New-York tribune., 1874-12-05
THE TRANSIT OF VENUS.

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DETERMINING THE DISTANCE OF The SUN
rux nAsIs OF OUR PRESENT KNowLEDGE-wHY
T11E TRANSIT Is oBsERvED-DElIsLEs METHOD
c,vpry srArEp-rHE DIRECT METHoD-THE
PHOTOGRAPHIC METHoD-HALLEY's METHOD
SUMMARY OF THE MEANS NEUTRALITY-SUSPICIOUS.
OF OBSERVERS
The prepaI ations that have been made by men of
science in all leading nations for several years. and
now include the organization and equipment of
large expeditions to distant lands indicate the value
set upOn the observations to be made the Sth of
December next concerning the transit of Venus In
THE article attempt will be made to show the
reasons for this deep and widespread interest to set
forth what astronomers hope to ascertain by OBJ
serving the transit aud to explain in simple phrase
and as far as possible, without technlcahties' now
the observations may give the information required
twill be conceded that long series of observa-
lions on the motions of the planets, due allowance
being made for that Of the EARTH would result after
suitable comparisons and calculations in giving the
hines during which the planets circle around the
ann A CeOmetrical study of these observations has
snabed astronomers to substitute for the apparent
paths of the planets the stars their real paths
around the sun that is to project the form of their
orbits BY calculations of the most laborOus nature,
s1ade from immense number of observations.
Johann Keper in the early part Of the seventeenth
".
movements of the planets which are known as KEEP
er's laws. Sir Isaac Newton afterwards demon
,trated that these laws a consequence of an
attractive force and thus developed the doctrine of
Tavity.
WHAT ASTRONOMERS ARE CERTAIN OF
1t is practicable to explain Oae of the methods
used during the transit of Venus for asCertaining
he distance of the from the earth, without alu-
ion to kepier'S laws; but it els necessary to have
some understanding Of at least one of them to com
prehend the anxiety of astronomers respecting this
Instance; TO why it appears to them so import
ant to have this measure accurately. Keplers third
law may be stated rs follows:
T >Gatrcs Of the peri times Of TIE planets have
amor -cr tine SIN" proportion have 10 each
the cuOes or their mean distances from the sun
In practical astronomy this rule has to be qualified
by taking also into the computation the mass of the
planet; but this Is a very small fraction and may
he dsrcgardel for the purposes of this article. BY
using round numbers it viii be easy to get clear
couceutiou of the meaning Of this law. The earth
goes around the sun in one year. Venus goes
around the sun in about fve-eighths of a year
Their respective periodic times may therefore be
represented bv one and by fve-eghths Square
these times. The square of l is l; the square of
fve-eichhs is about two-Gfths. Let Us call the dis
tance OF the earth from the sum- l. If this is cubed,
il is still l. The distance of Venus from the sun
will then be- according to Kepler's rule, something
which il cubed would be equal to twO-hfths; In
other words, the cube root of two-ffths or nearly
eievcn-fitecuths. That il. in round numbers Venus
Is eleven-hfteenths as far from the sun as is the
earth Ii may make the foregoing calculation little
to put it in tabular form in decmals; thus
Years. Squared
k~yOu''Oa OF Venus.--. 1,,,5,t S3 c.t.4 a. nearly
Distance Of earth ......... !.
Distance of Vcuus........ 73 A nearly
Il .ue a5oye tAbe II w ie tha, the ,ql,res Of
tn times thr 1s The cubes or le dis anccs
II he distance Of Venus from the sun is u-l5ths
that # the earth, the distance Of Venus from the
earth must be fonrffeenth of the distance Of the
earth from the sun If then We know the distance
of the earth from Venus, We can calculate the dis
tance from the earth the sun, and then The distance
Of al the planets, using that of the earth a basis.
For instance if we know that the distance of Venus
from the earth is 2LjJ0OO0 miles, this is four
ffteeuths Of the distance of the SUI, we may make
the calculation.
4: Is :: 2i.JOOO0O 9l,875OoO
thus obtaining a very fair approximation Of the
SIMS distance from the earth. Having the earths
distance from the sun, we have the measuring rod
far the distances Of tie other planets Their pro-
por.i1ual distances if Ills measuring rod is rcpce-
seu,cd by /, are, stated decimaIy, as follows:
Mercury. y......... .3s-lo Japi'er... --. 52.2-s -
N.ius...-..-.... 1859 : ~tnrn--.---..-.- .5'7s
M,ls............ L52369 Neptuue.........3O.O7o55
If9lS-50JO Is the length of our measurlng-rod, !
we have only t0mIltiply the above figures by 1t to
obtain the distances of those planets from the sun
But what if 92GSOOC0 s the true length of the rod
or o. go 0J0l A mistake of 100000 miles to the
eart's distance Is q mistake of 100000 miles as to
that Of Neptune: mistake of more than 10000 |
OO0OOO miles rs to the distance of the nearest fixed
star. Hence the need of obtaining accurately the
length of the measuung-rod.
WHY VENUS n TRANSIT BEST SUITS FOR OBsERYA-
TIONs.
From what has been said it is clear that II the
distance of any one planet from the earth were accu
rately known all these other distances would
become mere matters of calculation. Venus being
the nearest planet to the earth would ne the one
most naturally selected for measurement OF dis
taDce But why select the occurrence OF a
transtf Why not measure her distance at any
other time lust as well r. These are questions not
UnfrequeDtly asked To answer them it is necessary
10 mention what may appear sufficiently obvious
that the greater the distance the greater IS the dill
cuty Of measurement. 1t Is solely on account Oh
the suns immense remOteness that direct tngono-
retical methods of ascertaining its distance, such
as are applied to the moon, fail in the case OF the
entirely.
Is