Chamberss Edinburgh Journal, No. 449 | Page 9

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