Wood County reporter. [volume] (Grand Rapids [i.e. Wisconsin Rapids], Wis.) 1857-1923, 1873-09-11
The Suns Distance and How it is
Measured.

inaccessible object. The solution involves
the measurement of a base line and the
angles formed on it by two lines connect
ing It's eXtremities with the object whose
elevation is to be found. For example.
suppose the object to be balloon. If at
the same moment two persons, in line
with the balloon and considerable dis
lance apart, make a note of its angle of
elevation, the angles thus obtained, with
the distance between the observers. are all
the data required for calculating the
height of the balloon above the earth. In
like manner, if two observers, say, one at
Washington and the other at Lima, or one
at Paris and the other at the Cape of Good
Hope, observe the position of the moons
center at the same moment, they will have
two angles Of a triangle, which, with the
included side-the distance between the
observers,-wille enable them to determine
the length Of the remaining sides of the
triangle, that is, the distance between
either station and the moon. In this case,
the triangle is extremely long and narrow,
the longer base line mentioned giving an
angle at the moon of only about degree
ana half
1t IS obvious that an object much far
Their off than the moon would give with
any base line Obtainable on the earth, an
angle two small for direct measurement.
In the case Of sun, for instance, the dis
lance IS so great that the incest observa,
ton tails to show any measurable differ
ence IN his position, whether he is viewed
from one or another part Of the earth's
surface. The determination of his dis
lance must therefore be by other means
than by direct triangulation
Several ingenious attempts were made
by ancient astronomers to solve this pro-
bem Of the suns distance indirectly, but
| the limits of error by their method were
so wide that the results obtained by them
had no value even as approximations.
Indeed it was not until Kepler discovered
the proportions Of the solar system that
| it became possible to attack the problem
with any hope of success. As soon, how
ever, as kepler's third law made all the
distances of all the system calculable as
soon as one was exactly known, it was
clear that, if the distance from the earth
to one Of the nearer planets, say Mars or
Venus, could be found, then a simple
proportion would give the distance of the
sun.
Mars was the first planet to be studied
for this purpose. Venus approaches nearer
to the earth; but as her orbit lies within
that of the earth, her position during her
periods of conjunction is unfavorable for
observation, save at remote intervals
when she happens TO be exactly IN line
with the suns disk, that is, during her
| transits. OF these more hereafter. Since
Mars, when nearest the earth, is still too
far away to be reached by direct triangu-
| latiOn-that is, so far that two lines con
Meeting the eXtremitles of the longest
base line to be had on the earth with the
planets center would be so nearly paral-
lei that the angle OF their conveyance
could not be directly measured-it is OBJ
vi0usly necessary to devise some other
means of discovering the value of that
important angle. Omitting all but the
fundamental elements of the problem, the
plan adopted may be roughly illustrated
as follows: Hold a small object, say a
pencil, steadily at arms length and note
the spot on the wall which the pencil
point covers when looked at with the
right eye, the left being closed. Now
close the right eye and look at the pencil
point with the left eye. Its position is
shifted to the right, more or less accord
ing to the distance of the pencil from the
eve and from the wall The amount of
this shifting, in angular measurement,
may be called the pencils parallax.
Suppose that, instead of being held be
tween the eve and wall, the pencil is
placed before the moon at such distance
from the face that, when looked at with
| the right eye, its point covers the lett
horn Of the moon, and, when seen with
the left eye, the right horn We may now
imagine two similar triangles one having
for its base the distance between the eyes,
and for its sides two lines proceeding from
the ewes and meeting at the pencil point,
I z Z 10, fInr, n
the same lines to the opposite sides OF the
moons disk The measure Ofthc vertical
anple of the triangle standing on the
moons diameter is the portion OF the
GREAT circle of the heavens covered by the
moon. that is about half a degree. The
vertical angle of the triangle having for
| it's base the distance between the eyes IS
the same; hence the remaining sides OF
the triangle-that is, the distance OF the
| Dencil from either eye--can be determln-
| ed bv simple process Of calculation.
Precisely the same principles are IN
volved in the determination OF the DIS
lance of heavenly body like Mars, the
! disbursement OF the planet, as seen from
two distant observatories, being measured
| with reference to some star lying as near
1rr 40 nOssible in the same direction.
| ISince the distance of star is so extreme
! IL GREAT that its position Is not dPpleEi-
INV altered by any difference IN points
If view possible on the surface OF the
earth-in other words, since the star has
no navalaX-it answers peTeclly aS
fxed point of comparison) AS soon rs
i've distance Of the planet has been caiGo-
Iatnd the distance of the sun can DE oe-
tormined by an application OF hepler
MID law. Kepler made the calculation
on on the basis ofTycho Brahes observations
If Mars; but owing to the rudeness 90. o.
trosa observations, he could only say that
tho SIMS parallax could not be greater
han one sixtieth of q degree Il mln-} Wnich
1lonId make his distance not IeSS 't'"ii
, TGrt, and a half million miles
~ G,,h.aon,nt observations oi 2rr--ater eN-
80 nnahled Cassini to calcu'ate that
% .p,t e,,n,S nara;lax could not exceed ten
sdo OF arc 40 sec) and he was con
cS,. hat il was not greater than % SEE
ll" ,.as,,ondinr with distance not less liah
cor 10 600 miles. The establishments more
oo sly SEPARATED points OF observation,
VA Tho immense improvement haut u
a i.a ears in the construction o, ,5o OIL
a. a;ca1 instruments, have enaoieo iuoueI I,
ON itr,lors to make great improvements ..
1 ,Isa Am f,rnres which will be noticed al
these ny N the meantime, however the
recti!: If Venus in 1901 and 1909 Iu[-
n;Ihed data for another ana en'i'0'' >'''
ferent set of calculations .u,,si.s Of Venus
iei Tho nortance Of the transits ol on !Ls