St. Paul daily globe., 1895-12-22
TWICE IN THIS DECEMBER WILL
LUNA SHOW A FULL
FACE.
PHENOMENON NOT REREs
BUT NOT EASY OF EXPLANATfQN,
TO THE UNINiTlA- oaT
5>.~s4 r rvsD aiiii' u
e19fT
TO OLD-TIME ASTRONOMERS
The Division of Time Was u Puz-
zle-NO Mystery to the Night
ROunder. - ..

There will be two full moons. IN
December this year, one on the "a
and one on the 31st. A full moon is
supposed to occur once a month, but,
as a matter of fact, it is not so ae-
! commodating. The result is a com
plication of the calendar which gives
| the mathematical astronomers some
figures to keep them busy while they
are waiting for a new comet or
double star to be discovered.
The month, one of the twelve d1-
visions of the year according to mod
ern calendars, has no very exact
standing among astronomers. There
| is no astronomical reason why one
particular month should have thirty
one days and another thirty. And
yet the ASTRONOMERS are full of refer
ences to the month. They are not
satisfied with plain, simple month,
which is good enough for the aver
| age man, but they have solar months,
nodical months, anomalistic months,
| siderial months, synodic months and
no telling how many other varieties.
| w Ueu lie dIlCl6ulS COIIlulcuCSo "
reckon time they used the day and
the year, these being the more OBJ
vious divisions. Then they saw the
moon and invented the month. Then
they saw the stars and observed that
a belt around the heavens was
marked by twelve consultations,
! which they called the signs Of the
| zodiac. But their divisions were en-
| tirely too inaccurate for modern as
| tronomers, who add a restricting ad-
! jective whenever they speak of
| month"
MANY KINDS OF MONTHS.
The calendar month is the month
as it is known to business men and
others who talk plain English. The
solar month is an exact twelfth part
of a year, and is reckoned by the
signs of the zodiac. 1t amounts to
! hours. Thus there is the nodical
month, which is the time of the
moons revolution from one ofsits
! nodes to the same node again, and
/ is equal to 27.21222 days. The anoma1-
| istic month is the time of revolutfon
/ from perigee to perigee again and
| equals 27.5546 days. The sideriaI
month is the time occupied by the
moon in passing from one star to- the
same star again and is equal to
! 2732166 days. The synodic month is
simpler, this being the interval from
: n O1in If
F! varies somewhat on account Of the
eccentricity of the moons orbit, and
! of that of the earth around the suns
=, but it's mean value is 29.53059 days,
or 20 days IL hours Al min;
utes and 1854 seconds, with
the possible variation Of a hum
/ dredth part of a second. The rea-
son why this synodical month is long
/ er than the siderial is that during
each siderial month of twenty-three
and one-third days the sun has adj
vanced among the stars and must
| be caught up with.
! These figures appear to be suffi-
ciently accurate for all ordinary pur-
poses, but the astronomers are not
yet satisfied. They have discovered
that the month is trif1e shorter now
than in the days Of Ptolemy, and
this trifle must be computed down
to the fraction of a second. But that
is another question.
PHENOMENON NOT RARE.
Enough figures have been given to
show how it is possible for two full
moons, which mark the beginning of
two lunar months, to happen in a sin
gle calendar month. Owing to the diff
| ference in the length of the different
kinds of months the phenomenon must
| happen in some month or other about
once in three years.
Way back before Christ, Meton, a
! Greek, in an effort to reconcile lunar
and solar time, discovered the Metonic
! circle, which consists of 235 lunar
months and is equal within two hours
to nineteen common years Of 1854 days
Every nineteen years the new and
full moons recur on the same day of
the year, and about the same hour of
the day. The calendar of the phases of
the moon, therefore, is the same for
1895 that it was for 1876, and that it