Evening star. [volume] (Washington, D.C.) 1854-1972, 1882-12-04

METHODS or OBSERVATION USED On OCCASION
OF FORMER TRANSITs &c.
The approaching transit of Venus was the
subject Of very interesting lecture. deliver cd
by Liest. +. ". Sturdy. L. S y. in charge Of
the Naut,.-.l Almanac Office. before the Carroll
institute n I Thars lay evening. The lecture
was illust-itad with sketches and d.arrans.
which g2yt 3 c!ttT idea Of the work befor. the
astronone-s. v-ho VIII take part in the observa-
tions- If Ee3aesday should be clear day. he
said and o'c took the trouble to point small
telescope az a. sun. between nine O'Clock in the
morning and three in tn afternoon. he will see
q small round shallow passing over the face of
the sun from let to right because then the
planet Venus will pass directly between the
earIh and the sun. He referred to the unusual
character Of the phenomenon and the Great
value which astronomers attach to observations
OF it Astronomers are now stationed at many
and widely sta1t points. provided each with
all the appliances required for the most refined
observations. and Il num5cr Of assistants.
WHAT IL Is HOPED To OBTAIN
These preparations have been made in the
hope OF obtaining exact kn0Vlcdr ~ OF the dis
lance from the earth to the sun. While the in
yestigatiOns of modern mathcmatic.ans have
enabled Us TO predict with certainty the move
meats OF celestial bodies. their dimensions and
distances are not known with the same pre-
cision- The relative distances of all the
planets have been determined. One of the
methods OF attacking this problem is that
known as the indirect kcpler's third law is
that the square OF the time OF the revolution OF
each planet il proportional 70 the cube OF its
mean distance from the sun. Multiply the
Gr -
the sun by the same number and that gives the
hrst number. Then ind 4 second number
which multiplied by Itself iyice gives the
first number. This second III her is the dis
lance from the planet To the sun. the earth's
distance being called one. JU itcr taxes about
eleven years to go around he sun. Eleven
multiplied by eleven gives 12l Now by multi
plying five by five have p and 10y again
multiplying by i've we have 12j. which is very
nearly the same as l21. Hence ii is said that
Jupiter is about five times as far from the sun
Is the earth. BY applying these methods to ali
the planets. uSin-. of course exact figures their
orbits can be laid down upon plane kll we
shall then have to find is the scale upon which
The plane is drawn. If We knew the distance
from the earth To the san. or of any of the plan
cts. We would have the scale Various methods
have been employed to obtain
TIE UNIT OF THIS SCALE
but the one which makes use OF the transit of
Venus is considered a. the most correct. though
that Depending upon the velocity of light has
had no little prominence. and experiments are
still carried on by this method. 1t has been
found that il takes a sensible time for li-!lt to
travel through space. When one of JGpiter's
satellites is eclipsed. whea the planet is on the
side of the sun. away from the earth 3S min
utes-the time taken for light to pass from Ju-
pter to the earth-tiapSes before the eclipse
becomes visible here. Now. when Jupiter is on
the same side OF the su1 a. TIE earth the dis
lance is Sh1Ttt: by the Whole diameter OF the
earth's orbit. ana the difference in time gives
us about eight minutes Is TIE time required for
light 10 pass from tn" sun To the earth. When
the exact velocity Of li-sht has been determined
We shall have the means Of measuring the d.S-
tance between the sun and the planets The
object i'll view in making the observations of the
transit OF Venus. howtyts. is tine determination
Oh the solar parallax-
WHAT PARALLAX Is.
ParallaX is the difference between the direc-
lions of body as seen from two different
points. The lecturer to more clearly and prac-
tically Illustrate paTallay. supposed q street to
run east and west. II man stood on the pave
ment in front of Carroll hall anil looked at a
lamp placed on the same side. ct the corner of
10th street. il would appear in a direction west:
but ii he crossed the street it would be a little
south of west. aud that difference in direction
would represent the ParailaX. Il surveyor by
measuring the distance across the street and
the exact direction. would have sufficient
data for obtaining the distance of the lamp.
Suppose the lamp was placed as far uD the
street as 10Th street. the surveyor would pro-
coed III the same way The angle in the latter
case would be smaller than in the former FL
Rally point would De reached where the angle
would be 80 small that the accuracy of the meas
urements would be doubtful. That was lust
the problem presented in measuring the dis
tanee OF heavenly body. The term parallax.
as uSed by astronomers generally. means the
difference in direction of body as il appears
from the center of the earth and as seen from
the surface. The horizontal paralIax is the an
gle which the radius of tine earth subtends as
seen from the object. When we say the hOri-
zontal paTaaX OF the moon is tifty-Seven sec
onds, IL Il the same saying that the diameter
OF the earth a. seen from the moon subtenGs an
angie OF twice hGty-srYeT seconds. The eaua-
torial or longest diameter of the earth has been
adopted as the standard. The corresponding
parasiax. that Is. the equatorial radius of the
eartn as seen from celestial body. is called the
equatorial horizontal paraliaX of that BODY
Inc earth Is 20 small as compared with the
distances of the sun and the planets that the
parallaX
ALMOST DEFIEs MEASUREMENT
except in the case Of those planets which are
nearest the earth and even then it is so
minute that its accurate determination is one of
themost difficult problems ofmOdern astronomy.
htcoit II sOhlctlileS Htu 10 relative paraI!aX-
by this is meant the difference between the
parailaxcs of two bodies lying nearly in the
same direction. To determine the parallax of
venus, when nearest the earth, would not
always be practicable. because the observation
must be made in the daytime when the atnos-
phere is disturbed by the rays of the sun and also
becaIsc only small portion Of the planet can
then De seen IT, however. the planet is actually
between Us and the sun. a. 20 be seen pro-
)ected on the sun face. the apparent distance
OF the planet from the limb of the sun NAV be
found with considerable accuracy knowing
tne relative parallaX can by the same
law of kepler's find the suns paralIay.
The Phenomenon Of the transit Of Venus has re-
Cured four times every 2l3 years This cycle is
MADE up OF four intervals, the lengths OF which
are lool3 J. l2l5z and years after which the
intervals repeat themselves. IL has been only
IN comparatively Teceat times that this pheaom-
ena could oe predicted and observed
The FIRST PREDICTED TRANSIT.
kepler first predicted the transit of 1881 but
IL did not commence before sunset in Europe and
was over before sunrise the next morning. so if
passed uno5seryed- keplers calculations were
% far inaccurate that they failed 70 Indicate tn,
transit which occurred eight years later. The
transit of 1872. Ike all former ones would hava