The evening times., 1900-08-01

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will present the following phases: First
quarter on the Sd. full on the 10th last
quarter on the 18th, and new en- the 24th.
! On the 18th she will be in perigee and on
the 9th in apogee. The sun. in the course
or the month viii traverse an arc Of nearly
10 degrees north decliration, and be about
] S12 degrees distant from The equator at
the end of the month. On the last day 1t
viii rise and set little over II degrees
north of the line east and west points Of
the horizon Of Washington.
The planet Mercury during the greater
portion OF the month will be in the con-
steilation Cancer,i but toward the last in
| Leo. Venus throughout the whole month
viii BE in Gemihi and shine as a morning
/ star. She will rise between . and o'clock
in the morning.
! 1t has been mentioned once or twice be-
/ fore that Venus is at her maximum bright
ness neither when she is in inferior nor in
/ superior conjunction, since in the former
/ position too small portion of her disk IS
| visible. and in the latter position. though
1 Iull, sac subtends too small an angle She
assumes her greatest brilliance when her
I elongation is about 40 degrees-that is
about thirty-six days before and after inle-
| rior conjunction. In this favorable position
| she will be on the 14th of the month and
she will emit then such a brilliant light as
! to make II worth cne's while to rise early
In the morning and to have a look al her
At the period of her maximum bright
ness her light is so intense as to make
objects illumined by her cast pereeptibIe
shadows. The planet Mars. too. acts as a
morning star II is al present on the
| boundary line that separates the consteIla-
! tion Taurus and Gemini and rises a little
! past I O'Clock.
Jupiter an Evening Star.
The great planet Jupiter is at present
| the evening star par excellence. StiH ling
! ering in the summer constellation. Scor-
/ pio, this beautiful planet will be visible
until about 1890 o'clock in the evening
and unquestiOnably forms the most beau
! tiful object in the evening skies. East OF
Jupiter, in the constellation Sagittarius
! and near Ophiuchus. the planet Saturn viii
! not fail TO arrest the observers attention
/ enhancing by his presence the beauty Of
that portion of the sky. Saturn sets about
an hour after his brother planet Jupiter
Uranus is also in Scorpio. so that the
three great planeis-Uranus. Jupiter. and
Saturn-are near together, though the
/ great distance Of the first OF these renders
II very faint object for the unaided eve.
Not lar from Venus. also in The constella-
/ ton Gemini, is the planet Neptune, though
/ visible only through a telescope
! Many persons may recollect the trdnsits
OF Venus across the suns disk in the
years 1874 and 1852, which were predicted
] long before the phenomenon occurred. and
il has been calculated that the next two
! viii occur in the month Of June in the
years 200+ and 2Ol2. none happening in the
I twentieth century II may therefore be
I presumed that after the certainty with
/ which such predictions are invariably
I borne out by facts there is scarcely a Fer-
. son incredulous enough TO doubt the ac-
curacy Of such astronomical predictions
J based upon immutsble mathematical laws
and not mere speculation.
Now the question may arise in many a
I ones mind. what is necessary to render
prediction as 70 certain planetary positions
possible? This question is easily answered
In order TO find a planets place either in
the past or in the future at any given time
il is necessary 10 de ermine the planets
path around the sun or its orbit with the
I utmost eIactuess- This has been done with
! all the planets thus far known. Il cannot
/ but be Of great interest to know certain
! Quantities that must be given in order to
describe the orbit with precision. these
I quantities being known by the name of the
elements Of a planets orbit an express
| sion which many cannot fail to have heard
| often, especially in connection with com
/ cts. There are seven Of these elements.
| It may be presumed that nearly every
body knows that the planets DO not move
il circles around the sun. though some or
bits verge closely upon circles. but in an
oval closed curve called ellipse. and this
ellipsc is perfectly determined BY the
semi-majOr axis which is half its maxi
mum diameter. and the eccentricfty. which
is the ratio of the distance Of the focus
from the centre of the ellipse to the semi
major axis. These are the first two eie-
ments. The third is the inclination OF the
plane OF the planets orbit 10 the plane OF
the earth's orbit. the ecliptic The fourth
is what is called the longitude OF The as
cending node and is the angle which the
straight line that connects the sun with
the Vernal equinox in the plane of the
eCliptiC makes with the straight line that
conects the sun with the ascending node
in the plane Of the planets orbit The fifth
IS what IS technically called the longitude
, oI periheliOn and is simply the direction
in which the major axis Of the elliDse lies
/ on the plane Of the planets orbit The last
two are the periodic time and the epoch.
I the former being the period Of a revolution
of the planet about the sun and the Ia.-
[ ter the longitude Of the planet as seen
I from the sun at some given date usually
January /, 1850, or 1900. or else some pre-.
/ else date at which the planet passed the
perihelion or the node.
/ Planetary Perturbation.
How these different elements are deter
mined is, of course. the specific business Of
the professional astronomer. and any ex-
planation of the process would be entirely
out of place here but il certainly is useful
I to know what these clements are A uues-
Z tion-a very pertinent One-here obtrudes
/ itself upon the mind. namely, Whether the
planets do not disturb each other in such
| a degree as to change their elliptical mo-
tiOn around the SUM- The subject OF plan
etary perturbation. as these mutual dis
turbanees are called, is intrinsically very
| interesting one.
The central body that attracts all the
planets and causes them TO revolve around
| IL is the sun. and in regard to the above
question of mutual perturbation everything
depends upon the ratio Of the planetary
! attraction to that of the sun Now in no
case is the mass of the disturbing body
! as great as one -thOusandth of the suns
mass. in consequence of which the disturb
ing force arising from planetary attraction
is never more than a very small fraction
! OF the suns attraction. With the excep-
tion Of some of the minor planets the
greatest disturbing force in the planetary
system is that Of the two biggest planets
Jupiter and Saturn-at the time when
these are nearest to each other. In this
case ii amounts to a little less than the
one -hundredth part of the suns attraction.
/ tance from each other IT amounts only to
! somewhat less than the three -hundredth
part 1t may be understood then that the
] n a1a -l.s..s