Sacramento daily record-union. [volume] (Sacramento [Calif.]) 1875-1891, 1882-12-04
TRANSIT OF VENUS wRDnEspAY
How 1t Determines the Distance Of the
Sun ard Why the Problem is Difficult

Sun ard Why the Problem is Difficult
The present generation may as well get
all the possible interest out of the coming
transit of Venus, for it is something that
they will not see again, nor their children's
| children. After the 9th of December next.
there will not be another transit for 1911
years, the next following in the month of
June, in the year 2004. As very few peo-
pie will probably observe the phenomenon,
its chief interest for them must lie in the
same cause which gives it's Importance for
astronomers and men of science, the fact
that 1t Is expected to furnish a more &C
curate measurement OF the distance bet 9een
the earth and the sun than now exists. It
must be remembered that the opportuni.
ties already enjoyed for this observation
can still be counted ON the fingers of one
hand and that considerable errors have
been shown to exist The first predicted
and observed transit of Venus was in
1859, by Benjamin HORROR, of Lancashire,
England, and it hardly counts in the series.
The next transit, in l76l, was observed
from considerable number of stations,
but the results obtained varied widely,
ranging from II SAG, 900 to 96, l6S, 600
miles. In 1863 there was again considera.
BILE variation, the largest limit being, as be-
fore, SO, l63 900, while the smallest was
81, 049. eco. From a combination of the
two, Encke deduced probable result of
81, 049, GOO which was accepted for many
years. Other methods indicated THAT this
result was too great by :, 1000, 600 to 4, 000,-
600 miles, and the observations of the next
transit, that Of 1874, confirmed this, Civ.
ing distance of about 81, 600, 900 miles.
The approaching observations are awaited
with special interest, because, if in full
agreement with those last taken, they will
substantially determine this question.
The measurement Of the distance depends
on ascertainining the suns horizontal par
allax, that is, the angle which should be
MADE at the center of the sun by lines ex-
tending from point there, one to the
center Of the earth and one to Just touch
its curved surface. In the triangle formed
by these lines, the angles being known and
also one side, that is the semi-diameter of
the earth, the remaining sides can be cal
culated AID the distance between tine earth
and sun thus determined. How to find
the angle accurately is the great practical
difficulty, because it is too small for meas
urement by any ordinary means. In the
case of the moon. for instance, the hori.
zontal parallax is not far from one degree,
and is therefore large enough to be directly
observed without much difficulty. In the
case of the sun whose parallax is not
far from eight and half seconds, or about
one, 420sh part that OF the moon, the case
is very different. The probable error in
the direct observation is so great that
results obtained in this way are open to
grave suspicion in spite of the progress
that has been made in the construction of
instruments adapted to the work. Not
many years ago it was reckoned at four
seconds or half the whole approximate
parallax, and the result of such an error,
according to the side on which it occurred,
would be to make the distance anywhere
from third less than the true amount UP
to twice what it ought TO be. Tnat is TO
say, such a result would give the distance
of the sun anywere from 60, 000, 600 miles
to $50, 000. 600.
The occasional passage of the planet
Venus across the face of the sun relieves
this difficulty. It will be remembered that
Venus has distance from the sun of some
6S, 900, 0O0 miles as against 90 600, 600 for
the earth. When, therefore, she passes
between the earth and the sun she ia less
than one third as far from the earth as the
sun is, and consequently her parallactic
displacement as she passes across the sun
is considerable enough for tolerably accu-
rate measurement by modern appliances.
In passing across, her course describes a
line across the face of the sun, which is
differently situated according to the posi-.
ton of the observer. The lengths Of these
cords may be utilized as basis Of calcula-
| ton, and also by De!islo's method the COM
putation may he made from data obtained
by Observers at different points, noting the
beginning or the end of contact. Sill
other methods may be followed, and all
these various means were employed at the
last transit
As practical matter it is of course I'M
possible to take the observation from POINTS
ninety degrees apart, anu all the ohserya-
tious taken are suhiect TO long and CON-
plicated reduction and involve an immense
amount of calculation. There are means
independent of the transit of Venus by
which the suns distance may be computed
and the problem has been repeatedly
worked out In various ways. This, how
! ever, is the direct and most satisfactory
method. Ii perfectly accurate observa-
ton absolutely. Some idea of the dimen-
sious used and a corresponding notion of
| the delicaoy of the problem may be had by
remembering that if the lines from the sun
to the center and the circumference of the
earth could be laid down on paper the d.
vergence would be so small as to be practi-
cally invisible for eight or ten feet. To
ascertain an angle like this is an enormous
undertaking, which aside from the i'm
portant result to be obtained would attract