Melllsh's comet, of whose discovery we spoke last month, has already passed Its nearest approach to the earth, and Is now receding rapidly both from is and from the sun, so that It will not long remain visible. At Its best it was only a diffuse, hazy spot of light, but was fairly bright for a telescopic comet, and easily seen In a small telescope. There has been considerable popular discussion lately about certain bright spots that have been seen on Saturn's rings. The rings are at present so situated that we see their dark side; that Is, the one on which the sun does not shine. They are consequently Invisible, except for the thin edge, which can be seen In large telescopes as a faint and exceedingly delicate line of light, extending out on each side of the planet. On this line there appear brighter "knots" or "condensations," two on each side of the planet, symmetrical with respect to It, as was announced a few weeks ago from the Lick Observatory. This phenomenon Is, however, not new to science, and In fact an explanation of It was given by Fro'. Bond of Harvard rather more than fifty years ago, substantially as follows: The rings of Saturn are thin plane sheets, most probably less than 100 miles In thickness, though 168,000 miles In diameter. They do not, however, form an unbroken sheet, but are composed of three divisions an outer one, about 10,000 miles wide, separated from the second by a dark space of 1,600 miles; a second, about 16,000 miles In width, brighter than the first; and this shades gradually Into the third, which is faint and partly transparent; doubtless because the particles which compose It are so far apart that we can see through between them. We are looking at them at present almost, though not quite, edgewise; our distance from the plane of the rings being only about 180 of the distance of the planet. In consequence, as we look at the part of the rings which Is nearly In front of the planet, we see only the illuminated outer edge of the outer ring. The outer edge of the second ring is also illuminated by the sun, but we do not see It, for It Is concealed behind the dark Inner edge of the outer ring, which appears to overlap It, since the actual gap between them Is so narrow. But when we consider a series of points apparently farther from the planet. It Is easy to see that we will be looking more and more obliquely across the narrow gap between the rings, so that It will look wider, until finally we can see the bright edge of the second ring through It. In the same way we see the Inner edge of the outer ring, behind the planet, through the same gap, but only when we look across it very obliquely. The combination of these two (which are too close together to be seen separately) accounts for one of the brighter "knots" on the faint line of the rings. Its distance from the planet's center should evidently be equal to the apparent diameter of the division between the rings, and there will be another similar bright spot on the opposite side of the planet, at the same distance. The inner edge of the second ring accounts for the other pair of bright "knots" in a similar fashion. To see them satisfactorily requires a large telescope and good atmospheric conditions, so that they are all beyond the range of most amateur observers. Tirn HEAVENS. The winter constellations, which form probably the finest group In all the skies, are now appearing. Orion Is pretty well up, about east-southeast. The line of his belt points upward toward Aldebaran, and downward to Slrius, rather more accurately than our map would Indicate. Procyon In the Little Dog, Castor and Pollux In the Twins, and Capella In the CTliaiiolcci-, make up a second line of bright stars to the northward of the first. Perseus and Andromeda are overhead, with the Rani (Aries) and the small but ancient constellation of the Triangle. In the south are the large but faint star groups of the Fishes, the River Erldanus, and the Whale, which can best be Identified by aid of the map. The variable star Mira, which Is lettered ? In the last constellation. Is now a little past maximum and still visible to the naked eye. The southwestern sky contains three bright objects the star Fomalhaut and the planets Saturn and Mars. The former Is in Aquarius (the Water Bearer) south of the middle of the great square of Pegasus. The latter Is in the same constellation, a little farther west, and can be told by his red color. Pegasus and the Dolphin ar In the west, and the Swan and the Lyre In the northwest. Of the clr-cumpolar constellations, Cassiopeia Is almost overhead, Cepheus below her toward the northwest, the Little Bear and the Dragon below the Pole, and the Grer.t Bear coming up in the northeast. THE PLANETS. Mercury Is a morning star all through the month. He Is best visible during the first week, near his elongation, which occurs on the 1st. At this time he Is In Libra, and rises nearly two hours before the sun, so that he can easily be seen. Venus Is evening star In Scorpio and Sagittarius, but Is so far south that she Is not at all conspicuous, though she sets about an hour and a half later than the sun. Mars is In Aquarius. He Is moving rapidly eastward among the stars, and overtakes Saturn on the last day of the year, when the two planets are less than two degrees apart. Jupiter Is In Cancer, and rises about 8:30 P. M. in the middle of the month. Saturn Is In Aquarius, and is in quadrature with the sun on the 13th, and comes to the meridian at 6 P. M. Uranus Is In Sagittarius, too near the sun to be seen. Neptune Is In Gemini, approaching opposition, which occurs early next month. THE MOON. New moon occurs at d A. M. on the uth, first quarter at 9 P. M. on the 11 th, full moon at 1 P. M. on the 19th, and last quarter at 6 F. M. on the 27th. The moon Is nearest us on the 6th, and farthest off on the 22d. She is in conjunction with Mercury on the 3d, Venus on the 6th, Uranus on the 7th, Mars and Saturn on the 11th, Neptune on the 20th, and Jupiter on the 23d. The last conjunction Is fairly close. At 7 P. M. on the 22d the sun reaclus ila greatest distance south of the celestial cqualor, and enters the sign of Capricorn, and. In the phrase of the almanac, “Winter commences.” Princeton University Observatory.
This article was originally published with the title "The Heavens in December" in Scientific American 97, 22, 391 (November 1907)