MACULA:, in astronomy, dark spots appearing on the luminous faces of the sun, moon, and even some of the planets ; in which sense they stand contradistin guished from faculx. See FAcut7e.
These spots are most numerous and easily observed in the sun. It is not un common to see them in various forms, magnitudes, and numbers, moving over the sun's disc. They were first of all discovered by astronomer Galileo, in the year 1610, soon after he bad finished his new-invented telescope. It has been supposed that these spots adhere to, or float upon, the surface of the sun, for the following reasons. 1. Many of them are observed to break out near the middle of the sun's disc ; others to decay and vanish there, or at some distance from his limb. 2. Their apparent velocities are always greatest over the middle of the disc, and gradually slower from thence on each side towards the limb. 3. The shape of the spots varies according to their position on the several parts of the disc : those which are round and broad in the middle, grow oblong and slender as they approach the limb, according as they ought to appear by the rules of op tics.
By comparing many observations of the intervals of time in which the spots made their revolution, by Galileo, Cassini, Scheirer, Hevelius, Dr. Halley, Dr. Der ham, and others, it is found that 27 days, 12 hours, 20 minutes, is the measure of one of them at a mean ; but in this time the earth describes the angular motion of 26° 22', about the sun's centre : there fore say, as the angular motion of 360° + 26° 22', is to 360° ; so is 27 days, 12 hours, 20 minutes, to 25 days, 15 hours, 16 minutes ; which, therefore, is the time of the sun's revolution about its axis.
As to the magnitude of the spots, they are very considerable, as will appear if we observe that some of them are so large as to be plainly visible to the naked eye thus Galileo saw one of them in the year 1612 ; and Mr. Martin assures us, that he knew two gentlemen that thus viewed them several years ago ; whence he con cludes, that these spots must therefore subtend, at least, an angle of one minute. Now the diameter of the earth, if removed to the sun, would subtend an angle of but 20" ; so that the diameter of a spot, just visible to the naked eye, is, to the diameter of the earth, as 60 to 20, or as 3 to 1 ; and, therefore, the surface of the spot, if circular, to a great circle of the earth, is as 9 to 1; but 4 great circles are equal to the earth's superficies ; whence the surface of the spot is, to the surface of the earth, as 9 to 4; or as 2i to 1. Gassendus says, he saw a spot
whose diameter was equal to 23 of that of the sun, and therefore subtended an angle at the eye of 1' 30" ; its surface must have been five times larger than the surface of the whole earth. What these spots are, it is presumed, nobody can tell; hut they seem to be rather thin sub stances than solid bodies, because they lose the appearance of solidity in going off the disc of the sun : they resemble' something of the nature of scum or scoria.
swimming on the surface, which are gene ral ed and dissolved by causes little known to us : but whatever these solar spots are, it is certain they are produced from causes very inconstant and irregular ; for Scheiner says he frequently saw fifty at once, but for twenty years after scarce any appeared. And in the last century the spots were very frequent and nume rous till the year 1741, when, for three years successively, very few appeared; and now, since the year 1744, they have again appeared as usual.
These macula are not peculiar to the sun, they have been observed in all the planets. Thus Venus was observed to have several by Signior Blanchini, in the year 1726. As in Venus, so in Mars, both dark and bright spots have been ob served, first by Galileo, and afterwards by Cassini, &c. Jupiter has had his spots observable ever since the invention and use of large telescopes. Saturn, by reason of his great distance on one hand, and Mercury, by reason of his smallness and vicinity to the sun on the other, have not as yet had any spots discovered on their surfaces, and consequently nothing in re lation to their diurnal motions and incli nations of their axis to the planes of their orbits can be known, which circumstances are determined in all the other planets, as well as in the sun, by means of these macula.
The spots, or maculx, the moon's surface, seem to be only cavities or large caverns, on which the sun shin ing very obliquely, and touching only their upper edge with his light, the deeper places remain without light ; but as the sun rises higher upon them, they receive more light, and the shadow, or dark parts, grow smaller and shorter, till the sun comes at last to shine directly upon them, and then the whole cavity will be illustrated : but the dark dusky spots, which continue always the same, are supposed to proceed from a kind of matter or soil which reflects less light than that of the other regions. See