VII.

_THE LUNAR HALO._

There are some phenomena of nature which suggest false ideas. For

instance, when we look at the broad expanse of ocean on a moonlit night,

and see a path of glory on its surface, directed towards the moon's

place, we seem to be assured by the sense of sight that that broad track

is illuminated while the waters all around are dark. A little

consideration, however, assures us that the impression is a false one,

that in this case seeing is not believing. The moon's rays really

illumine the whole surface which lies before us, and we fail to receive

light from other parts than the track below the moon, _not_ because they

receive no light, but because the light which they receive is not

reflected towards us. An observer, stationed a mile or two towards the

right or towards the left of our station, sees a different track of

light, while the part which seems bright to us seems dark to him.

The rainbow is another phenomenon of this deceptive kind. We seem to see

an arch of many colours suspended in the air,--and when we learn that it

is due to the presence of drops of water in the air, we are apt to infer

that where we see the red arch there are drops lit up with red light,

where the yellow, green, or violet arch, that the drops are aglow with

yellow, green, or violet light. But in reality this is not so; the same

drops which seem green to us will seem red to another observer, violet

to another, and to yet other observers will show none of the prismatic

colours, but only the dull grey colour of the cloud on which the rainbow

is seen. We have here a pretty emblem of the varied aspects which events

of the same real nature present to different persons, or according to

the different circumstances under which the same person may see them.

One shall see events in rosy tints, or with the freshness of spring

hues, or with the melancholy symbolled by the

deeper indigo (as when

The heavy-skirted evening droops with frost)--

while to others the same events shall show only the ordinary tints of

common-place life.

The lunar halo is one of the phenomena thus deceptive to the view. We

see all around the moon a circle or arc of light, nearly white, though

sometimes faint tints of colour can be perceived in it, while the space

within the circle seems manifestly darker than the space outside. The

appearance of the halo as seen under favourable conditions is shown in

fig. 11, on the next page. In this country the dark space round the moon

is not generally so well seen as in countries where the air is clearer.

But this is in reality the characteristic feature of the halo, as its

name shows. For the name is derived from a Greek word signifying

threshing-floor (the old threshing-floors being round), and thus

naturally describes a round space relatively clear, surrounded on all

sides by a ring of aggregated matter.

We seem in looking at the lunar halo, then, to see the moon at the

centre of a dark space, surrounded by a ring of bright particles,

outside which again are particles not quite so brightly illuminated as

those forming the ring, but more brightly than those within the ring.

But in reality this impression, which, so far as the sense of sight is

concerned, seems _forced_ upon the mind, is entirely erroneous. There is

no real distinction between the space which looks dark all round the

moon, the space beyond which does not look dark, and the ring between

the two spaces which looks bright. These are all equally illuminated by

the moon, in the same sense, at least, that we say the surface of a

moonlit sea is all equally illuminated, neglecting slight differences

which do not concern the point we are specially dealing with. Precisely

as the path of light on the ocean is not a real path of illumination,

bounded on either side by dark spaces, so the ring of light round the

moon is not a real ring of light, bounded on one side by a less bright

region, and within by a dark space.

[Illustration: Fig. 11.--Lunar Halo]

Although my object in these essays is not specially to deal with

scientific matters, but rather with the thoughts (much more important in

my belief) which they suggest--so that, in dealing with my present

subject, I wish rather to call attention to the manifold ways in which

our senses may deceive us unless their evidence is carefully

cross-examined--yet it may be worth while to notice how the particular

illusion here considered has deceived even the scientific elect.

It had been noticed by Tyndall, in certain experiments, that a very

sensitive measurer of heat, when placed under the moon's rays, gathered

together by a powerful condenser, seemed to indicate cooling rather than

heating, as we should expect. On this a French student of science

pointed to the darkening under the moon where the lunar halo is seen as

evidence that our satellite possesses a certain power of clearing away

vaporous matter from the air. "_On peut dire_," he said, speaking of the

dark space within the halo, "_que la lune ouvre alors une porte par

laquelle s'échappe le calorique que l'action solaire a emmagasiné dans

les couches inférieures_." "One may say," that is, "that the moon then

opens a door through which the heat escapes, which the sun's action has

stored up in the lower layers" (of the air). It will be manifest, if we

remember that a lunar halo can often be seen at the same time from

stations hundreds of miles apart, that there can be no such opening of

clear air. For the cloud layer in which the halo is formed is but a few

miles above the observer; and therefore, if one observer saw a circular

opening in this layer, with the moon at its centre, another, a hundred

miles from him, would see the space in a very different direction. The

moon would not only not be at the centre of the space for this second

observer, but would not be visible through the space at all. Moreover,

the space could not possibly seem round to both observers; if it seemed

round to one, it would look like a very flat oval of darkness (almost a

mere line) to the other.

The real explanation of the lunar halo is very different. When you see

such a halo, you may be certain that there is, high up in the air, a

layer of light feathery cloud--the cirrus cloud, as it is

called--composed of tiny crystals of ice. These crystals, as we know

from those which in winter sometimes fall (not as snow, but as little

ice-stars), have all a definite shape. They are in fact little prisms of

ice, with angles like those of an equilateral triangle. These little

prisms deflect the light which falls upon them, just as one of the drops

of a chandelier deflects any light which falls upon it. If you hold a

prism-drop of a chandelier between the eye and a light, you will see

that the prism looks dark; it is really lit up, but it sends the light

away in such a direction that the eye receives none. Now move it

gradually away from the line of sight to the light, and at a certain

distance it appears full of light; or, to speak more correctly, it sends

the light it receives directly towards your eye. Beyond that position it

again looks dark, but not so dark as when it was nearly between the eye

and the light.

The little crystals of ice perform the same part with respect to the

moon, when we see a lunar halo. Those between us and the moon, or within

a certain distance from the line of sight to the moon, are, in reality,

lit up by the moon's rays; but they send off those rays in such

directions that we do not receive the light. Thus, all the space lying

towards the moon, and for a certain distance all round, looks dark. But,

at a certain distance, these little crystals send us light. If we could

see them separately, they would seem to be full of light. That is the

distance where ice-crystals of their known shape act most favourably in

deflecting light,--that is, send off most for all the varying positions

(not places) they can be in. At greater distances, a small proportion

send us light. Thus, at that distance we have a ring of light, and

outside the ring we have a gradual falling off in the quantity of light.

But the reader will be apt, perhaps, to say, How can all this be proved?

No one has ever been among the ice-crystals of the feathery clouds when

they are performing this work. When Coxwell and Glaisher made their

highest ascent, the feather-clouds seemed almost as high above them as

ever. Nor, if any one could reach those clouds, could he see the

ice-crystals at their work. Yet there are few points about which science

is more certainly assured than about this explanation of the halo. For

we know the shape constantly assumed by ice-crystals; we know according

to what precise law ice bends rays of light falling upon it; hence we

can calculate quite certainly where, if ice-crystals make the halo, its

rings should be seen. And the halo has the precise position thus

calculated from the known laws of optics, and the known facts about ice

and ice-crystals. The diameter of the halo should be, and is, about

eighty times the apparent diameter of the moon, or somewhat less than

half the arc which separates the point overhead from the horizon.

There is, however, yet stronger evidence. Haloes form around the sun as

well as round the moon,--in fact, more frequently. Solar haloes have so

much more light in them that we can recognise varieties of tint. Now, it

follows from the laws of optics that, for the red part of the sun's

light, the halo ring should have a smaller diameter than the halo ring

for the violet part, intermediate colours having their corresponding

intermediate halo rings. Thus, the halo ring, as a whole, should be

rainbow-tinted, red on the inside, then orange, yellow, green, blue,

indigo, and violet; and these colours are shown (under favourable

conditions) in this order.

The student looking out for haloes, solar or lunar, must be careful not

to confound them with solar and lunar coronas, that is, not the corona

of astronomy, but rings of light around the sun and moon, much smaller

than the true halo rings. What I have said above about the size of the

true halo will suffice to prevent such a mistake. Coronas are not nearly

so _easily_, though they have been quite as thoroughly, explained by

science, as haloes.

It is singular to observe how utterly unlike the interpretation of the

halo by science is from the natural interpretation. The observer would

say, There surely is a dark space all round the moon, and round that a

ring of light,--I see these things, and seeing is believing. Science

says there is no dark space, and there is no ring of light; while the

eye of science perceives something where the lunar halo shines which

ordinary vision cannot recognise. Up yonder, many miles above the earth,

science sees millions of crystals of ice, carried hither and thither--so

light are they--by every movement of the air. Science sees these ice

crystals deflecting the rays of moonlight, sifting the red rays from the

orange, and these from the yellow, yellow from green, green from blue,

blue from indigo, and indigo from violet. Science, in fine, perceives

processes taking place in those higher regions of air compared with

which the most delicate analyses of the laboratory are utterly coarse

and imperfect.

There is a purer and nobler poetry in the lunar halo as thus understood

than in its mere visible phenomena, attractive and beautiful though

these are. Idle indeed is the fear that the interpretation of this

special mystery of nature will leave the number of nature's mysteries

diminished by one. On the contrary, for the one mystery explained many

deeper mysteries are suggested. The phenomena discernible by the sense

of sight are explained, but only by bringing into the range of a purer

and more piercing vision phenomena infinitely more wonderful. If one

could see through some amazing extension of visual power, or if even the

imagination could adequately picture, the rush of light waves of all

orders of length upon the line of crystal breakers, their deflection in

all directions, their separation into their various orders of

wave-length; if one could perceive the actual illumination of the

ice-crystals, even where they seem dark to us, and the continual

fluctuations of the troubled sea of ether between the crystal breakers

and the earth below,--the scene would infinitely transcend in interest

and mystery, the picture would be infinitely more suggestive of solemn

thoughts, than the scene--beautiful though it doubtless is--presented

by the halo-girt moon to ordinary vision. Truly they know little of the

real meaning of science who regard it as depriving natural phenomena of

their effect on the imagination, as robbing Nature of her poetic

influence.

[Illustration]