New-York tribune., 1874-12-05

the minutes and seconds OF circle being con-
founded with the minutes and seconds used as Ill
visions Of time. But the power OF custom ~ very
strong and 80 many thousands OF books and tables
are printed full Of figures based on this old mode OF
computing that il is not easy to dispense with 1t nc
the present day. One form IN which it constantly
appears in astronomical estimates of the distance ot
the sum- is the statement that the Darallax Is -
seconds or 81. or and some decimal fraction. ms
phrase has caused a deal OF trouble to popular
writer's on the subject Among frantic endeaVOrs I'd
bring it before the minds OF readers fav0rte con-
ception is to suppose that an observer IS placed a1
the center of the sun, and is reading OF degrees
minutes and seconds on the orbit Of the earth
There is no more need for statoning observer on
the sun than for standing him on his head, TO make
this matter clear but it is worth while TO under
stand it, because the reports that will come 10 us
after astronomers have studied out the results OF
this transit will be that the "parallax" is eight
seconds aud fraction that is somewhere about hve-
sixths of second 1t will be represented in deci-
mails thus. S."85; perhaps it will be g. g or g. Sd
The meaning of this will be- that the orbit IN whlch
the earth travels around the sun is of such size
that when il is divided into 1296000 parts, there
will be eight and five-sixths for thereaboutsl OF
these parts seconds in the semi-diameter OF the
earth measured through the equator; or that, III
q her y,
1868 miles
To illustrate, let suppose the orbit to be 5's-
1GO 600 miles in circumference, according to the hg
ures used in this article in explaining Delise's
method Dividing this sum by $50, then by 60. and
then again by 60 TOR by 129G 900 at once) we should
find that one second Of such n circle is 446 miles,
Then in 1858 miles there would be about 8."S8. This
il the "paralax" for orbit of sly 10000 miles or
distance from the sun of 92,000000 miles Here
then is the whole mystery Of the "parallax;' II is
the semi-dameter of the earth stated in seconds Tor
tvelve-hundred and ninety-sx thOusandthsl of the
earth's orbit.
OTHER MEANS OF CALCULATING THE SUNS
DISTANCE.
The amount of the parallax is ascertained from
several other sources besides calculations from OBJ
servations at the last transit Of Venus. From what
has been said it is evident that if careful Observa-
tOns could determine the distance of any other
planet. this would serve quite as well as the distance
IF Venus for basis to calculate the distance OF the
sun. But only one of the planets Is ever near
enough for its distance to be accurately meaSura-
Bile; that s near enough so that observers Oppo-
site sides of the earth would see it at the same mo-
ment IN places in the heavens sufficiently wide apart
for measurement. This next nearest planet after
Venus is Mars How small the difference in posi-
ton. or the utmost apparent displacement Of Mars
is we NAV estimate from that of Venus at the time
of her transit since Mars further off and the dis
placement must be less The displacement of Venus
as represented in Our diagram at the head OF this
article is less than her diameter. 10 s so little
that if two stars separated by that apparent
space in the sky they would appear to an ordinary
observer like double star, but he could not dis
tinctly see with his naked eye the dark space be-
tween them Yet from this very small displacement
| astronomers have calculated the distance Of Mars
and from this obtained a solar parallax Of s..
If the moon circled directly around the sun instead
of around the earth its nearness to us would give
| easy means for determining the suns distance AS
if j. the attraction Of the alters the orbit of the
| moon around the earth considerably as to AFFORD
basis for this calculation which Of course involves
profound geometrical knowledge. How great IS the
deformity of the moons orbit may be judged from
| her uearness to Us during the recent eclipse.
! TRIBUNE readers may recollect that it was stated
then that her distance at that time only $4,000
miles-almost twelfth to the earth than her
average. To ascertain the effect of the suns at
traction upon an orbit of such rreCulary evidently
! involves difficulties of high order. From long
series Of observations and study of the moons mo-
tions. solar parallax Of g. So has been obtained.
! There are other exceedingly Ingenious methods #
| comparing the data which we possess concerning
l heavenly
from which the parallax has been obtained as some
where between S."S2 and s. S6. Very recent cal
culations made from observations on the little
asteroid Flora, have given an estimate Of 8. s..
But n method Of much greater significance
than these is that of measuring the We
Iocity of light by means of revolving
| mirrors rejecting q ray. The rate of their
revolution furnished the basis of calculation by
| which il was shown that light flies at the rate OK
900 miles in a second (of timel 1t has long since
been ascertaned by observations on the eclpses OF
Jupiers satellites that light travels from the sun
70 the earth in 40s seconds. Multiplying $4,000 by
4gs we have 600 miles for the suns distance;
| n parallax of g 50. Observations on what is known
rs the alteration of light and calculations there
from gave the same rate for its speed in travelling
from the sum- did observations on the satellites Oh
| Jupiter
Prof. Newcomb of Washington, who is high au-
! tboritv on this subject. and from whose writings
some of the foregoing estimates of the parallaN are
taken. saws that its exact amount is probably be-
tween 81 and g. So and the distance of the sun
il probably between 81 100000 and 100000 miles.
This years observations on the transit will doubt
less 90 far to determine the figures.
| Amonp other causes of failure 10 obtain precise
results in observing previous transts the phenome-
non known as the "black drop has been chiefly
operative This may be briefly stated as change
in the apparent shape of Venus at the moment when
its round black edge is inst coming to or recedng
from the bright edge of the sun. In approaching
the SUMS edge the edge of Venus goes out to it, in
retreating from the edge of the sun, either when
within the suns disk or after leaving it at the close
of the transit the edge of Venus seems to cling to
the edge of the sun like drop of muciage falling
from the lip of bottle. At the moment OF the con-
tact of the edges or rather at each Of the four mo-
ments Of contact two exterior and two interior)
Venus loses shape ceases to appear round, and thus
renders the time of the instant OF contact uncer-