VER our heads, as we look up on a clear August evening, shines a very bright bluish-white star. Even a novice in star-gazing will recognize this as Vega, the principal star of the constellation Lyra, and, according to the careful measures made at Harvard, the brighest in the northern heavens. For the latter honor it has, indeed, two close rivals, CapeUa being but seven per cent fainter, and Arcturus ten per cent. The very ' marked differences in color between these stars introduce great difficulties in such a comparisoll. The eyes of diffh-ent persons are unequally sensitive to light of different colors and an observer who was unusually sensitive to red light would undoubtedly estimate Arcturus to be brighter than Vega. In making the comparison it is necessary to choose a time when the two stars are equally high above the horizon, for otherwise the absorption of light in our atmosphere, even in clear weather, makes the star lower down seem the fainter. On photographs, which are far more sensitive than the eye to the blue and. violet rays, Vega appears nearly twice as bright as Capella, and [ully three times as bright as Arcturus. Three stars in the southern half of the heavens-Sirius, Oanopus and Alpha Centuri - exceed Vega in apparent brightness; but only the first is visible in our latitude. Measures of parallax show that Vega is four times as far away as Sirius so that as a matter of fact it exceeds the latter about three-fold in real brightness. Telescopically, it is interesting because of its great brilliancy, and on account of a faint companion, some 40 seconds distant, which does not share the motion of the bright star, and presumably lies far behind it. As our initial shows, Lyra Is one more example of a constellation wholly unlike the thing for which it is named. Though it boasts no other bright stars, it eontains seveml objects of interest. Close to Vega are two smaIl stars, forming with it a nearly equilateral triangle. The northernmost of these, Epsilon Lyrae, is just separable by a keen eye, and easily divided by an opera glass. A telescope oif three inches' aperture will show both components of this wide pair as close doubles, forming a splendid quadruple system. The two close pairs are in slow orbital motion, and all four stars are moving together in space. The other star of the triangle, Zeta Lyrae, is a wide telescope double, with separation of 43 seconds. A line from Vega through this star carried as far_ again points out Delta Lyrae-a very wide pair, separated by ten minutes of arc, which is pretty in a field glass. Some five degrees to the southwest are the brighter stars Beta and Gamma Lyrae. The former (which is nearest ' Vega) is a remarkable variable and spectroscopic binary. It is really a very close double, consisting of two unequally bright stars revolving almost in contact in a period a little less than thirteen days, which mutually eclipse one another. The closeness of a good comparison star makes the variations very easy to follow. At maximum it is nearly equal in brightness to Gamma Lyrae; at minimum less than By Henry Norris Russell, Ph. D. half as bright. Almost on the line joining these two stars is the Ring Nebula of Lyra-a beautiful object in a telescope of sufficient power. Not far from Lyra, and just north of the zenith, is the head of Draco, marked by two bright stars, of between the second and third magnitudes. The easterly olle of these, Gamma Draconis, is of considerable interest in the history of astronomy, since it was by observing it that the great English astronomer Bradley, early in the eighteenth century, made the important discovery of the aberration of light. Bradley's original purpose was to observe the position of this star in the heavens, with, an instrument of his own invention, much superior in accuracy to those previously used, in the hope of detecting a yearly change in its position, due to the earth's orbital At S'/i o'clock: July 30. NIGHT SKY: JULY AND AUGUST motion about the sun-in other words, what we now call an annual parallax. If such an effect of the earth's motion should exist, it is clear that the star should appear farther south than usual when the earth was north of the sun (as seen from the star) and farther north than usual when the earth was south of the sun (or the sun north of the earth, which happens in June). But the actual observations showed a different and very perplexing state of things. The star's apparent position changed, but it was farthest north, not in June, as apparently it ought to have been, but in September-three months late! In June and December, when the effect of parallax should be a maximum, the star's position was almost exactly the same, but between September and March, when no difference owing to parallax could be expected, a large difference was actually observed. After much puzzling over this strange phenomenon, the true explanation occurred to Bradley while sailing on the Thames, upon noticing that the wind appeared to change in direction every time the boa: tacked and changed her course. Suppose the wind to be blowing from the north, as observed on shore, or on a vessel at anchor. Now suppose the ship to get under way, and proceed due east. In a caln this would apparently produce, for an observer on board, a wind blowing from the direction in which the vessel was heading. The combination of this with the actual north wind would make the wind appear to come from some point east of north. The faster the ship's motion, compared with that of the wind, the greater would be the deviation of its apparent direction. Now suppose the ship turned about and headed due west. The wind would now seem to come from a point as far west of north as the direction previously observed was east of north. But if the ship was steering north or south, the apparent direction of the wind would be unchanged, though its velocity would seem to be increased or diminished. Now the light which reaches us from a star, though in other respects quite unlike wind, possesses like the wind a finite velocity; and the combination of this with the velocity of the (bserver, who is carried along on the moving earth, produces similar effects. When the earth is moving in any given dii'ection — say northward-the light of a star which is not in the line of motion will seem to come from a point farther north than its true position; that is, the star will apparently be displaced to the north ward, and vice versa if the earth is moving southward. Now this is exactly what Bradley had observed—the star being apparently farthest north just at the time when the earth is moving northward fastest-or the sun apparently moving southward at the most rapid rate. This occurs in September and all the other observations are equally well explained. The mystery was thus cleared up. What is more, a conclusive proof had been given that the earth is not fixed in space, but moves around the sun. The rest of Draco winds about, first to the right, then to the left and downward, inc los i n g Ursa Minor and separating it from the Great Bear, which is descending in the northwestern sky. On the other side of the Pole, Cassiopeia is rISIng and Cepheus is higher up. Following the Milky Way southward, we reach Cygnus, and then Aquila, in the sbutheast. The great square of Pegasus has just risen and the line of stars which runs from it through Andromeda and Perseus is near the northeastern horizon. The lower southeastern sky is dull, containing only Capricornus and Aquarius, but the region of Sagittarius and Scorpio, in the south, is one of the finest in the heavens, and contains the brighest part of the whole Milky Way. Spica Virginis is low in the southwest and much less conspicuous than the planet .upiter, which is a little to the left. Libra, Ophiuchus and Serpens fll a large area of the southwestern sky. Hercules is west of the zenith, Corona Borealis below, and Bootes lower still, Arcturus being almost due west of the zenith. THE PLANETS. Mercury is evening star throughout August, and is best visible before the 12th. His apparent distance from the sun (27 degrees) is unusually large, but, being far south of the latter, he remains in sight for only an hour after sunset and will not be easy to see. Conditions wiII be most favorable in the early part of the month, wilen he is farther north. Venus is likewise evening star, but being farther from the sun than Mercury, and very much brighter, is a conspicuous object. She reaches her greatest brilIiancy on the lOth, after which the rapid narrowing of heT crescent, as she comes between us and the sun, more than makes up for the increase in her apparent si7e as she approaches us. At the beginning of the month she sets at 8.40 P. M., and on the 15th a liltle before 8 P. M. After this she approaches the sun very rapidly, and by the 31st she will be hard to see as she sets about 6.45. At the be.inning of the month she appears as a crescent some 35 seconds in diameter, and 12 seconds m Width. On the 31st her diameter has increased to 54 seconds, but her crescent is only 4 seconds wide, so that she sends us less light than before. Mars is in Aries, and rises a little before 11 p, M on the 15th. He moves slowly eastward, and increases in brightness and apparent size as the earth overtakes him. Saturn is close by, but is moving more slowly, so that Mars overtakes him early on the morning of the 17th. At this time the two planets are apparently separated by only 21 seconds—about 3 the moon's diameter. Tile moon herself is close by—about four degrees to the southward—and the triple conjunction will be a pretty spectacle in the morning sky. Jupiter is evening star in Virgo, setting about 10 P. M, in the middle of the month. Uranus is in Sagittarius just past opposition, his position on the 15th being R. A. 19 h. 54.1 m,; Dec, 21 deg. 26 min. Neptune is in Gemini, and rises about two hours before the sun. The' moon is in her first quarter at 6 P. M. on the 1st, full at 10 P. M. on the 9th ' in her last quarter at 7 A. M. on the 17th, new at 11 p, M. on the 23rd, and in the first quarter once more at 11 A. M, on the 31st. She is nearest us al the 21st, and farthest off on the 5th. As she passes round the skies, she comes into conjunction with Jupiter on the 1st, Uranus on the 8th, Mars and Saturn on the morning of the 17th, Neptune on the 21st, Venus and Mercury on the 25th, and Jupiter again on the 29th. KIESS'S COMET. A camet, visible in an operaglass and possessing a tail, was discovered by Kiess at the Lick Observatory on July 6th. At that time it was in the southern part of Auriga, and rose about four hours before sunrise.
This article was originally published with the title "The Heavens in August"