The Catholic World, Vol. 07, April 1868 to September, 1868

by Various

The penumbra is not always composed exclusively of threads like those you see. The centre is often surrounded by a uniform pale color, over which the currents are disseminated. These currents are not always continuous, and their different parts present an appearance which may be compared to elongated grains.

In spite of the increased power of the instruments we employ to observe the sun, the detached parts of the spots often appear to us as microscopic objects. In order to form an exact idea of their real dimensions, we must always remember that, at this distance, four fifths of a second is equal to 140 kilometres, and consequently these apparent threads, whose seeming width is at most not more than one or two seconds, are in reality immense currents, being, about the middle, of 600 or 700 kilometres in width, while their length is at least equal to the diameter of the terrestrial globe.

The drawings which you have just seen represent some of these spots in their complete form and exactly defined. But they present themselves oftener under fantastic and irregular forms. They are sometimes accompanied by a kind of tail, itself formed of black spots, and which seems to follow the centre in its motion. We have here a curious example. The centre is not quite black; we meet with shadows there—some gray, and others red; the filaments on all sides fall toward the centre, and their edges are turned back and bent, as if they had experienced some resistance, or as if they had encountered a whirlwind.

Figure 2.

Here is a spot of this kind, (Figure 2,) the details of which are most instructive, and most important in a theoretical point of view. We find the centre divided in several parts by the luminous threads. This appearance was remarked by the ancient astronomers, who explained it by supposing that on the surface of the sun solid crusts were formed, which broke into shivers like glass under a blow from a stone. Modern observations, however, do not admit of this explanation. They show us clearly that these divisions are produced by currents which, leaving opposite edges, meet in the middle of the centre, and thus divide the spot into several parts.

The formation of a spot is never instantaneous. It is ordinarily announced by the appearance of several black points, and by a kind of diminution in the thickness of the luminous bed. These little cavities multiply themselves; one of them develops itself, absorbing the others, and the process ends in the formation of a black spot in the centre. In this first phase the movements of the spots are very irregular, and their advance is always to the front, by reason of the solar rotation.

The drawing which is now before you represents the first appearance of a great spot which was formed almost suddenly on the 30th of July, 1865. The day preceding that of its appearance, in observing the sun as usual, we had remarked only three little cavities, of which we noted the position. On the 30th of July, at mid-day, we found in the place of these cavities an enormous spot, the surface of which was equal to at least ten times the size of our globe. It was so mobile, and its form changed so constantly, that we could scarcely draw it. We could discover in it four principal centres, where the movement of the matter was visible in the form of a whirlwind. In an interval of 24 hours it had undergone some considerable changes. On the 31st of July, the four centres were completely distinct, and the matter which separated them seemed as if it were stretched out. During the days which followed, this form became more and more marked. Soon there were four spots clearly defined, which ultimately assumed the form of four independent craters or cavities. In the interior of these craters we perceived some light shadows, whose form reminded us of that of the clouds we call cirrus. Their color was different from that of the other part of the sun which presented itself to view. As the polariscopic eye-glass does not change the color of objects, we are enabled to see that these clouds are often of a very decided red; and, as this tint is clear and well marked, it is impossible to confound it with the effects due to the achromatism of the instruments. You see here a great number of spots presenting this appearance, and especially in Figure 2, where the red shadows seem intertwined with the white shadows. I have more than once seen these luminous tongues, so to speak, transform themselves into red veils.

This hasty view is, however, so complete as to convince us that the spots cannot be compared to clouds, their aspect not warranting such a comparison. If any part of them may be compared to clouds, it is more the luminous matter; for we see it precipitate itself in the obscure space, and there dissolve in much the same way as we see the vapor which forms the mist dissolve into thin air. All that we are required to believe is, that these apparently black masses are but rents made in the luminous veil which covers the solar body, and to which we give the name of photosphere. It is this bed which transmits light and heat to us. It is suspended in the solar atmosphere, just as clouds in the terrestrial atmosphere. What appear to us as spots in the sun is simply the effect of the rents which take place in it. We are confirmed in this view by the well-ascertained fact that the spots are depressions in the solar body, and that they have the form of a funnel. This form becomes very perceptible, when the spots are drawn by the rotary movement toward the solar disk. When we examine a spot situated toward the centre of the sun, we find that the shape of the penumbra is more regular. But when the spot moves toward the edge, we see the penumbra diminish on the side of the centre, and increase on the opposite side, in which case it presents the appearance of a cavity in the form of a funnel looked at obliquely.