There is a certain degree of satisfaction to the inquiring mind in
knowing that, even in these days of aptness for discovering and
explaining everything, there yet remains something to be found out;
something to excite speculation and recompense research. Such a
subject is the zodiacal light, which, for nearly two centuries past,
has at different times occupied the attention of astronomers and other
observers of celestial phenomena, though it is only of late years that
the theories concerning it have acquired anything like a precise
character. Many ingenious hypotheses have been thrown out, which may
perhaps be accepted as steps towards a true explanation; and while
waiting the result of further inquiry, we shall endeavour to make our
readers acquainted with the interesting phenomenon.
The zodiacal light is a peculiar brightness, pyramidal or wedge-like
in form, seen at certain periods of the year in the eastern or western
sky, before sunrise and after sunset. Its direction is in the line of
the zodiac, whence its name--not perpendicular to the horizon, but at
a varying angle, being in the spring from 60 to 70 degrees. The base
of the wedge, which has a breadth generally of from 10 to 12 degrees,
is below, and the sides rise in a line, curving outwards, to the apex,
but so vague and diffuse as to be frequently indefinable. In our
latitudes, it is best seen at or just after the equinoxes; before
sunrise in autumn, and after sunset in spring; and becomes invisible
as twilight increases, or if the moon shines; the light even of Venus
and Jupiter is sufficient to render its discovery difficult. It is
brightest at the base, and grows fainter the further it stretches from
the horizon, vanishing entirely at the point. Unpractised observers
would be apt to overlook it altogether, and those accustomed to watch
the heavens are at times obliged to fix one eye on a dark space of
sky, while they search for the light with the other, and discover it
only by the contrast. A stratum of black cloud resting on the horizon
often affords a means of detection, as the light can then be seen
shooting from it with comparative distinctness. The soft, clear
atmosphere which usually precedes or follows rain, is very favourable
to a view of the light.
The luminous wedge varies in length with the progress of the seasons:
sometimes but little more than its point is visible; at others, it is
seen extending over a space of 120 degrees. Astronomically speaking,
the axis of the zodiacal light is said to lie in the plane of the
solar equator, with an angle of more than 7 degrees to the ecliptic,
which it consequently intersects, the points of intersection becoming
its nodes, and these nodes are the parts through which the earth
passes in March and September. The light travels forward along the
zodiacal signs from Gemini to Cancer and Leo from August to November,
keeping pace with the sun. It grows dim towards the end of November,
and fades more and more until January; but while this decrease has
been going on in the east, and in the morning, the light has presented
itself with increasing brightness in the west, and in the evening, and
pursues its course until the end of February at about the same rate of
motion. In March, it is slow, and travels through not more than one
sign, and fades in April, and is lost in May, to reappear again at the
end of summer, and perform the same route.
Lengthened twilight is not favourable to the appearance of the
zodiacal light; it can, therefore, be observed successfully in the
temperate latitudes only by patient and long-continued watching. But
in tropical regions, the deep azure of the sky, and the brief
twilight, give it a distinctness and luminosity never witnessed
elsewhere. In Egypt, we are told it is clearly 'visible every night,
except when the light of the moon is too great, from January to June;'
and in India its appearance is described as that of 'a pyramid of
faint aurora-borealis like light' usually preceding the dawn. Humboldt
tells us, that he has seen it shine with greater brightness than the
Milky Way, from different parts of the coast of South America, and
from places on the Andes more than 13,000 feet above the sea-level.
'Those who have dwelt long,' he writes, 'in the zone of palms, must
retain a pleasing remembrance of the mild radiance of this phenomenon,
which, rising pyramidally, illumines a portion of the unvarying length
of the tropical nights.' And once, during a voyage from Lima to
Mexico, he saw it in greater magnificence than ever before. 'Long
narrow clouds, scattered over the lovely azure of the sky, appeared
low down in the horizon, as if in front of a golden curtain, while
bright varied tints played from time to time on the higher clouds: it
seemed a second sunset. Towards that side of the heavens, the light
diffused appeared almost to equal that of the moon in her first
quarter.'
The zodiacal light can hardly fail of having been observed by
astronomers in the past ages of the world; but the earliest known
mention of it occurs in the _Britannia Baconica_, published by
Childrey in 1661. The writer says: 'There is another thing which I
recommend to the observation of mathematical men--which is, that in
February, and for a little before and a little after that month--as I
have observed for several years together--about six in the evening,
when the twilight hath almost deserted the horizon, you shall see a
plainly discernible way of the twilight, striking up towards the
Pleiads, and seeming almost to touch them. It is so observed any clear
night, but it is best _illæ nocte_. There is no such way to be
observed at any other time of the year. But what the cause of it in
nature should be, I cannot yet imagine, but leave it to further
inquiry.' The further inquiry followed soon afterwards, for Cassini,
the eminent French astronomer, having carefully observed the
phenomenon from 1683 to 1688, communicated the results to the Académie
des Sciences. Some of his views and determinations were well founded;
and from them we gather that the zodiacal light was nearly or quite
the same in his day as at present. Others also devoted considerable
attention to it, and noticed the variations in brightness in different
years, which subsequent observations have verified. Since then, it has
been made more or less a subject of investigation by modern
astronomers, and has been observed in many parts of the world; the
first observations in the southern hemisphere being those made by
Professor Smyth at the Cape of Good Hope, from 1843 to 1845. In that
latitude, the zodiacal light is best seen in spring evenings, at an
angle of 30 degrees, visible long after sunset; its opposite peak is
discernible at daybreak, but has scarcely come into view before the
rising sun overpowers it. In autumn, the reverse takes place; the best
appearance is in the morning.
To understand what is meant by the 'opposite peak,' we are to regard
the zodiacal light, of which we see only one end in our latitudes, as
a body extending all round the sun in the same form, presenting at a
distance the appearance of one of those flat elongated oval nebulæ
seen in the heavens. Its direction is at right angles to that of the
sun's rotation, a straight line drawn from either pole of the great
luminary divides it in the centre. From its outline resembling that of
a lens in section, it is frequently described as a 'cosmical body of
lenticular form.'
From this account of what the zodiacal light appears to be, we proceed
to consider what it is. Some inquirers--arguing from the 'nebular
theory,' which assumes the formation of the several planets, one after
another, from nebulous matter--have supposed the zodiacal light to be
a remnant of that matter yet unconcentrated. In this view, it may be a
nebula, brightest in the centre, as is the case with most, and fainter
towards the margin. According to Humboldt, 'we may with great
probability attribute the zodiacal light to the existence of an
extremely oblate ring of nebulous matter, revolving freely in space
between the orbits of Venus and Mars.' On several occasions he
witnessed its fluctuations, night after night, from the plains of
South America, shewing itself at times greatly collapsed or condensed,
with intermittences of vividness and faintness, in the course of a few
minutes, as is observed of the aurora. The light of the stars, of even
the fifth or sixth magnitudes, can be seen through it: the same has
been remarked of comets; and it is known also that the tails of comets
undergo frequent flashings or pulsations, so that the two phenomena
may be analogous in character. It is necessary, however, to
distinguish the fluctuations from such effects as may be produced by
movements in the lower strata of the atmosphere.
Mairan, who wrote in 1731, was of opinion that the zodiacal light
consisted of particles thrown off from the sun by its rapid rotation,
or a species of atmosphere peculiar to the central orb. Others have
supposed the luminosity to be composed of 'revolving planetary
particles,' shining by a direct or reflected light. But, according to
Professor Olmsted, of Yale College, Massachusetts, it is something
which has a motion of its own around the sun, notwithstanding that the
general steadiness of its movements had warranted the notion that it
was in some way attached to the body of the sun itself. Olmsted's
conclusions are drawn from a diligent observation of the light during
a period of six years, and are on this account, as well as from his
scientific reputation, entitled to respect. He states the light to be,
in constitution, colour, and density, similar to that of the tail of a
comet, the portion nearest the sun being brightest, and both admitting
of stars being seen through them. We may, therefore, infer it to be a
nebulous ring surrounding the sun, in the same way that the
magnificent rings of Saturn surround that planet. Of such nebulæ as
this there are from 2000 to 3000 visible in the regions of space,
compared with which the dimension of ours is insignificant: at the
same distance, and sought for with the same instruments, it would be
invisible.
In one point, Professor Olmsted's views are particularly interesting,
as, out of one mysterious phenomenon, he endeavours to explain
another, and inquires: 'Whether or not the zodiacal light is the
origin of the meteoric showers of November and August, and especially
those of November?' Many readers know that for some years past great
numbers of falling-stars, or showers of meteors, have been observed
periodically in November: the fall seen in the United States in
1834--when, as is estimated, more than 240,000 stars fell as thick as
snow-flakes, in the space of nine hours--being the most remarkable
hitherto known. The explanation is, that the zodiacal light is a
nebulous body revolving round the sun, and arrives at its aphelion on
the 13th November in that part of the earth's orbit which the earth
then reaches, and, coming into contact with our atmosphere, portions
of the nebulous matter are detached, and, taking fire as they pass
through, appear to us as shooting-stars. This explanation of the
phenomenon in question is one not hastily conceived; the reasoning on
which it is founded is altogether satisfactory, as well with regard to
the movement of the nebulous matter, as to that of the earth.
Professor Olmsted, in a communication addressed to the 'American
Association for the Advancement of Science,' sums up his views as
follows:
'1. The zodiacal light, as we have found, in our inquiry into its
nature and constitution, is a _nebulous body_.
'2. It has a revolution round the sun.
'3. It reaches beyond and _lies over the earth's orbit_ at the time of
the November meteors, and makes but a small angle with the ecliptic.
'4. Like the nebulous body, its periodic time is commensurable with
that of the earth, so as to perform a certain whole number of
revolutions while the earth performs one, and thus to complete the
cycle in one year, at the end of which the zodiacal light and the
earth return to the same relative position in space. This necessarily
follows from the fact, that at the same season of the year it occupies
the same position one year with another, and the same now as when
Cassini made his observations nearly 170 years ago.
'5. In the meteoric showers of November, _the meteors are actually
seen to come from the extreme portions of the zodiacal light_, or
rather a little beyond the visible portions.'
There is much that is suggestive in this summary, and, as we said at
the commencement, the subject is one of a nature to stimulate inquiry
and research, and to lead to further explanations of cosmical
phenomena. M. Mathieson's observations, published in the _Comptes
Rendus_ of the Académie des Sciences for 1843, shew, that when tested
with the thermo-multiplier, the zodiacal light was found to radiate
heat as well as light--a fact which, if further verified, will support
the evidence in favour of an independent luminous ring.