The Sun.
London, England, 1860-10-25 00:00:00

THE SECULAR CHAT GE OF THI EARTAPJ
corBIT.

Mr. J. R. Hind, in a letter to the Times, says:—
Several inquiries have been lately addressed to me
with reference to the present state of our knowledge
of the secular inequalities of the earth's orbit, and
its bearing upon the general change of climate
which some geologists recognise and attribute to a
great variation in the obliquity of the ecliptic.
I have reason to believe that the results of the
most recent and complete investigation on the
subject, that of M. Leverrier, are but little known
except to astronomers in this country.
Independently of those inequalities in the orbits
of the planets which are termed "periodic," and
which go through their variations in a few hundred
years at the utmost, there is a class requiring periods
of vastly greater length for their entire exhibition,
which are yet within the grasp of the physical
astronomer, who can define their limits without the
chance of material error. They arise from the slow
but accumulative attraction of the planetary system
upon each of its members, whereby in a cycle of im-
mense duration every orbit is more or less dis-
turbed, though from the nature of the perturba-
tions the planes and eccentricities are necessarily
kept within certain limits. This disturbance it
has been M. Leverrier's object to define.
In the case of the earth it is shown by the French
mathematician that the greatest possible deviation
of the plane of the orbit from the position it occu-
pied in 1800 is 4 deg. 52 min. one way or the
other, and it is easy to see from the form adopted
in the calculations that this result is not liable to
I an uncertainty of more than five or six minutes.
Hence it follows, that the greatest possible dis-
tance of the tropics from the equator is 28 deg. 20
min., and the least 18 deg. 36 min. The eccentri-
city is found to attain a maximum included be-
tween the limits 0.06 and 0.08, the most probable
value being 0.078; consequently, assuming the
earth's mean distance from the sun to be
95,365,000 miles, the difference between her
greatest and least distances may amount to nearly
15,000,000 miles ; while, on the other hand, there is
apparently nothing to prevent her orbit becoming,
for a time, sensibly circular. This, however,
will not take place during the present oscilla-
tion. After the lapse of about 24,000 years
from this epoch, a minimum of 0.003 will be at-
tained, whereby the difference between the earth's
extreme distances will be reduced to 500,000 miles.

The following table, exhibiting the distances of the
tropics from the equator, and the effect of the ellip-
ticity of the earth's orbit upon her distance from the
sun, for periods of 10,000 years, reckoning back-
wards from 1800, is founded upon M. Leverrier'e
theory. I give his results in this form as being
more suitable for a popular communication than
that employed in the original memoir :

Differences between
Distance of the the Earth's greatest
Tropics from and least Distances
the Equator. from the Sun.
100,0008900,000000
.. : : : **
_: : D 22222117095:
444884152 m.
53253'6637:
.• ............. 89;1460222111,e000:®.
70,000
6,0'27,000
60.000
50,000
40,000 ........ 27 30 56 • • .
2,079,000
30,000 .. • • .... 27 0 46 2,880,000
20,000 ..... ... 26 12 7
10,000 .... „• • 24 52 30 ...... , 3,567,000
000
0
It is evident from the above that they astronomi-
cal conditions which affect climatologynge of such
ve never
undergone, nor will ever suffer,
magnitude as would lead to the subversion of the
a cha
Present order of things.
---------7-- -- '°'—_

Years
before
1600.