These black spots have a sort of shadowy fringe (_penumbra_) which gives them the appearance of being openings through a luminous surface, discovering a surface less luminous, with openings in it through which some still darker surface is seen; that this really is the case, and that these are not merely dark spots or surfaces, is shown by the fore-shortening of the nearest edge as they arrive at the verge of the sun's disc, and are consequently presented edgewise to the eye, so that a spot may have the appearance of fig. 2 when seen in front-view near the centre, and of fig. 3 when seen fore-shortened near the edge of the sun's disc. These spots (although but specks in comparison with the vast surface of the sun) are yet of prodigious magnitude, some having been calculated to measure 18,000 miles across--large enough for two such worlds as ours to pass through side by side! From the rapid motion of the "penumbra" or edges of these spots, which alter, open, and close up very rapidly, and from some of them being semi-dark, others quite dark, &c., it has been concluded that the sun has at least two surrounding atmospheres, that the body of the sun is not luminous, nor the first atmosphere or that nearest to it, but that the outer one is, and that the dusky appearance of the inner, as seen through openings of the luminous outer atmosphere, results from its being very strongly illuminated by the outer one; this luminosity may be (and probably is) only comparative, for the spots which appear black may be yet very bright, appearing black only in comparison with the extreme brilliancy of the outer atmosphere, just as a bar of red-hot iron appears black when held up to the sun. The course which the planets take round the sun is not exactly circular, but deviating from it more or less, never being quite a circle. The orbit of each planet, if accurately computed and laid down on paper, proves to be that form of curved line called an "ellipse," or that form which instead of having one centre, as in the circle, from which all lines radiating to the circumference are equal in length, has a longer and shorter diameter, two centres called "foci," and the circumference or boundary so placed that the sum of any two lines, drawn one from each centre, and meeting at any part of the circumference, shall be equal to any two other lines so drawn and meeting at any other point of the circumference, and to the major axis or long diameter of the ellipse. In fig. 4, each pair of lines drawn from A A and meeting repectively at B are each equal. An ellipse may be very much elongated, or almost circular, but still have these properties which are essential to it, and in the case of the orbits of the planets, they are so nearly circular, that if drawn a few inches wide, would hardly be detected to differ from the circle. The orbit of the earth is about one thirtieth part longer than broad. [Illustration: FIG. 2.] [Illustration: FIG. 3.] [Illustration: FIG. 4.]