HAD the Moon been in her first quarter, instead of her third, Kiesss comet, which was within twenty million miles of the Earth on August 16th, would have been fairly conspicuous to the naked eye. As , it was, the moonlight drowned it out pretty thoroughly. By the beginning of September this comets rapid motion carries it into 52 degrees south declination, making it quite invis;ble in our latitude. As it is rapidly reeeding from the Sun, and still faster from the Earth (owing to its retrograde orbital motion), it will doubtless soon disappear from the gaze even of southern observe r s. Brooks comet-discovered at Geneva, N. Y., on July 20th, by a veteran observer of such bodies-is: appro aching the Sun, and will not be in perihelion1 until October 27th. During the early part of September it will be fairly near the Earth (so.me thirty' million miles away) and will be high in the northern sky, and above the horizon most of the night during which time it will be favorably located for observation and may even be visible to the naked eye. Still a third comet is now in sight-Encke's, which is noteworthy for several reasons. Firstly, it has the shortest period of any known Gomet, returning to the Sun at intervals of slightly less than three and one-third years. Secondly, it varies in brightness, at successi ve returns, in a very singular manner, being sometimes easily visible in small telescopes (as is reported at the present return), while at other times, though equally near the J:arth and Sun and apparently as favorably placet, it is excessively faint, and can only be observed with powerful j elcscollES, as was the ( ::se pt its last return in 1908. Prof. B1ack1und of Palkowa, Russia, who has spent much time in the study of this comet, finds that it always appears faint when it lies in certain directions from the Earth and Sun, behaving as if it was a thin flat disk, almost vanishing when seen edgewise, but fairly' conspicuous if not viewed too obliquely. The actual explanation of its changes must be more complete, for, like other faint comets, this one appears as a roundish mass of light. It wou l d be possible to explain the facts by assuming that the comet was composed of a great number of separate particles all strongly flattened parallel to the same plane; but some less artificial explanation will doubtless in time be available. Third among the peculiarities of this comet, and most iIteresting of all from the theoretical standpoint, is the fact that its period of revolution is decreasing. This proves that some force, other than the attraction of the Sun, must act upon it, for otherwise, like the planets, it would always take exactly the same time to complete a circuit of its orbit. Tha attraction of the planets themselves would indeed give rise to slight differences in the duration of successive revolutions; but when the laborious calculations necessary to determine these “perturbations” have been, carried out, and checked so as to exclude any chance of error, it is found that there remains an . unexplained shortening of the comet's period. It is one of the apparent paradoxes of theoretical astronomy that the return of a comet earlier than the predicted time (provided this can be accurately determined) means that some force has been at work which has retarded the comet's motion. A full explanation cannot be given by elementary methods; but the principle involved is this: The retarding force decreases the rate of the comet's motion, as might be expected; but the Sun's attraction acting on the now slower moving comet draws it into an orbit smaller than before-so much smaller, indeed, that even at the slower speed it gets round it in less time than formerly. By refined methods, taking into account the change in the form of the orbit, as well as the period, it is possible to discriminate between the influence of a minute retarding force acting steadily all along the comet's orbit, and a more powerful one exerted along NIGHT SKY: AUGUST AND SEPTEMBER only a part of it. In this way Blacklund comes to the conclusion that the resistance acting on Encke's Comet is concentrated in. a re l atively small· region in the part of the orbit nearest the Sun. lL3 actual amount is, however, so small that the retardation cannot be detected by direct observation immediately after the comet leaves this region. It is only at the following return when the cumulative, results during the whole following period become evident, that the extent to which the comet has been retarded can be calculated. Still another curious circumstance must .be mentioned. The amount of the retardation is not always the same. In the middle of the last century the period was shortened by about two and a half hours in each revolution; but since then the influence was twice abruptly diminished. Prof. Blacklund is indined to explain this by supposing that the comet's motion is hindered by passing through a doud of meteorities-or something of that sort-which is not of the same density at the successive times when the comet strikes it. Enough has been said to show what an interesting body this little comet is to the a stronom e r. Accurate observations are now secured for as long an interval as possible at each return, and great mathematical skill, and enormous labor in computation, expended in the calculation of its motion; but it may still be long before the nature, and especially the variations, of the strange resistance to which its motion is subjected are fully comprehended. The Heavens. Turning now to our map of the evening skies, we fnd that the splendid Cygnus right overhead. To the west is Lyra, with the resplendent star Vega, and southward shines the less luminous Altair. A brilliant region of the Milky Way, below this, leads us to Sagittarius and Scorpio-the latter almost setting in the southwest. Ophiucus and Serpens are farther to the right; then Hercules, Corona and Bootes, in an almost vertical line below Lyra. The Great Bear is well down in the northwest, with the Dragon and the Little Bear above. The vacant region below the pole on the other side contains the modern constellation Camelopard us-made to fllll a gap between the ancient ones. Farther to the right, in the Milky Way, is another fine series of star groups-Auriga, just rising, then Perseus, next Cassiopeia, with the less brilliant Cepheus between her and Cygnus. The great square of Pegasus is well up in the east. Andromeda is on the left, and Aries below. A few stars of Cetus have risen to the south of east. About southeast is a bright star, which from its southern declination and isolation is at once recognizable as Fomalhaut, the only conspicuous member of the constellation of the Southern Fish. Above this is Aquarius and •farther to the right Capricornus. Tbough one of tlw fainter zodiacal constellations this last is a fairly definite group of stars, and can easily be made out on a dear night. Its principal stars which bear the Greek letters Alpha and Beta, are close together in the northwestern part of the eonstellation, at the head of the Sea-Goat, as our initial shows. About 20 degrees to the eastward the monster's tail is marked by the stars Gamma and Delta Capriconi, the 1 a tt e r, though fourth in alphabetical order, being the brightest in the whole constellation. A slightly curved line of faint stars, only ^conspicuous when there is no moonlight, connects t'hese two groups, and two similar lines run downward .from ihem, fOl:ming a tri:mgle whose third vertex is f arked by two stars (in the Goat's fore p a ws) which, though not much fainter than those already men-betfr the last letters .f the Greek alphabet - Psi and Omega'.- Alpha- Capricorni is a fine naked-eye double star, with components 61 minutbs of arc apart. Beta Capricorni is also a wide double, but a: the components are very unequal in brightness and [eparated by but 3Y minutes of arc, the aid of a fielL-glass is needed to see them separately. The Planets. Mercury is evening star at the beginning of the month, but is too near the Sun to be seen. On the 9th he passes through conjunction, and thereafter is a morning star, reaching his greatest western elongation on the 25th. About this time he rises at 4: 30 A. M. and can easily be seen. Venus is also in conjunction with the Sun on the 15th and changes from evening to morning star. She can on'y be well seen toward the end of the month, (Cuntinued on page 217.)
This article was originally published with the title "The Heavens in September 1911"