or PARHELUM, in natural philosophy, a mock-fun or meteor, in form of a very bright light, appearing on one side of the sun.

Appearances of this kind have been made mention of both by the ancients and moderns. Aristotle observes, that in general they are seen only when the sun is near the horizon, though he takes notice of two that were seen in Bosphorus from morning to evening; and Pliny has related the times when such phenomena were observed at Rome. Gasendi says, that, in 1635 and 1636, he often saw one mock-fun. Two were observed by M. de la Hire in 1680; and the same number by Caffini in 1693, Mr Grey in 1700, and Dr Halley in 1702: but the most celebrated appearances of this kind were seen at Rome by Scheiner, by Muffchenbroeck at Utrecht, and by Hevelius at Sedan. By the two former, four mock-funs were observed, and by the latter seven.

Parhelia are apparently of the same size with the sun, though not always of the same brightness, nor even of the same shape; and when a number appear at once, there is some difference in both respects among them. Externally, they are tinged with colours like the rainbow; and many have a long fiery tail opposite to the sun, but paler towards the extremity. Parhelia are generally accompanied with corona's, some of which are tinged with rainbow-colours, but others are white. They differ in number and size; but all agree in breadth, which is that of the apparent diameter of the sun.

A very large white circle, parallel to the horizon, generally passes through all the parhelia; and, if it were entire, it would go through the centre of the sun. Sometimes there are arcs of lesser circles concentric to this, touching those coloured circles which surround Parhelion surround the sun. They are also tinged with colours, and contain other parhelia. There are also said to have been other circles obliquely situated with respect to all those we have mentioned; but of this we have met with no authentic account. The order of the colours in these circles is the same as in the rainbow; but on the inside, with respect to the sun, they are red, as is also observed in many other corona's.

Parhelia have been visible for 1, 2, 3, and 4 hours together; and in North America they are said to continue some days, and to be visible from sun-rise to sun-set.

When the parhelia disappear, it sometimes rains, or there falls snow in the form of oblong spicules, as Mavaldi, Weidler, Krafft, and others, have observed; and because the air in North America abounds with such frozen spicules, which are even visible to the eye, according to Ellis and Middleton, such particles have been thought to be the cause of all corona's and parhelia.

Mr Wales says, that, at Churchill in Hudson's Bay, the rising of the sun is always preceded by two long streams of red light, one on each side of him, and about 20° distant from him. These rise as the sun rises; and as they grow longer begin to bend towards each other, till they meet directly over the sun, just as he rises, forming there a kind of parhelion or mock-fun. These two streams of light, he says, seem to have their source in two other parhelia, which rise with the true sun; and in the winter-season, when the sun never rises above the haze or fog, which he says is constantly found near the horizon, all these accompany him the whole day, and set with him in the same manner as they rise. Once or twice he saw a fourth parhelion directly under the true sun; but this, he says, is not common. These facts being constant, are very valuable, and may throw great light on the theory of these remarkable phenomena.

Sometimes parhelia appear in a different manner; as when three suns have been seen in the same vertical circle, well defined, and touching one another. The true sun was in the middle, and the lowest touched the horizon; and they set one after the other. This appearance was seen by M. Malezew in 1722. Other appearances similar to this are recited by M. Mutchtenbroeck.

Sometimes the sun has risen or set with a luminous tail projecting from him, of the same breadth with his diameter, and perpendicular to the horizon. Such an appearance was seen by Cassini in 1672 and 1692, by de la Hire in 1702, and by Mr Ellis in Hudson's Bay.

As M. Feuille was walking on the banks of the river La Plata, he saw the sun rising over the river with a luminous tail projecting downwards, which continued till he was six degrees high.

Parafelence, or mock-moons, have also been seen, accompanied with tails and coloured circles, like those which accompany the parhelia. An account of several, and a particular description of a fine appearance of this kind, may be seen in Mutchtenbroeck.

The Roman phenomenon, observed by Scheiner, is famous on account of its having been the first appearance of the kind that engaged the attention of philosophers. It is represented in fig. 4.; in which A is the place of the observer, B his zenith, C the true sun, AB a plane passing through the observer's eye, Parhelion the true sun, and the zenith. About the sun C, there appeared two concentric rings, not complete, but diversified with colours. The lesser of them, DEF, was fuller, and more perfect; and though it was open from D to F, yet those ends were perpetually endeavouring to unite; and sometimes they did so. The outer of these rings was much fainter, so as scarcely to be discernible. It had, however, a variety of colours; but was very inconstant. The third circle, KLMN, was very large, and all over white, passing through the middle of the sun, and everywhere parallel to the horizon. At first this circle was entire; but towards the end of the appearance it was weak and ragged, so as hardly to be perceived from M towards N.

In the interjection of this circle, and the outward iris GKI, there broke out two parhelia or mock-funs, N and K, not quite perfect; K being rather weak, but N shone brighter and stronger. The brightness of the middle of them was something like that of the sun; but towards the edges they were tinged with colours like those of the rainbow; and they were uneven and ragged. The parhelion N was a little wavering, and sent out a spiked tail, NP, of a colour somewhat fiery, the length of which was continually changing.

The parhelia at L and M in the horizontal ring were not so bright as the former; but were rounder, and white, like the circle in which they were placed. The parhelion N disappeared before K; and while M grew fainter, K grew brighter, and vanished the last of all.

It is to be observed farther, that the order of the colours in the circles DEF, GKN, was the same as in the common halos, namely, red next the sun; and the diameter of the inner circle was also about 45 degrees; which is the usual size of a halo.

Various hypotheses have been framed by philosophers to account for this phenomenon. None of them are satisfactory; but the reader who chooses to know them may consult Dr Priestley's History of Vision, Light, and Colours, vol. ii. p. 619.