National Republican., 1873-11-15
History OLE ASTRONOMY.

second Lecture by Professor Ncwcomb-
solar aud Lunar Rciipses-Thc Measure
ment OF Timc-T Formation OF the
Calendar.
Tho second lecture Of the course on the HIS
tory OF Astronomy wa, delivered last evening by
Professor simon Newcomb, at the Medical Col
lege OF the Columbian University, II street. The
subject was the motions NJ the moon and the
phenomcna growing out OF thcm, including She
measurement Of time and the formation OF The
calendar.
The Professor commenced by saying thai
among the most remarkable phenomena con.
nected with the motions OF the moon are tnyse
OF solar and lunar eclipses. Nothing IN .no
ancient progress Of knowledge Is nnoIe ,u, ,.,n,.,
able than that men were perfectly aoHuaneen
with the causes OF ecllpscs In the earliest ages
Of which wo have an exact history. .,, a,
From noticing that eclipses OF We lnoon ~. NJ
WAYS occurred at full moon, and eclipses OF tue
sun at new moon, it was dlyined that the former
were due 10 the passage OF the moon through
the dark shadow o. nc eaTn % anu sne me Nv ~, ,o
Tne nrst difficult question which must have
arisen IN this connection is, why In that case
should we not have an eclipse OF the moon A.
every full moon. and an eclipse OF the sun at
every new moon:
we nnd tnat Hipparcus, the father Of astron-
omv, was able TO answer this question. The
reason is that the sun and moon do not follow
the same apparent path In the heavens, but
their arbits cross each other At two opposite
points, called the nodes. The Inclination o.
In consequence the moon may pass i've degrees
above or Delow the sun, and the same distance
above or below the earths shadow.
IL Ia only when the two bodies happen to be
together near a node, that m eclipse can occur.
since there are two nodes on opposite sides OF
the ecliptic, the sun passes the node OF the
moons orbit twice In year, whence eclipses AZ
For example: an eclipse OF the moon occurred
on May llth last, and one Of the sun on May Yoth.
During the present month the sun reached the
opposite node, so that there was an eclipse OF
the moon on the Ith Of this month, while one Of
the sun, visible only near the south pole, will
occur next Thursday. After this the sun moves
occIr until next spring, when he again Ap-
pToaches the other node. IL the sun II very near
ine node At the time Of an eclipse, an eclipse Of
the moon VIII ee total, and one 81 the sun , wiz
parent diameter subtended by the moon 10
greater or less than tina, subtended By the sun.
There are reall, more eclipses Of the sun than
Of the moon, hut We see more OF the moon than
OF the sun because an echpse Of the moon u al
ways visible over more than half the earth, while
ono OF the sun is visible only over . compara-
lively small portion OF the earth.
Tie phenomeus Of eclipses are rendered more
complicated from the fact that the moons orbit
IA continually In motion, so that the nodes make
ilttle less than twenty years. In consequence
the average interval between eclipses Ia some
what less than six months, and the season OF
eCHHseb OrCufb hDOut tWeuty tNcty
year. This year the season Is Nay and Noveu,-
Der. but nve years hence II will he in February
and August.
The lecturer then explained the mode In which
the ancients were enabled to predict eclipses by
the period called the Saros. IL the sun and moon
start out together from the moons node, We
shall mud that after nineteen revolutions OF the
sun and two hundred and twenty-thrce reyolu-
tons Of the moon they will both again be found
together very near the node and all the conf unc-
tons Of the sun anil moon will recur In the same
order that they did before. The real period Is
Il eighteen years and eleven or twelve days, ac-
cofdlng to the number Of leap years In the inter
Il val; so whatever eclipse occurs today, an
eclipse, very nearly corresponding to ", will
occur at the end Of that period Of time.
The subject Of the measurement Of months and
years was then considered by the lecturer. The
original month was founded on the returns OF
the new moon, which take place at an average
interval Of twenty.nine and = half days. AZ
/ twelve OF these months do not make year,
great confusion Is found In all the ancient cal.
| endars In the endeavor to make the months and
| Years correspond. The formation OF the difer.
ent calendars was described, ending with an &C
| count OF the Julian and Gregorian calendar,
now adopted by all civilized nauons.