The daily gazette., 1882-12-06

For the last time for 122 years the planet
Venus is to-day seen traversing the disc of
the solar orb as a tiny black spot, Just per-
ceptibleto the naked eye. There must be
few indeed who have not noted the beauties
Of the brilliant star that for months past has
adorned the Western sky, till about two
weeks ago 1t was seen nightly nearing the
sun, flying towards it, dropping into it as 1t
were. OF course such an appearance was
purely optical, Venus remaining at or near
the same distance from the sun at all sea-
sons and times The real cause of the ap-
proximation was the alteration in the rela-
tive position of the planet, the earth and
the sun. Finally to-day the suns face is
decorated with tiny perfectly circular
black spot passing across 1t in the direction
shown In the engraving. But as seen from
different parts of the illuminated side of the
earth, from which alone the phenomenon
will be visible, the chord apparentlydescribed
on the suns disc will be different, and it IS
just this difference which the governments
of the civilized nations Of the earth are
spending to-day thousands of dollars to
ascertain.
Although the problem sought to be
accomplished, viz the distance of the sun
from the earth, Is one of the most delicate
and difficult operations in Plane Astronomy,
the general considerations entering into the
matter are fortunately sufficiently easy Of
comprehension and susceptible of illustra-
ton. 1t is matter of familiar note that
the apparent location of an object varies
with the position of the spectator. Sup
pose a tall staff TO he set up on the ground
cutting the face of a distant hill two OBJ
servers stationed hundred yards apart
would reer the staff 10 very different
position on that hill face. Now the distance
between the observers and the width of the
staff being known, the apparent angular
separation of the lines on which the staff
will appear on the hill face being measured
by suitable instrument the distance of the
staff from the hill face becomes matter of
simple calculation. Another illustration
may be given Let coin, say hall dol
lar, be held up between the eye and the
moon. 1t will be found that To
make the coin appear just to cover
the moon and no more 1t must
be placed at distance OF 10 feet or l20
inches from the eye. But the width Of the
coin is about one inch, that is to say that the
width OF the coin is to its distance as one to
l20. Now the diameter of the moon is 1900
miles and under the conditions specified its
distance from the earth will be in the same
proportion to its diameter as the known dis
lance of the coin from the eye is to Its
known diameter. The latter being 1000
miles and the scale derived from the coin
being 100 to one, the moons distance will be
| . 2,O00x1 20-240, 900 miles. Now from this It
| follows that If we know any two OF the
quantities in the problem we can calculate
the third, and the coin plays just the part in
| this simple experiment described that the
planet Venus does today in its passage
across the sun, but unfortunately this Inn
portant unit of measurement is only avail
able on very rare occasions as VIII Dc
presently seen.
Let the reader imagine two hoops nearly
eoual in size laid one within another; repre-
| senting the orbits of the Earth and Venus
| and a ball IN the center to represent the sun.
| Then If the orbits were parallel as repre-
sented in this case by the hoops, there
would be transit of Venus atevery inferior
conjunction Chat is when Venus passes be
tween the Earth and the sun)-tie period
being 5S4 days, or barely two years. Bul
instead of being parallel the orbits are really
inclined, To illustrate this let the hoops be
placed one within the other but slightly
| lifted apart SO as to touch each other inter.
nally in two places. These are the only
conditions under which transit can take
place, viz. when Venus, (the ball travelling
round the inner hoop is at one or other OF
the crossing points. The one Of these, called
the descending node is reached in the month
of June, at intervals of 8, l05, 8, ~ years
| the others ab INC asceHulIS Huuc III nliiiIidl
intervals. Practically the transits occui
in pairs of s years, separated by the Longer
intervals. Thus the present transit is at tht
ascending node and is the duplicate of the
one which occurred in 1889, the first ont
ever observed. The transit of 1874 also oc
Cured at the ascending node and was tht
duplicate of that OF 1851. The next transit
viii occur at the descending node on tht
sth of June, 20O4 and will be repetition o.
the transit of 1761; that of June q, 20l2 vii
he A recurrence of the celebrated transit O
1863, by which the distance of the eartl
from the sun was first determined.
Mercury being like Venus an inferio
planet, whose orbit lies within that OF tn
earth, necessarily passes over the disc ofth'
sun in the same manner, and the nodei
being crossed in May and November transit
are restricted to those months. Thes'
phenomena are of much more frequent oe
currence than the transits of Venus, but arr
unfortunately oflittle usein determining tn
same problem, because the distance OF Mer
cury from the earth is greater than th
distance of Mercury from the sun, while il
the case of Venus the opposite condition