The New York herald. [volume] (New York [N.Y.]) 1840-1920, 1878-05-05
THE TRANSIT OF MERCURY.

THROUGH THE PLANETS SHADOW CONE
| Tfansts OF Mercury over the SUMS disk although
separated from each other by such long Intervals
transts OF Venus still sufficiently be OF
great interest astronomers On oil the dis
lance OF Mercury from ths earth they worthless
means Of determining the solar paraax; but 1t Is
thought that tho phenomena presented by them
analogous to those presented by Venus under sim
liar circumstances DO capable OF throwing much
light upon OF tho obscure and difficult
points connected with trsnsts OF The latter planet
and. furlhermors, they present exceptional facilities
for determining the exact longitude OF Mercury, AID
almost the only possible opportunities Of accurately
measuring Its equatorial diameter and polar compress
sion.
FIGURES TRANSIT
The crcums tancies of a transit ined somewhat
more.in detail are fo.lows:-Morcury, like all The
planets OF the solar system, intsrcepts tho SUMS rays
and casts two van conical shadows, one within the
othsr-the having It's base resting upon the
planet and It's pex about 32 900 miles away, while
tho larger has planet nearly NJ lis apex and
stretches far ipsp space, enarRing as 1t goes TO
person anyw tere within tho smaller shadow cone
Mercury wouid appear larger than the sun and ITS
transits woulf be total solar cchpses; while
TO person thin larger, but outside the smaller
shadow cone Mercury could never wholly eclipse the
The earth sonnet nes passes through the larger
shadow cone, but never enters the smaller Que.
Now, what will be tao condition Of affairs the
morning OF the 9th Of AtayY The earth Will be boom
Ing along in it's orbit at the rate OF 656 $40 miles per
hour, while ercury, sssfyng his ancient repuia-
ton switness. will be rushing through space at
the rate OF IS, 200 miles per hour, aud his SHADOW
cone point wheavt intersects the THIS orbit
viii actually be goin 2r9,9 900 miles per hour Inc
shadow IS thousands oil miles behind The earth, but
is rapidly gaming on il. The hands OF the Washing
ton clock creep iowa Ri 10 A. NJ. Tho shadow nears
The earth The clio TX points 10 1904 The shadow
touches the earth 81 swiftly closes around n-tnree
minutes later IT n,,uits 1t, and tho transit has
begun all poii "a where the visible
Where 1t fir e. seenY Clearly ut the point
where Tho shAc Ow cone nrst touched The
earth, and there he must have been very
The horizon. As Ae shadow closed around
tho transit comm enceA AT places whore the sun
high er and until IN less than two
it reach ed thl long where the sun was
the meridan. Alter thati began at places where
the sun was /, avcr ana lower until finally the last
point to be ,np allIed tho shadow placc
almost on the opposite side OF the earth from that
where the sr' asil began ana mere also the sun was
that although us centre does not pass over The earn
II s neverti aless seven hours aud halt before ITS
further edge res aches her. and she emerges by pro-
cess the revsrs OF her eng ulimenI
rnE ons sRvArsoxs Aso THEIR METHODS :,
many ways 1t TINS br1et article viii be confined 10
description; Of the methods which Will be adopted
by the Unitnd States Naval Observatory and by the
gentlemen CO operating with that INSTITUTION. The
Opucai obss .vat,ons will DO made with powerful
crolneteAs Of Sir George Alry's pattern. When the
transit Is " Jout commence the sun will be most
carefully tened through those instruments and the
exact ns,n when the smallest psrc:pbIo notch Is
nrst its edge by The planet will be noted very
accurately. This IL the first contact. About Two mia
utes later dorcury will have advanced Tar that the
sunlight will close around him and make him
appear as a very small, round, black spot upon the
solar disis This IN tho second contact and
instant " aeR II happens VIII be most carefully oo-
served. For than hours this little black
spot Win slowly crossing the SUM- and Will finally
break In rough lis edge a. point nearly opposite 10
re ii entered This Is mo th,rd contact and
tho ex.a instant of occurrence Will be noted. The
psnots m continue to move forward aud about two
minute later the last trace Of II will disappear from
the ss OF the sun. Tho instant when tnis happens
IS the ourth contact and II also Will be carefully oO-
ssrv~d Tho first and fourth contacts are often
cailDa the EXTERIOR and the second and third tho
inta1 n contacts. II tho sun had q hard, smooth
oulm q, aud II Mercury were destitute Of
tnosphere, all these contacts would place
shag py would be possible to observe the
inAt ant OF their occurrence to witnin one.tenh Of
nd or time but Unfortunately for this purpose
sun outline, and Mercury possesses
atmosphere and the Two together destroy the
sn rpnsss OF the contacts aud give rise 10 a whole
tn in OF phenomena concerning whose character, ana
ev exstence there are great differences Of opinion
MEASURING TnE DiAMgrxRs NRRcuRr
During tho seven and hall hours that the planet
rossing the lace tue sun double Image ncrom-
s,ters come into play. They instruments some
'Shat Ike the eye piece ~ an ordinary spy glass,
the object glass, cut into halves which
past each other by microneter screw. On looking
through telescope to which one OF these Instruments
Is sEacocd, the halves Of the divided lens arc IN
Tho same positions rcatveXfo each other AZ ociorc
they cut apart only single image Of Mercury
VIII be bat II the halves of the lens moved
little each other two images viii APPEAR ana ny
turning the mcromctsr screw until these images Just
touch each other the diameter OF Mercury Is
measured. The polar compression is obtained by
COMPARISON oil the measured polar and equatorial
rHE LONGITUDE or TnK PLANET.
To dcterinlue Tho longitude Of Mercury we must
determine AS exact position upon The lace OF tho Gun
a known instant Of time, ana this IL best ny
means eitlser oil hciIomctcr or of photoheograph.
As the Naval Observatory does not possess an exam
pie Of the former instrument IT'S observers win en-
pioy tue horizontal pnotoheliogrnpns which were
uarsd In observing tho last transit oil Venus Each
OF thom consists essentially OF iorty loot tcle
scope placed horizontally and pointed TO the north,
tho suns rays being reflected INTO by very per.ect
mirror suitably mounted and uriven by clockwork.
sensitive photographic plates placed at the eye end
otthe telescope receive tue image oi be sun ana thus
pictures OF IL ratner III Tour aiame-
ter are oDtaiued These pictures, NO matter where
MADE. VIII all be taken TO Washington anu IN assured
upon designed Tor that purpose by Professor
| Hardness, and IRON tue measurements so made tho
position Mercury upon the lace Of the sun AT The
instant when ths pictures were made VIII become
known.
HINTS ron AMATEUR OBSERVERS.
Having now shown nov tue astronomers will MAKO
their observations perhaps some oi the HERALD's
readers would like 10 be told how they can ses the
transit Gu the otn Of May Mercury viii nrst touch
the rs northeast side at about 1905 A. NJ,
washing ton mean Time aud will slo%sy pass across
the sun disk little north Of as centre Gualy leav
ins II It's northwest side ABOUT 5:so Il N, wash
Ington time Inc times oil contact Tor any other
place than Washington can be iouna by applying
| 10 the Washington time the difference between tho
lime at Tue place IN question aud sshington time.
For pic--Sew York cay time - twelve
minutes faster than Washington time, conse-
quentiy the transit viii Dcgn In New York
~ 1918 A. NJ. ana end 5:52 T.. NJ.
| while the sun Mercury Will appear a round
black point small TIT there ~ doubt whether
or lot Will be visible TO the naked eye, but ~ can
easily be with an opera glass or small spy glass
6i course a piece OF smoked glass must De used la
observing WIN the naked eye. Large telescopes
always suitable coiored glasses Tor observing the
| sun, but spy glasses are rarely and opera glasses
never proviaen with anything OF the xind Howevcr.
this need prevent lueir owners from using them
10 observe tue transit because little smok. applied
citney 10 the object glass or the eyo lens will make
| them answer every purpose
j IRXING uAlI. IL MlNEAlN