The most severe test is, however, Epsilon Lyræ, the northern of two small stars which form a little triangle with the brilliant Vega. This, to some eyes, appears double. The famous German astronomer Bessel is said to have seen it at thirteen years of age. To most people, however, it will perhaps appear only elongated. This is a very remarkable star, as each of the components is seen to be a close double when examined with a good telescope; and between the pairs are several fainter stars.
Among those interesting objects, the variable stars, are several which may be well observed without optical assistance. Of these may be mentioned Algol, of which all the fluctuations of light may be easily observed with the naked eye; Mira Ceti, which may be well observed when at its brightest; Lambda Tauri, a variable star of the Algol type; Betelgeuse (Alpha Orionis), which is slightly variable; Zeta Geminorum, a fourth magnitude star, which varies about three-quarters of a magnitude in a period of about ten days; R. Hydræ, which is visible to the naked eye at maximum; Beta Lyræ, period about thirteen days; Eta Aquilæ, period about seven days; and Delta Cephei, which varies about one magnitude in a period of a little over five days. Of all these stars useful observations may be made without optical assistance of any sort.
Observations, and even discoveries, of new or “temporary” stars may also be made with the naked eye. This occurred in the case of the “temporary” stars of 1572, 1604, 1670, 1866, and 1870, but, of course, these were bright objects at the time of their discovery. Hind’s “new star” of 1848 in Ophiuchus was, however, only of the fifth magnitude when it appeared, and it might have escaped detection with the naked eye. A star of this magnitude might, however, be easily detected by an observer who is familiar with the principal stars of a constellation.
The Milky Way may, perhaps, be better seen with the naked eye than with any instrument, although an opera-glass brings out well, in some places, its more delicate details. A mere passing glance might lead a casual observer to suppose that the Galaxy stretched as a band of nearly uniform brightness across the heavens. But good eyesight, careful attention, and a clear sky will soon disclose numerous details previously unsuspected; streams and rays of different brightness, intersected by rifts of darkness, and interspersed with spots and channels of comparatively starless spaces. An excellent drawing of the Milky Way—the result of five years’ observations with the naked eye alone—has recently been completed by Dr. Otto Boeddicker at Lord Rosse’s observatory in Ireland. This beautiful picture is exquisitely drawn, and shows a wonderful amount of detail. A writer in the Saturday Review of November 30, 1889, says: “His maps are in many respects a completely new disclosure. Features barely suspected before come out in them as evident and persistent; every previous representation appears, by comparison, structureless.” This shows what can be done with the naked eye in the study of this wonderful zone.
Among the nebulæ and clusters there are not many objects visible to the naked eye. A hazy appearance about the middle star in Orion’s “sword” indicates the presence of the “great Nebula,” one of the finest objects in the heavens. The “great Nebula in Andromeda,” aptly termed “the Queen of the Nebulæ,” is distinctly visible to the naked eye on a very clear night. It lies near the four and a half magnitude star, Nu Andromedæ (a few degrees north of Beta Andromedæ), and may be well seen in the early evening hours in the month of January, when it is high in the sky. It somewhat resembles a small comet. This nebula was known long before the invention of the telescope, and it was described by one of the earlier astronomers as resembling “a candle shining through horn,” a not inapt description.
Of star clusters visible without optical aid may be mentioned the double cluster Chi Persei, which appears to the eye as a luminous spot in the Milky Way; the cluster known as 35 Messier, a little north of Eta Geminorum, just visible to the naked eye on a very clear night; and there are others in the Southern Hemisphere, notably the globular cluster known as Omega in the Centaur, which shines as a hazy star of the fourth magnitude. Among the clusters may perhaps be included the Præsepe, or the “Beehive,” in Cancer, which has a nebulous appearance to the naked eye.
Coming now to the Solar System, the sun and moon, of course, first attract attention. Cases of sun-spots visible to the naked eye are recorded, but, of course, spots of such enormous size are of rare occurrence. Of lunar detail little can be seen without a telescope of some sort, but the larger markings are sufficiently distinct to good eyesight to convince the observer that they do not alter perceptibly, thus showing clearly that the moon always turns the same side to the earth.
Of the planets, nothing of their appearance in the telescope can, of course, be seen with the naked eye, but it is easy to identify the brighter planets. Mercury, owing to its proximity to the sun, is rarely visible in Europe and North America, but when favorably situated, it may sometimes be detected near the sun shortly after sunset or a little before sunrise. Notwithstanding the difficulty of seeing it, it was well known to the ancients, an observation of the planet dating back to 264 B. C. It is easier, however, to see in more southern latitudes, and I have frequently observed it as bright as a star of the first magnitude in the clear air of the Punjab sky. I have also seen it on several occasions in Ireland, and the Rev. S. S. Johnson, F.R.A.S., tells me he has seen it with the naked eye no less than one hundred times in the south of England. The brilliant planet Venus can hardly be mistaken when seen in the morning or evening sky. When at its brightest it considerably exceeds Jupiter and Mars, and far surpasses Sirius, the brightest star in the heavens.
If a very bright planet is seen rising at sunset, it can not be Venus, which is never seen beyond a limited distance from the sun. The observer may, therefore, conclude with certainty that the planet is either Jupiter or Mars. The latter, which occasionally rivals Jupiter in brilliancy, may be easily distinguished from the “giant planet” by its distinctly reddish color. Saturn shines with a yellowish light, and is never so bright as Mars or Jupiter when at their brightest. The planet Uranus is just visible to the naked eye, and may be found without optical assistance when its position is accurately known.
Some observers think that they can see the crescent of Venus with the naked eye when the planet is in that phase, but this seems very doubtful. Cases have been recorded of one or two satellites of Jupiter having been seen with the unaided eyesight, but few are gifted with such keen vision.