Year: 1874

What Prof. Simon NeVEcomb Knows


About Them.


I have said that the sun pursues a
certain definite path among the stars,
about half a degree wide, which you
could see if he left any trace so, also,
if you could mark the position of the
moon tonight among the stars, and
mark its position at every hour during
her whole course, you would find that
she has alse pursued q definite path
among the stars, but you would not find
this path to be the same as the suns
path. If it were the same, we should
have an eclipse of the sun every time
the moon crossed the sun, and an
eclipse of the moon every time that the
moon passed on the other side of the
earth from the sun. But the two paths
are inclined to each other about five de-
grees. They cross each other at point
which, in the month of October, 1874,
is very near the sun. The moons path
is south of the suns in nearly all that
part of the heavens which we can see
In the evenings of that month but it
approaches the suns path and crosses
1t near the eastern horizon, and in most
Of the invisible half Of the sphere. or
thai part below the horizon, the moons s
path is farther north. All this viii be
clear on examining the star maps,
where the dotted line shows the path of
the moon during 1874, crossing the
suns path Ia the consultations Aries
and Libra.


These two opposite points in which
the moons path crosses the path of the
sun are called the nodes. 1t is very
clear that unless the sun is near one of
the moons nodes, when the moon her
self passes by, the moon will pass above
or below the sun, according as her path
is above or below that of the sun at this
point, and consequently there will be no
eclipse. But if the sun happens to be
near the node, then the moon will
necessarily pass over his face and
eclipse some portion of him. Now, as
1 have just explained, there are two op-
posite nodes the one set few hours
ago, and the other has Just risen. Since
the sun makes the whole circuit of the
heavens in the course of a year, he
crosses the moons nodes twice in that
time. In 1874 he crosses one node in
May, and will cross the other node in
November consequently it is only
about these two times that any eclipse
can take place during this year.


If the moon always followed the same
path in the heavens, we could Lever
have any eclipses but at those two sea-
sons. But if we watch the motions of
the moon for several years in success
sion, we shall find that her path is con-
tinuRlly changing. At the present time
she passes seven degrees north of
Aldebaran a month hence she will
seem to pass the star at almost exactly
the same distance but if you continue
your observations for four or five
months, you will find that she passes it
perceptibly farther north, and in three
years you will find that she crosses it
at the distance of about ten degrees.
After that she would begin to cross
farther south, passing near Aldebarau
at every revolution for eight years, un-
til in 1885 and 1886 she will pass right
over it. If the moons path were painted
on the heavens, you would see that
every time the moon came round to the
same point in her path, which takes
place about every twenty-seven days,
she would cross the suns path about
three of her own diameters sooner than
the month before. In the course of a
year, therefore, she will have crossed
about thirty-six diameters sooner, or
farther to the west. In the course of
twenty years y u will find that this mo-
ton has been kept until she crosses at
the same point she does now, and thus
the nodes have made a complete circuit
of the heavens. The seasons of eclipses
vary, therefore, in the same manner.
This year they are in May and October;
in five years from now they will occur
three months earlier, and we shall have
them in February and August; five
years more, and they Will be in October
and May; five years more, in August
and February; five years more, and
they will correspond once more to what
they are now.-HGr70er's Magazine.