HALO, or CORONA, is a luminous circle, sometimes containing all the prismatic colours, which occasionally appear about the sun and moon, and other luminous bodies.
In the northern regions of the globe, the sun and moon frequently appear surrounded with halos, or coloured cir cles, having their diameter about 44° or 92°. When a horizontal white circle intersects these halos, narhelia or brighter spots appear near their intersections, and also portions of inverted arches of various curvatures. In the horizontal circle, there are often anthelia or bright spots nearly opposite to the sun. • In order to lay before our readers a pretty full account of this curious class of phenomena, we shall begin with describing the most interesting halos that have hitherto been observed, and then give an account of the theories which have been employed to explain them. As the most mi nute accuracy is necessary in the description of the phe nemena, we shall generally give it in the words of the ob servers themselves.
On the 20th March 1629, Scheiner observed at Rome a singular halo, which Huygens describes in the following manner from the wt itings of Descartes and Gassendi : " A is the place of the observer at Rome, B the vertex or point over his head, C the true sun, AB a vertical plane passing through the observer's eye, the true sun and the vertex B, which are all projected in the straight line ACB (Plate CCLXXXVII. Fig. I.) About the sun C, there appeared two concentric rings not complete, but diversified with colours. The lesser and inner of them, DEF, was fuller and more perfect ; and though it was open from D to F, yet these ends D and F were perpetually endeavour ing to unite. Sometimes they did unite and complete the ring, and then opened again. The other exterior and fainter, and scarce discernible circle, was GKI; it had a variety of colours, but was very inconstant. The third circle KLMN was very large, and all over of a white co lour, such as are often seen with paraselenx about the moon. This was an eccentric circle passing through the middle of the sun, at first entire, but towards the end of the ap pearance it was weak and ragged, and scarce discerni ble from M towards N. In the common intersection of this circle, and of the outward iris GKI, there broke out two parhelia N and K, not entirely perfect ; K was some what weak, but N shone and stronger. The brightness in the middle of them both resembled that of the sun; but towards their edges, they were tinged with colours like those of the rainbow. They were not perfect ly round and even at their edges, but uneven and ragged. The parhelion N was a little wavering, and sent out.a spiked tail NP, of a colour somewhat fiery, which had a continual reciprocation. The parhelia at L and M, beyond the ze nith B, were not so bright as the former, but rounder, and white like the circle which they were placed in. They resembled milk or clean silver. The parhelion M was al most quite extinct at half an hour past two o'clock, except ing that some faint remains would revive now and then, and the circle itself vanished in that place. The parhelion N
disappeared before K did, and while M became fainter K grew brighter, and vanished last of all." This has been generally called the Roman Phenomenon.
On Sunday February 20th 1661, new style, Hevelius observed at Dantzic a very curious halo, which he thus de scribes in the appendix to his Mercurius in Sole Vim°, page 174. " A little before 11 o'clock, the sun being to wards the south, and the sky very clear, there appeared seven suns together in several circles, some white and some coloured, and these with very long tails, waving and point ing from the true sun, together with certain white arches crossing one another. 1st, The true sun at A (Fig. 2.) being about 25° high, was surrounded almost entirely by a circle whose diameter was 45°, and which was coloured like the rainbow with purple, red and yellow, its under limb being scarce 21° above the horizon. 2d, On each side of the sun at B and C, towards the west and east, there ap peared two mock suns, coloured especially towards the sun, with very long splendid tails of a whitish colour, and terminating in a point. 3d, A far greater circle YXHVZ, almost 90 degrees in diameter, encompassed the sun and the former lesser circle GBIC, and extended itself down to the horizon. It was very strongly coloured in its upper part, but was somewhat duller and fainter on each side. 4th At the tops of these two circles at G and H were two in verted arches, whose common centre lay in the zenith, and these were very bright and beautifully coloured. The di ameter of the lower arch QGR was 90°, and that of the up per one THS was 45°. In the middle of the lower arch at G, where it coincided with the circle BGC, there appear ed another mock sun ; but its light and colours were dull and faintish. 5th, There appeared a circle BEFDC much bigger than the former, of an uniform whitish colour, pa rallel to the horizon at the distance of 25°, and 130° in di ameter, which arose as it were from the collateral mock suns B and C, and passed through three other parhelia of an uniform whitish colour like silver: one at D, almoit 90° from the true sun, towards the east ; another at E, to wards the west ; and a third at F, in the north, diametrically opposite to thd true sun, all of the same colour and bright ness. There passed also two other white arches EN, DP, of the greatest circle of the sphere through the eastern and western mock suns E, D, and also through K, the pole of the ecliptic. They went down to the horizon at N and P, crossing the great white circle obliquely, so as to make a white cross at each parhelion; so that seven suns appear ed very plain at the same time ; and if I could have seen the phenomenon sooner from an eminence, I do not ques tion but I should have found two more at H and I, which would have made nine in all ; for there remained in those places such marks, as made this suspicion not im probable.