Evening star. [volume] (Washington, D.C.) 1854-1972, 1891-05-02
SUN
THE CONTACTS.

II these four contacts could be observed with
absolute accuracy the observations would have
q much greater value. But the external con-
facts are difficult to note. while the internal
contact is rendered uncertain on account of
faulty atmospheric and faulty optical effect,
known as the "black drop"-a distortion Of the
image of Mercury. If the orbit of Mercury
and the earth were both in the same plane the
transit would be seen at each con junction.
But that of the innermost planet is inclined
some seven degrees to the ecEptic (the earth's
path around the SUM, which plane it crosses
twice in each revolution. at points termed the
modes" and only if the conjunction happen
when the planet is very near a node will
transit result 1t can only happen about May
for one node, and November 9 for the other
In the former case Mercury is moving toward
the south lat the descending node and in
the latter he is travelling north (the Ascending
node"). We might expect the November and
May transits to occur with similar frequency.
but in point of fact the November transits are
nearly twice as frequent.
At the time of the May transit Mercury is
very near aphelion, or its greatest distance
from the sun, and the relative parallax being
greater conjunction must happen at q time
when the planet is very close to the node in
order to have q transit. That is why May
transits do not occur as often as November
transits.
OBSERVATIONS.
The first actual observation Of transit Of
Mercury was made in 1881 by Gassendi; the
first record Of any scientific accuracy was made
by HALEY on the island Of St. Helena in 1877.
Since then twenty-three different transits have
been observed and given astronomers q vast
array Of figures to work upon.
Transits Of Mercury happen at intervals Of
never less than three years nor more than thir-
teen. Usually the intervals run about as follows:
Three, thirteen, seven, ten, three, ten, three,
thirteen, DC. OF course the planets path across
the sun is not identical at each transit The
nearer Mercury is to the node at the time of
conjunction the more nearly central will be the
transit. The longest possible duration of q
November transit is about five and q half hours,
and Of May transit nearly eight hours When
the planet merely grazes the suns edge the
entire period may occupy much less than an
hour. The shortest transit ever observed
throughout was that OF 1852, November L. II
was recorded by several early American astron-
omers, including Winthrop and Rittenhouse,
and lasted only one hour and twenty minutes.
A still shorter one on November g. l776,
was not observed. In studying the different
paths Of the planet in transitu for o number of
centuries it is curious to observe how Q series
viii commence near the southern limit for No
vember, and near the northern border for May,
and gradually cross the face Of the sun at per-
fectly uniform distances and ct intervals of
regularity of the mechanism Of the universe, in
which the ponderous bodies forming our plan
etary system are held in such perfect control
by the force Of grsvitation that their relative
movements, both past and present may be
ascertained with absolute certainty.
SOME FONTS ABOUT MERCURY.
Mercury revolves around the sun at mean
distance of Sc, 000, 900 miles from that central
orb. When nearest the sun it IL % 0oO, 900
miles and when farthest II il a, 000, 900 miles
Its diameter il about one-third that Of the
earth and its year IL Lightweight days, or, in
other words, it makes revolution around the sun
in Lightweight days. In this latitude. owing to
the planets proximity to the sun, II u not often
seen. II sets 20 soon after or rises 20 soon be-
fore the sun that il 10 commonly % near the
horizon - to be obscured by the vapors seen
in that direction. Viewed with . telescope
Mercury appears 70 have phases Ike the moon,
sometimes appearing like . half moon and