PARHELION, or Parhelium, formed from παgα, near, and ηκa>ς, sun, in Natural Philosophy, a mock sun or meteor, in the form of a very bright light, appearing on the one side of the sun.

Appearances of this kind have been mentioned both by the ancients and by the moderns. Aristotle observes, that in general they are seen only when the sun is near the horizon, though he takes notice of two which were observed in the Bosphorus from morning to evening; and Pliny has related the occasions when such phenomena appeared at Rome. Gassendi says, that in 1635 and 1636 he often saw one mock sun. Two were observed by Μ. de la Hire in 1689, and the same number by Cassini 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, at Utrecht by Muschenbroeck, and at Sedan by Hevelius. By the two former four mock suns were observed, and by the latter seven.
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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 in both these respects some difference amongst 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 coronas, some of which are tinged with rainbow colours, but others are white. They differ in number and size; but all agree in point of 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 run through the centre of the sun. Sometimes there are arcs of lesser circles concentric to this, touching those coloured circles which surround the sun. They are also tinged with colours, and contain other parhelia. Other circles are said to have been seen obliquely situated with respect to all those we have mentioned; but of these 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 coronas. Parhelia have been visible for one, two, three, and four hours together; and in North America they are said to continue several days, and to be visible from sunrise to sunset.

When the parhelia disappear, it sometimes rains, or there falls snow in the form of oblong spiculae, as Maraldi, Weidler, Krafft, and others, have observed; and in fact the air in North America abounds with such frozen spiculae, which are even visible to the eye. According to Ellis and Middleton, such particles have been supposed to be the cause of all coronas and parhelia.

Mr Ellis 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 they 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 sun. These two streams of light, he says, appear 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 is constantly found near the horizon, all these accompany him during the day, and set with him in the same manner as they rise. Once or twice Mr Ellis saw a fourth parhelion directly under the true sun; but this, it seems, is not common. These facts being constant, are very valuable, and may throw great light on the theory of such 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, the lowest touched the horizon, and they set one after the other. This appearance was seen by Μ. Maleziew in 1722, and others similar to it have been related by Μ. Musschenbroeck.

Sometimes the sun has been observed to rise 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 observed by Cassini in 1672 and 1692, by De la Hire in 1702, and by Mr Ellis in Hudson’s Bay. As Μ. Feuillée 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 bad attained an altitude of six degrees.

Paraselenae, or mock moons, have also been observed, accompanied with tails and coloured circles, like those which accompany the parhelia. An account of several of these, and in particular a description of a fine appearance of the same kind, may be found in Musschenbroeck.

Various hypotheses have been framed by philosophers to account for this phenomenon, more particularly by Marriotte, Descartes, and Huygens. None of these, however, seems satisfactory; but those readers who wish to become acquainted with them may consult Huygens’s Dissertation on this subject, in Smith’s Optics (book i. chap, xi.), Musschenbroeck’s Introduction (vol. xi. p. 1038, &c. 4to); and Dr Priestley’s History of Vision, Light, and Colours (vol. ii. p. 613, et seq.).