The Pacific commercial advertiser. [volume] (Honolulu, Hawaiian Islands) 1856-1888, 1874-09-19


Suppose one observer is at the eaTth's equator,
b, and the other at the pole, A. The observer at
B would see the sun in the direction Of G., and the
other in the direction Of ID. The two lines of vision
would meet and form a small angle at the suns
center. This angle would be what is called the
suns horizontal parallax, and evidently it would
be the angle under which the earth's semidiameter
would appear If viewed from the sun. 1t is seen
that the line B c forms a right angle, or 800, with
the earth's axis. bat the line A G does not. L. is
Just as much less than 800 as it defects from the
plane Of AS horizon. This would be found by
measurement. Now it is known that three angles
in any triangle formed by straight lines are equal
to 1902. The sum of the two angles referred to
would be subtracted from 1895, and the remainder
would be the value of that at G.
Until quite recently the SUMS horizontal parallax
was believed to be about $685.. At this rate the
earth's diameter would subtend at an angle of
ZIG" if seen from the sun or, in other words
that is the length of an arc on an imaginary circle
which the earth would occupy. That imaginary
circle, like every other, great and small, contains
$500 or 1896, OOO'. Dividing this by l7.I6 we
obtain 1884 and a fraction for the %quotient. Null
tiplying this by 1912, the earth's mean diameter,
we have the circumference of the earth's orbit in
miles, whence its radius or distance from the sun
is found by dividing by 6.2s31s. These 6gures
were accepted as the most authentic obtainable.
until recently.
Two different methods of observation will be
employed in December next. The first is called
Halley's method, after the great English ~astrono-
mer OF the last century. According to this method
the observer merely notes the duration of her
r~ansit-which is necessarily different at different
stations. Having the exact time occupied by the
I nIanaI IN fFavarsinO har chad of traua;t tho Ia1ctb
of the chord can be calculated, and consequently
Its distance from the suns center. A comparison
of two chords obtained from observations made at
two stations sufices to show the planets displace-
ment for the two stations.
The distance between the stations being known,
the planets distances from the earth and the sun
follow as before
This method requires simply that each observer
shall note the exact beginning and end of tbe
transit as seen at his station, and that his clock
shall measure the interval correctly but as the
planet appears as a comparatively broad disk on
the suns face, it is no easy matter to determine
the precise instant when her center crosses the
suns edge at the beginning and end of the transit.
To do this the observer has TO note with INFINITE
precision the moment when the edge of the planet
FIRST touches the suns rim, that is, her first external
contact; next, the instant when the planet is Just
uAOl uCIoC IL ~ OEOhcu Cuk6O Vi the Sun
appears to close that is, her arst internal contact /;
and lastly, he must repeat both observations in
reverse order when the planet leaves the suns
disk. The obstacles to the easy and exact observ-
alton of these phenomena are numerous and appar-
emily insurmountable. To secure the requisite
distance between points of observation it is neces-
sary that one station be as far north, the other as
far south as possible hence the sun cannot fail to
be near the horizon, at which times the outlines of
his disk is greatly distorted. Atmospheric causes
frequently aggravate this difficulty by giving the
suns edge an uneven or rippled appearance.
Then the planet, when just in contact with the
suns edge, always assumes a peculiar pear-sbaped
aspect which makes it all but impossible to fix the
exact moment of contact, and this is at a time
when q fraction OF a second is all-important.
The other, known as Delisle's method, is valu-
able in that it aims to determine the suns distance
in a manner entirely different from any thus far
noticed, and is applicable at times and places alto.
gether unfavorable to Halley's method. To illus
trate Suppose that in q photographers gallery
two cameras are placed side by side, facing, let us
say, a white, circular screen. Let a ball be sus-
pended between the cameras and the screen, two
yards from the former and Eve yards from the lat
ter, this being approximately the relative distances
Of the earth and Venus. If the ball be made to
pass from left to right it is obvious that it will
come in line between the edge of the circle and
the left hand camera FIRST. After an interval. de-
pending upon the balls rate Of motion and the dis
tance between the points of view, the ball will
come in line between the right hand camera and
the edge Of the screen. When it reaches the other
edge of the screen the appearance will be reyers-
ed, that Is, II will be seen 10 the observer at the
left hand camera te leave the circle sooner than to
the one at the right. Let US conane our attention
to the first case. If each observer notes the exact
time when the ball appears to him to touch the
rim OF the circle. the difference, say for seconds.
will measure the interval occupied by the ball in
passing q certain distance. Now, if that distance
can be accurately calculated, the balls rate of mo-
ton can be easily ascertained. Say the distance
between the cameras is twelve inches. The posi-
screen is known, since it divides the whole space
into parts having to each other the ratio of two TO
Eve. Hence, by q simple geometrical principle,
the space passed over by the ball in the given in
terval is to the distance between the cameras as )
to 7, which gives 8 At seconds, or 2 IL inches per
second. In like manner the rate at which Venus
moves in her orbit is calculated from the time be-
tween two observations Of the same phase of tran
sit as seen from two stations.
The time which the planet takes to make a CON-
plete circuit Of her orbit is known, as we have
already stated this time multiplied by the planets
rate of motion gives the circumference of her orbit
in miles-whenee its radius, or the distance Of We
nus from the sun, is easily found. The ratios of
the planetary distances being known by kepleIs
Third Law wiz. the squares of the times of the
planets revolutions are proportioned 10 the cubes
of their di~tances), the suns distance from the
earth or any other planet, in miles, follows by a
simple proportion as soon as his exact distance
from one is determined. The difficulties with this
method are the fixing Of the exact moment of in
gress and egress the clocks must show absolutely
tFne time and The l0uoituda of fha efati0ns must