By Professor C.A. YOUNG.
One of the most interesting inquiries relating to the moon is that
which deals with the heat she sends us, and the probable temperature
of her surface. The problem seems to have been first attacked by
Tschirnhausen and La Hire, about 1700; and they both found, that even
when the moon's rays were concentrated by the most powerful
burning-lenses and mirrors they could obtain, its heat was too small
to produce the slightest perceptible effect on the most delicate
thermometers then known. For more than a hundred years, this was all
that could be made out, though the experiment was often repeated.
It was not until 1831 that Melloni, with his newly-invented
"thermopile," [1] succeeded in making the lunar heat sensible; and in
1835, taking his apparatus to the top of Vesuvius, he obtained not
only perceptible, but measurable, results, getting a deviation of four
or five divisions of his galvanometer.
[Footnote 1: Probably most of our readers know that the
thermopile consists of a number of little bars of two different
metals, connected in pairs, and having the ends joined in a
conducting circuit with a galvanometer. If, now, one set of the
junctures is heated more than the other set, a current of
electricity will be generated, which will affect the
galvanometer. The bars are usually made of bismuth and antimony
though iron and German silver answer pretty well. They are
commonly about half or three-quarters of an inch long, and about
half as large as an ordinary match. The "pile" is made of from
fifty to a hundred such bars packed closely, but insulated by
thin strips of mica, except just at the soldered junctions. With
an instrument of this kind and a very delicate galvanometer,
Professor Henry found that the heat from a person's face could be
perceived at a distance of several hundred feet. There is
however, some doubt whether he was not mistaken in respect to
this extreme sensitiveness.]
Others repeated the experiment several times between this time and
1856, with more or less success; but, so far as I know, the first
quantitative result was that obtained in 1856 by Piazzi Smyth during
his Teneriffe expedition. On the top of the mountain, at an elevation
of ten thousand feet, he found that the moon's rays affected his
thermopile to the same extent as a standard candle ten feet away.
Marie Davy has since shown that this corresponds to a heating effect
of about 1/1300 of a Centigrade degree.
The subject was resumed in 1868 by Lord Rosse in Ireland; and a long
series of observations, running through several years, was made by the
aid of his three-foot reflector (not the great _six_-foot instrument,
which is too unwieldy for such work). The results of his work have,
until very recently, been accepted as authoritative. It should be
mentioned that, at about the same time, observations were also made at
Paris by Marie Davy and Martin; but they are generally looked upon
merely as corroborative of Rosse's work, which was more elaborate and
extensive. Rosse considered that his results show that the heat from
the moon is mainly _obscure, radiated_ heat; the _reflected_ heat,
according to him, being much less in amount.
A moment's thought will show that the moon's heat must consist of two
portions. First, there will be _reflected solar heat_. The amount and
character of this will depend in no way upon the temperature of the
moon's surface, but solely upon its reflecting power. And it is to be
noted that moon-_light_ is only a part of this reflected radiant
energy, differing from the invisible portion of the same merely in
having such a wave-length and vibration period as to bring it within
the range of perception of the human eye.
The second portion of the heat sent us by the moon is that which she
emits on her own account as a warm body--warmed, of course, mainly, if
not entirely, by the action of the sun. The amount of _this_ heat will
depend upon the temperature of the moon's surface and its radiating
power; and the temperature will depend upon a number of things
(chiefly heat-absorbing power of the surface, and the nature and
density of the lunar atmosphere, as well as the supply of heat
received from the sun), being determined by a balance between give and
take. So long as more heat is received in a second than is thrown off
in the same time, the temperature will rise, and _vice versa_.
It is to be noted, further, that this second component of the moon's
thermal radiance must be mainly what is called "obscure" or dark heat,
like that from a stove or teakettle, and characterized by the same
want of penetrative power. No one knows why at present; but it is a fact
that the heat-radiations from bodies at a low temperature--radiations
of which the vibrations are relatively slow, and the wave-length
great--have no such power of penetrating transparent media as the
higher-pitched vibrations which come from incandescent bodies. A great
part, therefore, of this contingent of the lunar heat is probably
stopped in the upper air, and never reaches the surface of the earth
at all.
Now, the thermopile cannot, of course, discriminate directly between
the two portions of the lunar heat; but to some extent it does enable
us to do so indirectly, since they vary in quite a different way with
the moon's age. The simple _reflected_ heat must follow the same law
as moonlight, and come to its maximum at full moon. The _radiated_
heat, on the other hand, will reach its maximum when the average
temperature of that part of the moon's surface turned toward the earth
is highest; and this must be some time after full moon, for the same
sort of reasons that make the hottest part of a summer's day come two
or three hours after noon.
The conclusion early reached by Lord Rosse was that nearly all the
lunar heat belonged to the second category--dark heat _radiated_ from
the moon's warmed surface, the _reflected_ portion being comparatively
small--and he estimated that the temperature of the hottest parts of
the moon's surface must run as high as 500° F.; well up toward the
boiling-point of mercury. Since the lunar day is a whole month long,
and there are never any clouds in the lunar sky, it is easy to imagine
that along toward two or three o'clock in the lunar afternoon (if I
may use the expression), the weather gets pretty hot; for when the sun
stands in the lunar sky as it does at Boston at two P.M., it has been
shining continuously for more than two hundred hours. On the other
hand, the coldest parts of the moon's surface, when the sun has only
just risen after a night of three hundred and forty hours, must have a
temperature more than a hundred degrees below zero.
Lord Rosse's later observations modified his conclusions, to some
extent, showing that he had at first underestimated the percentage of
simple reflected heat, but without causing him to make any radical
change in his ideas as to the maximum heat of the moon's surface.
For some time, however, there has been a growing skepticism among
astronomers, relating not so much to the correctness of his measures
as to the computations by which he inferred the high percentage of
obscure radiated beat compared with the reflected heat, and so deduced
the high temperature of lunar noon.
Professor Langley, who is now engaged in investigating the subject,
finds himself compelled to believe that the lunar surface never gets
even comfortably warm--because it has no blanket. It receives heat, it
is true, from the sun, and probably some twenty-five or thirty per
cent. more than the earth, since there are no clouds and no air to
absorb a large proportion of the incident rays; but, at the same time,
there is nothing to retain the heat, and prevent the radiation into
space as soon as the surface begins to warm. We have not yet the data
to determine exactly how much the temperature of the lunar rocks would
have to be raised above the absolute zero (-273° C. or -459° F.) in
order that they might throw off into space as much heat in a second as
they would get from the sun in a second. But Professor Langley's
observations, made on Mount Whitney at an elevation of fifteen
thousand feet, when the barometer stood at seventeen inches
(indicating that about fifty-seven per cent. of the air was still
above him), showed that rocks exposed to the perpendicular rays of the
sun were not heated to any such extent as those at the base of the
mountain similarly exposed; and the difference was so great as to make
it almost certain that a mass of rock not covered by a reasonably
dense atmosphere could never attain a temperature of even 200° or 300°
F. under solar radiation, however long continued.
It must, in fact, be considered at present extremely doubtful whether
any portion of the moon's surface ever reaches a temperature as high
as -100°.
The subject, undoubtedly, needs further investigation, and it is now
receiving it. Professor Langley is at work upon it with new and
specially constructed apparatus, including a "bolometer" so sensitive
that, whereas previous experimenters have thought themselves fortunate
if they could get deflections of ten or twelve galvanometric divisions
to work with, he easily obtains three or four hundred. We have no time
or space here to describe Professor Langley's "bolometer;" it must
suffice to say that it seems to stand to the thermopile much as that
does to the thermometer. There is good reason to believe that its
inventor will be able to advance our knowledge of the subject by a
long and important step; and it is no breach of confidence to add that
so far, although the research is not near completion yet, everything
seems to confirm the belief that the radiated heat of the moon,
instead of forming the principal part of the heat we get from her, is
relatively almost insignificant, and that the lunar surface now never
experiences a _thaw_ under any circumstances.
Since the superstition as to the moon's influence upon the wind and
weather is so widespread and deep seated, a word on that subject may
be in order. In the first place, since the total heat received from
the moon, even according to the highest determination (that of Smyth),
is not so much as 0.00001 of that received from the sun, and since the
only hold the moon has on the earth's weather is through the heat she
sends us (I ignore here the utterly insignificant atmospheric _tide_),
it follows necessarily that her influence _must_ be very trifling. In
the next place, all carefully collated observations show that it _is_
so, and not only trifling, but generally absolutely insensible.
For example, different investigators have examined the question of
nocturnal cloudiness at the time of full moon, there being a prevalent
belief that the full moon "eats up" light clouds. On comparing thirty
or forty years' observations at each of several stations (Greenwich.
Paris, etc.), it is found that there is no ground for the belief. And
so in almost every case of imagined lunar meteorological influence. As
to the coincidence of weather changes with changes of the moon, it is
enough to say that the idea is absolutely inconsistent with that
progressive movement of the "weather" across the country from west to
east, with which the Signal Service has now made us all so familiar.
Princeton, April 12, 1884.
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