Omaha daily bee., 1896-11-08

the tail of the comet, as II Is depicted on
the plates, is three times as extensive as the
tail OF the same body as 1t is displayed
through a telescope.
An interesting comet, which has afforded
much occupation to the photographer, was
discovered on July s, 1868, by Alfred Ror-
dame, an astronomer residing in Salt Lake
City. w. J. Hussey obtained some admirable
photographs of this object at the Lick OBJ
servatory. and we are also indebted to the
same astronomer for very interesting ac-
count Of the physical characteristics of this
body.
THE NEW RoRDAME COMET.
On looking at the photograph of the comet
Rordame, on July -, and comparing 1t with
that taken on the following night, the OBJ
server will be astonished at the difference
in the structure OF the two tails. II would
seem as II some violent dislocation of the
material of the tail must have taken place
in the interval which has elapsed between
the times when the two pictures were taken.
There is no doubt that visual observation
would never have established this point so
clearly the photographs have done
1t will be noticed that the plates are
marked over by numbers of bright streaks;
these are the photographs of the stars which
happened to lie In the same field OF view as
the comet. But IL may well be asked how
it has come to pass that the stars are rep
resented by streaks instead of the round
images, which we should expect from their
sun-like character. The explanation of this
circumstance is not a little curious and in
structive. The comet is in motion, and 1t
moves so rapidly that in the course of such
a protracted exposure as that on July H.
which lasted for one hour and twelve min
utes, the comet changes its position on the
sky through a distance which is quite APP
parent If the camera had been directed
throughout the exposure to the same part
of the heavens the comet, like the unquiet
sitter, would only have permitted us to OBJ
tain a very blurred and indistinct portrait
To obviate the effects of this motion 1t was
therefore necessary for the astronomer who
was engaged in taking the picture to shift
the camera slowly during the course of the
exposure and in that way to neutralize the
influence of the comets motion. The picture
is thus made to represent the comet as If
that body had remained at rest during the
exposure But the stars which were strewn
over the background remained quiet all the
time: as, however, the camera was shifted,
for the reason just mentioned, 1t follows that
each of the stars, instead of being repre-
scented by point as II would have been in
an ordinary sideral picture Is manifested
in this plate by streak
HOW TO READ THE STREAKS.
Such streaks, if useless as stellar pictures,
are, nevertheless, very instructive. They
reveal to us the nature and the extent Of
which the exposure of the streak expresses
the apparent distance through which the
comet has moved while the direction of the
streak indicates the direction in which the
comet is moving. At first sight this latter
circumstance may appear somewhat puzzling.
We are accustomed to see a shower OF sparks
extending out behind skyrocket and this
tail to the rocket follows generally the track
along which theorocket has Itself advanced.
But the tail of comet bears relation to
the comet very different from that which the
stream OF sparks behind rocket bears to the
rocket itself The position OF the comets
tail Is governed by the remarkable law that
It must be turned away from the sun. In
fact, It would generally be found that line
drawn through the comet from the tail TO
the head would, when continued around the
heavens, point to the sun.
In the present case IL Is plain, from the
position OF the star tracks, that the motion
OF the comet happens TO be almost perpen-
dicular to the direction OF its tail. Consid-
ering the very flimsy character OF the tail
of comet II may seem rather surprising
that this structure should not be swept BACK
ward so as to stream out along the comets
track. Especially Is this the case when we
think OF the enormous velocity at when the
object is moving. II must, however, be re
membered that no atmosphere exists In the
open space through which the comet wings
its flight There would be no other medium
to offer any resistance to the flight OF the
tail, and therefore there is no difhculty In
explaining how the object moves sideways