Flowers of the Sky

by Richard A. Proctor

Here, then, we seem to find the evidence of some cause external to the earth, as producing auroras, or at least as tending to make their occurrence more or less frequent. The earth has remained to all appearance unchanged in general respects during the last three centuries, yet in the sixteenth her magnetic poles have been frequently surrounded by auroral streamers; during the seventeenth these streamers have been seldom seen; during the last two-thirds of the seventeenth century auroras have again been frequent; and during the present century they have occurred sometimes frequently during several years in succession, at others very seldom.

Let us inquire a little more closely into the circumstances attending auroral displays, in order to ascertain what external cause it is which thus influences their occurrence.

Connected as auroras are with the phenomena of terrestrial magnetism, we may expect to find some help in our inquiry from the study of these phenomena.

Now it appears certain that magnetic phenomena are partly influenced by changes in the sun's condition. We may well believe that they are in the main due to the sun's ordinary action, but the peculiarities which affect them seem to depend on changes in the sun's action. It is found that the daily oscillation of the magnetic needle corresponds with the diurnal change in the position of the sun owing to the earth's rotation. An annual change affecting that oscillation depends on the varying distance of the sun as the year proceeds. The daily change is not only greater than the annual, but is characterized by irregularities, when the face of the sun shows the greatest number of spots. It was found by General Sabine, says Mr. Balfour Stewart, "that the aggregate value of magnetic disturbances at Toronto attained a maximum in 1848, nor was he slow to remark that this was also Schwabe's period of maximum sun-spots. It was afterwards found, by observations made at Kew, that 1859 (another of Schwabe's years) was also a year of maximum magnetic disturbance.... There is also some reason to believe that on one occasion our luminary was caught in the very act. On the first of September, 1869, two astronomers, Carrington and Hodgson, were independently observing the sun's disc, which exhibited at that time a very large spot, when, about a quarter past eleven, they noticed a very bright star of light suddenly break out over the spot and move with great velocity across the sun's surface. On Mr. Carrington sending afterwards to Kew Observatory, at which place the position of the magnet is recorded continuously by photography, it was found that a magnetic disturbance had broken out at the very moment when this singular appearance had been observed." The dip of the magnetic needle, its deflection from the north, the inferiority of its directive force, were all three simultaneously and abruptly altered, and continued so for many hours.

Nor are we left in any doubt as to the connection between such well-marked disturbances of the magnetic needle. While the needle was thus violently displaced, vivid auroras occurred over the greater part of both the northern and southern (magnetic) hemispheres. They were seen in latitudes where usually auroras are as infrequent as rain in Peru,—at Rome, in the West Indies, even within eighteen degrees of the equator.

The disturbance of the earth's electrical condition was well shown in other ways. Mr. C. V. Walker, the telegraphist, found that strong electrical currents affected the various telegraphic lines throughout England. These currents changed in direction every two or three minutes. In many places it was impossible to send telegraphic messages. In America some of the signalmen received severe electric shocks. "At a station in Norway," says Sir J. Herschel, "the telegraphic apparatus was set fire to; and at Boston, in North America, a flame of fire followed the pen of Bain's electric telegraph (which writes down the message upon chemically prepared paper)."

Many of my readers will doubtless remember the auroras of May 13, 1869, and October 24, 1870, both of which occurred when the sun's surface was marked by many spots, and both of which were accompanied by remarkable disturbance of the earth's magnetism.

It may, then, fairly be assumed that the occurrence of auroras depends in some way, directly or indirectly, on the condition of the sun. But what the real nature of that connection may be is not to be easily determined. It is clear that the eleven-year-period of sun-spots is not the only, or even the chief period affecting auroras, for we have seen that sometimes for a full century, or even more, very few auroras are seen. It is not by any means certain that the connection between the sun's condition and the occurrence of auroras is of the nature of cause and effect; quite probably sun-spots and auroras depend on some common cause as yet undetected,—and possibly never to be detected by man.

Regarding the auroral streamers as terrestrial lights only, but in some sense like the light reflected by planets in having their real source in the sun, we can no longer speak, as Humboldt was wont to do, of our planet possessing a power of emitting light of its own. Yet his manner of dealing with auroral light still possesses interest for us, especially in relation to the question whether these polar lights are emitted by other planets and may possibly be discerned from our earth. "It results from the phenomena of the aurora," said Humboldt, "that the earth is endowed with the property of emitting a light distinct from that of the sun. The intensity of this light is rather greater than that of the moon in its first quarter. It is at times, as on January 7, 1831, strong enough to admit of one's reading printed characters without difficulty. This light of the earth, the emission of which towards the poles is almost continuous" (this, however, is not strictly the case), "reminds us of the light of Venus, the part of which not lighted by the sun often glimmers with a dim phosphorescent light. Other planets may also possess a light evolved out of their own substance."

I would venture, however, to express strong doubts as to the possibility of discerning, either on Venus or on any other planet, the auroral gleams which may very probably illuminate at times their nocturnal skies. It must be remembered that the aurora, when at its brightest and covering a large part of the sky, only gives about as much light as the moon in her first quarter,—that is, as one half of a disc so small that 180,000 such discs would not equal the entire sky. The luminosity of the aurora is then in reality very small; probably far less than that of the earth's surface when illuminated by the full moon. A distant hill on which the rays of the full moon are falling seems strongly illuminated, and yet its light is really so faint that we could scarcely discern it at all save for the favouring effect of contrast. We know this, because we often see portions of the moon's surface which are illuminated by earthshine (when we see what is called the old moon in the new moon's arms), and these portions are quite faint by comparison with the rest of the moon; yet earthshine exceeds moonshine at least twelve times, and probably more nearly twenty times in splendour.