The Providence news. (Providence [R.I.]) 1891-1906, 1893-12-29
HARVEST AND HUNTERS MOONS.
Brief Statements rs 20 the Causes OF These
Celestial Phenomena.

jricI StotcmeutS ~ to the OhUsee o. 4nee0
Celestial Phsnonena.
The time of moonrise on any day Is on
the average about 81 minutes later than
it was on thepreceding day. This retarda-
tion Is, however, by nomenns constant In
amount, being at times very much less
than tho average and at times very much
greater. A very marked diminution In
the time Of retardation occurs about the
time of the full moon which falls In Sep
tember. This moon Is called the harvest
moon. The same phenomenon in a less
degree occurs at the time OF the next fol-
lowing full moon. which is known as the
hunters moon. The time of rctardation
during the harvest moon Ii less than half
an hour In our ATTITUDES, instead of its av-
erage value of nearly an hour, and for sew
eral successive nights the moon seems to
rise at nearly the same time.
Briefly stated, the cause OF the phenom-
enon is this: At the time Of the rising Of
the harvest moon the apparent path of
the moon among the stars il much less in
clined to the horizon than it is at other
times, and the ordinary days motion of
the moon along this path makes an un-
usually small change OF position of the
moon with reference TO its distance from
the horizon.
A more detailed explanation may make
the matter little more easily understood.
In addition to the ordinary apparent daily
motion which the moon has In common
with all other heavenly bodies, it has
another motion by which it completes the
circuit of the heavens relatively to the sun
once a month, and the direction of this
motion is generally not parallel with the
direction of the diurnal motion, it being
generally northward or southwark as well
as eastward. Any one may see this mo-
tion by an hours watching of the relative
positions Of the moon and a star near it.
The diurnal motion Is always on line at
right angles to a line drawn from the body
to the celestial pole, the point in the heav-
ems approximately indicated by the pole
star. 1t is always perfectly uniform about
the pole as u center, and il is this which
we take "a our ordinary measure of time.
Now consider the position OF the moon
and the sun at the time Of full moon. The
moon rises just as the sun sets. On the
next evening at sunset the moon will still
be below the horizon, because il has moved
eastward among thes stars relatively to the
sun, and it will net rise until the diurnal
motion of the heavens brings il above the
horizon. II the moons motion were un-
form and always along the line of ordi-
nary diurnal motion, this retardation of
the time of rising would always be the
same, and would, rs stated at the outset,
be 81 minutes per day. But at the time
of harvest moon the direction of motion of
the moon among the stars is considerably
northward as well as eastward, and at the
time of moonrise this line makes a COM
paratively small angle with the horizon,
very much less than it does at other times
of full moon during the year. Therefore,
at sunset on the day after the full, the
moon having moved along line which is
inclined to the horizon ut a much smaller
angle than usual, its distance below the
horizon will be less than the average, and
hence a smaller amount of diurnal motion
will bring it into view-that is, the retar-
dation OF time of rising is less than it is
at other times. This condition continues
for several days.
Several other things, notably the incli-
nation Of the moons orbit to that of the
earth, and the varying rate of motion in
the orbit due to variation of distance from
the earth, tend 10 change the amount Of
daily retardation, but they do not depend
on the time OF the year, and they sometimes
intensity and sometimes diminish the pc
culiar phenomenon of harvest moon. Lat
itude has strong effect, and in northern
Europe the phenomenon Is much more
noticeable one than it is in the United
States. In fact, IL one goes far enough
north, the harvest moon may rise even ear
lier on any night than it did on the night
preceding. Popular Science Monthly.