Light viewing in a balloon

SOME weeks ago there appeared in Weather Eye a digest of The Adventure of One Hans Pfaall

SOME weeks ago there appeared in Weather Eye a digest of The Adventure of One Hans Pfaall. It is a far fetched tale, as one would expect from the pen of Edgar Allen Poe, and it tells how the eponymous balloonist soared to unprecedented heights, right to the surface of the moon and back again. He describes one of the sights of his ascent as follows:

"The lenticular phenomenon, also called the zodiacal light, was a matter worthy of attention. This radiance, so apparent in the tropics, extends from the horizon obliquely upwards, and follows generally the direction of the sun's equator. It appeared to me evidently in the nature of a rare atmosphere extending from the sun outwards, beyond the orbit of Venus at least, and I believed indefinitely farther. It was easy to imagine it pervading the entire regions of our planetary system, condensed into what we call atmosphere."

Poe's description is astute, especially considering that it was written in the 1830s. Zodiacal light is a faint illumination sometimes to be seen in the western sky a little after sunset, just after the fading of the pinky glow of twilight. In shape it resembles an elongated pyramid, rounded at the top and, as Hans describes, rising obliquely from the horizon.

In texture it seems like a kind of luminous mist reminiscent of the Milky Way but somewhat "milkier"; it is brightest and broadest at the base and becomes fainter and narrower as you view it higher in the sky.

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The orientation of the zodiacal light relative to the horizon is dictated by the plane of the earth's orbit around the sun. At these latitudes, the pyramid makes only a small angle with the horizon for much of the year and is very difficult to see, but in the few weeks coming up to the vernal equinox the axis of the pyramid is not too far from vertical.

Hans Pfaall is less correct when it comes to what might cause this interesting phenomenon. Zodiacal light was once thought to be sunlight shining on the very high atmosphere of the earth. Now, however, we know that it is caused by sunlight reflected from debris left over after the formation of the planets, most of which lies close to the plane of the earth's rotation around the sun.

The colour of zodiacal light is similar to that of sunlight, which tells us that the debris is not composed of gaseous atoms or molecules which would produce a bluish colour like the sky in daylight but ranges in the size of its constituents from microscopic dust to mini asteroids that may be several feet across.