, in Astronomy, are dark places observed on the disks or faces of the sun, moon, and planets.

The Spots on the sun are seldom if ever visible, except through a telescope. I have indeed met with persons whose eyes were so good that they have declared they could distinguish the solar Spots; and it is mentioned in Josephus à Costa's Natural and Moral History of the West Indies, book 1, ch. 2, before the use of telescopes, that in Peru there are Spots to be seen in the sun, which are not to be seen in Europe. See a memoir by Dr. Zach, in the Astronomical Ephemeris of the Acad. of Berlin for 1788, relating to the discoveries and unpublished papers of Thomas Harriot the celebrated algebraist. In that memoir it is shewn, for the first time, that Harriot was also an excellent astronomer, both theoretical and practical; that he made innumerable observations with telescopes from the year 1610, and, amongst them, 199 observations of the solar Spots, with their drawings, calculations, and the determinations of the sun's revolution round his axis. These Spots were also discovered near about the same time by Galileo and Scheiner. See Joh. Fabricius Phrysius De Maculis in Sole observatis & apparente eorum cum sole conversione narratio, 1611; also Galileo's Istoria e Demonstrazioni intorne alle Machie Solare e loro accidenti, 1613.

Some distinguish the Spots into Maculæ, or dark Spots; and Faculæ, or bright Spots; but there seems but little foundation for any such division. They are very changeable as to number, form, &c; and are sometimes in a multitude, and sometimes none at all. Some imagine they may become so numerous, as to hide the whole face of the Sun, or at least the greater part of it; and to this they ascribe what Plutarch-mentions, viz, that in the first year of the reign of Augustus, the sun's light was so faint and obscure, that one might look steadily at it with the naked eye. To which Kepler adds, that in 1547, the Sun appeared reddish, as when viewed through a thick mist; and hence he conjectures that the Spots in the sun are a kind of dark smoke, or clouds, floating on his surface.

Some again will have them stars, or planets, passing over the body of the sun: but others, with more probability, think they are opake bodies, in manner of crusts, formed like the scums on the surface of liquors.

Dr. Derham, from a variety of particulars, which he has recited, concerning the solar Spots, and their congruity to what we observe in our own globe, infers, that they are caused by the eruption of some new volcano in the sun, which pouring out at first a prodigious quantity of smoke and other opake matter, causeth the Spots: and as that fuliginous matter decays and spends itself, and the volcano at last becomes more torrid and flaming, so the Spots decay and become umbræ, and at last faculæ: which faculæ he supposes to be no other than more flaming lighter parts than any other parts of the sun. Philos. Trans. vol. 23, p. 1504, or Abr. vol. 4, p. 235.

Dr. Franklin (in his Exper. and Observ. p. 266.) suggests a conjecture, that the parts of the Sun's sulphur separated by fire, rise into the atmosphere, and there being freed from the immediate action of the fire, they collect into cloudy masses, and gradually becoming too heavy to be longer supported, they descend to the sun, and are burnt over again. Hence, he says, the Spots appearing on his face, which are observed to diminish daily in size, their consuming edges being of particular brightness.

For another solution of these phenomena, see MACULÆ. Various other accounts and hypotheses of these Spots may be seen in many of the other volumes of the Philos. Trans. In one of these, viz, vol. 57, pa. 398, Dr. Horsley attempts to determine the height of the sun's atmosphere from the height of the solar Spots above his surface.

By means of the observations of these Spots, has been determined the period of the sun's rotation about his axis, viz, by observing their periodical return.

The lunar Spots are fixed: and astronomers reckon about 48 of them on the moon's face; to each of which they have given names. The 21st, called Tycho, is one of the most considerable.

Circular Spots, in Electricity. See Circular Spots and Colours.

Lucid Spots, in the heavens, are several little whitish Spots, that appear magnified, and more luminous when seen through telescopes; and yet without any stars in them. One of these is in Andromeda's girdle, and was first observed in 1612, by Simon Marius: it has some whitish rays near its middle, is liable to several changes, and is sometimes invisible. Another is near the ecliptic, between the head and bow of Sagittarius; it is small, but very luminous. A third is in the back of the Centaur, which is too far south to be seen in Britain. A fourth, of a smaller size, is before Antinous's right foot, having a star in it, which makes it appear more bright. A fifth is in the constellation Hercules, between the stars <*> and h, which is visible to the naked eye, though it is but small, when the sky is clear and the moon absent. It is probable that with more powerful telescopes these lucid Spots will be found to be congeries of very minute fixed stars.

Planetary Spots, are those of the planets. Astronomers find that the planets are not without their spots. Jupiter, Mars, and Venus, when viewed through a telescope, shew several very remarkable ones: and it is | by the motion of these Spots, that the rotation of the planets about their axes is concluded, in the same manner as that of the sun is deduced from the apparent motion of his maculæ.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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