advance Of Jupiter, be |
has lost ground very perceptibly In the past |
month, and now sets ct about 8 o'clock. |
Jupiter has practically closed his present |
season. He IL visible but for . short time |
after sunset. g. 8. JoNHs.