492 PROFESSOR PIAZZI SMYTH ON THE 
A few days after the last observation I left the Cape, and the passage thence 
by St Helena through the tropics, was so uniformly cloudy, that I was unable to 
obtain another satisfactory look at the zodiacal light, and ever since my residence 
in Edinburgh in the middle of a city glowing at night with gas, and reeking with 
smoke, and under a sky but rarely clear, and when it is so, not unfrequently 
illuminated by the Aurora Borealis, I have been equally unfortunate. 
To be able to make a good observation of the zodiacal light, the sky should be 
quite free from clouds, the air pure and transparent; not the slightest vestige of 
the twilight should remain, the milky way should be far from the neighbour- 
hood, the moon should not be visible, or the brighter planets, such as Venus and 
Jupiter. If these circumstances be secured, and a person look out at that period 
of the year, as hereinafter detailed, when the ecliptic makes a large angle with the 
horizon of the place of observation, he can hardly fail to see the phenomenon in the 
most marked degree. A beginner must be especially cautioned not to begin looking 
too soon in the twilight to discern the “ Sun’s atmosphere,” under the idea, happily, 
of catching it before all traces of the sun’s light on the horizon are completely 
gone; and he should also be forewarned of the immense influence which climate 
and geographical position have on the visibility and apparently on the form and size 
of the phenomenon. Thus, in 56° N. Lat., and still less further north, even were 
the elongation of the light E. and W., equal in every respect, it would still never 
appear equally visible, and would but seldom be seen either way. In the summer 
the twilight would render the sight impossible, and in winter, the sun’s path is 
too low and oblique. In the spring evenings, the light would be well seen, be- 
cause then the twilight is of a moderate length, and the zodiacal light rises at an 
angle to the horizon of 30° greater than the equator, and therefore does not set 
till long after the twilight has disappeared; but as the other end rises in the 
morning at that season, necessarily at an angle 30° less than that of the equator, 
the apex in its standing position, hardly rises above the mists of the horizon be- 
fore the twilight illumines the sky. In the month of September matters are just 
reversed, and the zodiacal light rising in the morning at a greater angle than the 
equator, is then well seen, but is not at all visible in the evening, when, from the 
standing position of the body, the apex sets very soon after the sun. And in these 
two short appropriate seasons, so many of the nights may be rendered untoward 
by clouds, strong moonlight, and other causes, that an opportunity of seeing the 
zodiacal light may even then but very rarely be enjoyed. 
Similarly in the southern hemisphere in 56° S. Lat., supposing also, as before, 
that the zodiacal light stretches out equally from the sun on every side in the 
plane of his equator, the two most favourable opportunities in the course of the 
year for viewing the body would be, in the evening in the month of September, 
and in the morning in the month of March. And that this would really be the 
case, the observations made in Lat. 33 S. sufficiently attest. 
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