The Pacific commercial advertiser. [volume] (Honolulu, Hawaiian Islands) 1856-1888, 1882-11-25
The Transit of Venus.

disc Of that orb. Such an event will not happen
again for 122 years. Parties of astronomers are
already at their posts in various parts Of the
world, preparing to take careful observations of
the rare event. 1t is confidently expected that
the Improvements which have been made in the
art of celestial photography since the transit OBJ
served in 1874 will so greatly aid in obtaining
precise results, that the great object which is
sought by these observations, the determination
Of the distance between the earth and the sun,
will be so well secured as to leave but q small
margin for future observers to deal with. 1t is
indeed expected that the approximation to the
truth will be so close that there viii be little
hope of improving upon it unless the march of
science shall disclose for US some improved
method and at the same time enable US to rem
der our instruments Of observation much more
perfect than they are. For this there is, indeed,
ample time before the astronomers of 2004 take
up their stations to repeat and review the OBJ
seryations of the present year.
As the event of 9th December next is not to
be repeated for 122 years to come, the planet
which takes the principal part In the celestial
drama naturally acquires for the time being a
prominent place in popular interest. The pri-
waite lives of great people are always matters on
which the public delights to be informed, and as
1t is the duty of the journalist to provide for his
readers the fullest information on the subjects
of the day, We propose to devote a few para
graphs to the vie intime of Madame Venus.
The transit of Venus across the disc OF the
Sun is not looked upon by astronomers as an i'm
portant event simply because of its rarity. The
interest which it cxeites depends almost entirely
on the excellent opportunity IT affords of obser-
ing, with a close approximation to accuracy. the
parallax of the planet and of the Sum- From the
parallax, the distance of each of these orbs (frOm
the centre of the earth) is deduced by calcula-
tons of a comparatively simple nature.
To understand how the distance of any object
is to be deduced from its observed parallaX, it is
necessary in the first place to know what this
word parallax means. a simple illustration Will
render this clear. Amongst the crowds which
thronged the wharves on Thursday last to wit
naGG fha hoof raqac khara vyara m 1Oubt many
who had to content themselves with back places,
and whose view of the boats was interfered with
by the heads and hats of those in front of them.
Such persons had, in order to follow the course
of any boat, TO shift their positions occasionally.
When the boat was hid from view by an inter
vening hat n slight change of position would
cause the hat to obsure some otter point of the
field of view and so let the boat be seen again.
The arc of the horizon between the APP
parent place of the boat and that of the objects
which the hat would hide after the spectator ha2
changed his position is the parallaX of the hat.
That arc once accurately ascertained and the dis
tance which the spectator moved, and the direction
in which he moved being measured, nothing fur
Their is required to enable the mathematician to
calculate the precise distance between the spec
tator and the hat. The method by which the
calculation is made is taught to every schoolboy
who learns the first rudiments Of trigonometry.
The earliest problems given to him are solutions
of triangles; the solution of the problem of par
allax is nothing more than one Of these. The
NAACAoa A'NA WI A0N II
pears to have moved when the spectator changes
his position, gives the measure of the vertical
angle of the triangle whose three corners are the
hat and the two positions from which it was
seen. Another angle of the triangle is known
from the observation taken of the direction in
which the spectator moved, and the base of the
triangle is the measured distance that he moved.
4 The lengths of the other two sides of the tri-
angle, which give the distance from the specta-
tor to the hat are to be found by quite simple
calculations, or by the aid of published tables by
the simple process of multiplication.
So, exactly, is the distance of Venus or of any
other heavenly body determined, except that in
stead of relying on consecutive observations by
one spectator who changes his point of view,
simultaneous observations are made by obser-
yers placed far apart. All the heavenly bodies
are so far away from us, that it requires n large
difference in the position of two observers to
give any measurable parallax. And at the best
there is always a divergence in the obser-
vations, caused by the want of absolute per-
lection in the instruments. and by certain
differences in the observers themselves, which
leave a margin of doubt as to the perfect aecu-
racy of the determinations.
OF all celestial objects, with the exception of
the moon, Venus at certain epOchs comes nearest
to the earth. : Her parallax is therefore greater
at such times than that of any other planet and
being so can be determined with a greater
approach to accuracy by such instruments as
we possess than that of any other planet. More
over, at the rare times when she passes
as a dark spot across the face of the Sun,
Il she can De observed Win closer accuracy
than at any other time. AS her position
on the Suns disc is different for each of two
observers who are placed at different points on
the surface of the earth, so the length of her
path across it must vary. If one observer sees
her on the apparent equator of the Sun, another
will see her nearer his upper or lower margin.
To the one, the track of the planet will be the
longest possible Tor it, being the whole diametlr
of the Sun. To the other, the time occupied in
the transit must necessarily be less. The OBJ
servations of time, therefore. form n check upon
those of position, and assist to give accuracy 10
the determination of the planets parallax.
Hence the observation of a transit has many
advantages over ordinary observations for paral-
lax. This is the fact which gives TO the rare
transits of Venus across the Sun their extreme
scientific value.
OF late years a new method of observation has
been added to the resources of the astronomer
that afforded by the art of photography. In
Simultaneous pictures of the Sun and Venus,
taken during the transit, are, if the time st
which they are taken be noted with accuracy,
invaluable records of the phenomena they de-
pict. Important use wns made of this method
when the transit of 1874 was observed. At
Kerguelen's Island, for instance, where the
duration of the transit was more than an hour
and a half, no less than 200 successive images
of Venus on the solar disc were taken. Since
1874 the art of photography has made many
strides. and in no department have these been
greater than in that of celestial photography.
1t is expected that during the transit of next
month, the work done by the lens will be of at
least double the value of that of 1874 1t is
quite probable that n first place as recorded
observations Will have to be given to these pic
lures which the Sun himself will contribute to
the cause of science. The mean result Of all
the photographic recorIs of 1874 was a close
| confederation of that derived from the personal
observations of experts.
The question may arise in the minds of some
of our readers how a knowledge of the distance
Of Venus from the earth on any particular day
will assist us to tell the distance of the sun. For
a reply TO this We must refer the enquirers to
Kepler's second law that the squares of the
times of revolution of the planets are proper
tional to the cubes of their mean distances from
the Sun" When the astronomer has secured a
measurement of the distance between the centre
of Venus and the centre of the earth by the
aid of transit observations, he has to deduce the
''mean distance between their orbits from it
by taking into account what positions the two
planets were in at the given time in regard to