The Astronomy of Milton's 'Paradise Lost' by Thomas Nathaniel Orchard (easy books to read in english TXT) 📕

Professor Henderson’s detection of the parallax of α Centauri was communicated to the Astronomical Society two months after Bessel announced his determination of the parallax of 61 Cygni.

The parallax of 61 Cygni assigns to the star a distance of forty billions of miles from the Earth, and that of α Centauri—regarded as the nearest star to our system—a distance of twenty-five billions of miles.

It is utterly beyond the capacity of the human mind to form any adequate conception of those vast distances, even when measured by the velocity with which the ether of space is thrilled into light. Light, which travels twelve millions of miles in a minute, requires 4-1/3 years to cross the abyss which intervenes between α Centauri and the Earth, and from 61 Cygni the period required for light to reach our globe is rather less than double that time.

The parallax of more than a dozen other stars has been determined, and the light passage of a few of the best known is estimated as follows:—Sirius, eight years; Procyon, twelve; Altair, sixteen; Aldebaran, twenty-eight; Capella, thirty; Regulus, thirty-five; Polaris, sixty-three; and Vega, ninety-six years.

It does not always follow that the brightest stars are those situated nearest to our system, though in a general way this may be regarded as correct. The diminishing magnitudes of the stars can be accounted for mainly by their increased distances, rather than by any difference in their intrinsic brilliancy. We should not err by inferring that the most minute stars are also the most remote; the telescope revealing thousands that are invisible to the naked eye. There are, however, exceptions to this general rule, and there are many stars of small magnitude less remote than those whose names have been enumerated, and whose light passage testifies to their profound distances and surpassing magnitude when compared with that of our Sun.

Sirius, ‘the leader of the heavenly host,’ is distant fifty billions of miles. The orb shines with a brilliancy far surpassing that of the Sun, and greatly exceeds him in mass and dimensions. Arcturus, the bright star in Boötes, whose golden yellow light renders it such a conspicuous object, is so far distant that its measurement gives no reliable parallax; and if we may infer from what little we know of the stars, Arcturus is believed to be the most magnificent and massive orb entering into the structure of that portion of the sidereal system which comes within our cognisance. Judging by its relative size and brightness, this star is ten thousand times more luminous, and may exceed the Sun one million times in volume.

Deneb, in the constellation of the Swan, though a first-magnitude star, possesses no perceptible proper motion or parallax—a circumstance indicative of amazing distance, and magnitude equalling, or surpassing, Arcturus and Sirius.

Canopus, in the constellation Argo, in the Southern Hemisphere, the brightest star in the heavens with the exception of Sirius, possesses no sensible parallax; consequently, its distance is unknown, though it has been estimated that its light passage cannot be less than sixty-five years.

By establishing a mean value for the parallax of stars of different magnitudes, it was believed that an approximation of their distances could be obtained by calculating the time occupied in their light passage. The light period for stars of the first magnitude has been estimated at thirty-six and a half years; this applies to the brightest stars, which are also regarded as the nearest. At the distance indicated by this period, the Sun would shrink to the dimensions of a seventh-magnitude star and become invisible to the naked eye; this of itself affords sufficient proof that the great luminary of our system cannot be regarded as one of the leading orbs of the firmament. Stars of the second magnitude have a mean distance of fifty-eight light years, those of the third magnitude ninety-two years, and so on. M. Peters estimated that light from stars of the sixth magnitude, which are just visible to the naked eye, requires a period of 138 years to accomplish its journey hither; whilst light emitted from the smallest stars visible in large telescopes does not reach the Earth until after the lapse of thousands of years from the time of leaving its source.

The profound distances of the nearest stars by which we are surrounded lead us to consider the isolated position of the solar system in space. A pinnacle of rock, or forsaken raft floating in mid-ocean, is not more distant from the shore than is the Sun from his nearest neighbours. The inconceivable dimensions of the abyss by which the orb and his attendants are surrounded in utter loneliness may be partially comprehended when it is known that light, which travels from the Sun to the Earth—a distance of ninety-three millions of miles—in eight minutes, requires a period of four and a third years to reach us from the nearest fixed star. A sphere having the Sun at its centre and this nearest star at its circumference would have a diameter of upwards of fifty billions of miles; the volume of the orb when compared with the dimensions of this circular vacuity of space is as a small shot to a globe 900 miles in diameter. It has been estimated by Father Secchi that, if a comet when at aphelion were to arrive at a point midway between the Sun and the nearest fixed star, it would require one hundred million years in the accomplishment of its journey thither. And yet the Sun is one of a group of stars which occupy a region of the heavens adjacent to the Milky Way and surrounded by that zone; nor is his isolation greater than that of those stars which are his companions, and who, notwithstanding their profound distance, influence his movements by their gravitational attraction, and in combination with the other stars of the firmament control his destiny.

Ancient astronomers, for the purpose of description, have mapped out the heavens into numerous irregular divisions called ‘constellations.’ They are of various forms and sizes, according to the configuration of the stars which occupy them, and have been named after different animals, mythological heroes, and other objects which they appear to resemble. In a few instances there does exist a similitude to the object after which a constellation is called; this is evident in the case of Corona Borealis (the Northern Crown), in which there can be seen a conspicuous arrangement of stars resembling a coronet, and in the constellations of the Dolphin and Scorpion, where the stars are so distributed that the forms of those creatures can be readily recognised. There is some slight resemblance to a bear in Ursa Major, and to a lion in Leo, and no great effort of the mind is required to imagine a chair in Cassiopeia, and a giant in Orion; but in the majority of instances it is difficult to perceive any likeness of the object after which a constellation is named, and in many cases there is no resemblance whatever.

The constellations are sixty-seven in number: excluding those of the Zodiac, which have been already mentioned, the constellations of the Northern Hemisphere number twenty-nine. The most important of these are Ursa Major and Minor, Andromeda, Cassiopeia, Cepheus, Cygnus, Lyra, Aquila, Auriga, Draco, Boötes, Hercules, Pegasus, and Corona Borealis.

To an observer of the nocturnal sky the stars appear to be very unequally distributed over the celestial sphere. In some regions they are few in number and of small magnitude, whilst in other parts of the heavens, and especially in the vicinity of the Milky Way, they are present in great numbers and form groups and aggregations of striking appearance and conspicuous brilliancy. On taking a casual glance at the midnight sky on a clear moonless night, one is struck with the apparent countless multitude of the stars; yet this impression of their vast number is deceptive, for not more than two thousand stars are usually visible at one time.

Much, however, depends upon the keenness of vision of the observer, and the transparency of the atmosphere. Argelander counted at Bonn more than 3,000 stars, and Hozeau, near the equator, where all the stars of the sphere successively appear in view, enumerated 6,000 stars. This number may be regarded as including all the stars in the heavens that are visible to the naked eye. With the aid of an opera glass thousands of stars can be seen that are imperceptible to ordinary vision. Argelander, with a small telescope of 2½ inches aperture, was able to count 234,000 stars in the Northern Hemisphere. Large telescopes reveal multitudes of stars utterly beyond the power of enumeration, nor do they appear to diminish in number as depth after depth of space is penetrated by powerful instruments. The star-population of the heavens has been reckoned at 100,000,000, but this estimate is merely an assumption; recent discoveries made by means of stellar photography indicate that the stars exist in myriads. It is reasonable to believe that there is a limit to the sidereal universe, but it is impossible to assign its bounds or comprehend the apparently infinite extent of its dimensions.

Scintillation or twinkling of the stars is a property which distinguishes them from the planets. It is due to a disturbed condition of the atmosphere and is most apparent when a star is near the horizon; at the zenith it almost entirely vanishes. Humboldt states that in the clear air of Cumana, in South America, the stars do not twinkle after they reach an elevation of 15° above the horizon. The presence of moisture in the atmosphere intensifies scintillation, and this is usually regarded as a prognostication of rain. White stars twinkle more than red ones. The occurrence of scintillation can be accounted for by the fact that the stars are visible as single points of light which twinkle as a whole, but in the case of the Sun, Moon, and planets, they form discs from which many points of light are emitted; they, therefore, do not scintillate as a whole, for the absence of rays of light from one portion of their surface is compensated by those from other parts of their discs, giving a mean average which creates a steadiness of vision.

The stars are divided into separate classes called ‘magnitudes,’ by which their relative apparent size and degree of brightness are distinguished. The magnitude of a star does not indicate its mass or dimensions, but its light-giving power, which depends partly upon its size and distance, though mainly upon the intensity of its luminosity. The most conspicuous are termed stars of the first magnitude; there are ten of those in the Northern Hemisphere, and an equal number south of the equator, but they are not all of the same brilliancy. Sirius outshines every other star of the firmament, and Arcturus has no rival in the northern heavens. The names of the first-magnitude stars north of the equator are: Arcturus, Capella, Vega, Betelgeux, Procyon, Aldebaran, Altair, Pollux, Regulus, and Deneb. The next class in order of brightness are called second-magnitude stars; they are fifty or sixty in number, the most important of which is the Pole Star. The stars diminish in luminosity by successive gradations, and when they sink to the sixth magnitude reach the utmost limit at which they appear visible to the naked eye. In great telescopes this classification is carried so low as to include stars of the eighteenth and twentieth magnitudes.

Entering into the structure of the stellar universe we have Single Stars, Double Stars, Triple, Quadruple, and Multiple Stars, Temporary, Periodical, and Variable Stars, Star-groups, Star-clusters, Galaxies, and Nebulæ.

Single or Insulated Stars include all those orbs sufficiently isolated in space so as not to be