The Astronomy of Milton's 'Paradise Lost', Thomas Nathaniel Orchard [best selling autobiographies .TXT] 📗

of the Earth’s orbit must be measured by millions of miles, and yet there was no perceptible motion or change of position of the stars when viewed from any point of the vast circumference which she traverses. Consequently, the Earth, if viewed from the neighbourhood of a star, would also appear motionless, and the dimensions of her orbit would be reduced to that of a point. This seemed incredible to Tycho, and he therefore concluded that the Copernican theory was incorrect.

The conclusion that the stars are orbs resembling our Sun in magnitude and brilliancy was one which, Tycho urged, should not be hastily adopted; and yet, if it were conceded that the Earth is a body which revolves round the Sun, it would be necessary to admit that the stars are suns also. If the Earth’s orbit, as seen from a star, were reduced to a point, then the Sun, which occupies its centre, would be reduced to a point of light also, and, when observed from a star of equal brilliancy and magnitude, would have the same resemblance that the star has when viewed from the Earth, which may be regarded as being in proximity to the Sun. Tycho Brahé would not admit the accuracy of these conclusions, which were too bewildering and overwhelming for his mental conception.

But the investigations of later astronomers disclosed the fact that the heavenly bodies are situated at distances more remote from each other than had been previously imagined, and that the reasons which led Tycho to reject the Copernican theory were based upon erroneous conclusions, and could, with greater aptitude, be employed in its support. It was ascertained that the distance of the Sun from the Earth, which at different periods was surmised to be ten, twenty, and forty millions of miles, was much greater than had been previously estimated. Later calculations determined it to be not less than eighty millions of miles, and, according to the most recent observations, the distance of the Sun from the Earth is believed to be about ninety-three millions of miles.

Having once ascertained the distance between the Earth and the Sun, astronomers were enabled to determine with greater facility the distances of other heavenly bodies.

It was now known that the diameter of the Earth’s orbit exceeded 183 millions of miles, and yet, with a base line of such enormous length, and with instruments of the most perfect construction, astronomers were only able to perceive the minutest appreciable alteration in the positions of a few stars when observed from opposite points of the terrestrial orbit.

It had long been the ambitious desire of astronomers to accomplish, if possible, a measurement of the abyss which separates our system from the nearest of the fixed stars. No imaginary measuring line had ever been stretched across this region of space, nor had its unfathomed depths ever been sounded by any effort of the human mind. The stars were known to be inconceivably remote, but how far away no person could tell, nor did there exist any guide by which an approximation of their distances could be arrived at.

In attempting to calculate the distances of the stars, astronomers have had recourse to a method called ‘Parallax,’ by which is meant the apparent change of position of a heavenly body when viewed from two different points of observation.

The annual parallax of a heavenly body is the angle subtended at that body by the radius of the Earth’s orbit.

The stars have no diurnal parallax, because, owing to their great distance, the Earth’s radius does not subtend any measurable angle, but the radius of the Earth’s orbit, which is immensely larger, does, in the case of a few stars, subtend a very minute angle.

‘This enormous base line of 183 millions of miles is barely sufficient, in conjunction with the use of the most delicate and powerful astronomical instruments, to exhibit the minutest measureable displacement of two or three of the nearest stars.’—Proctor.

The efforts of early astronomers to detect any perceptible alteration in the positions of the stars when observed from any point of the circumference of the Earth’s orbit were unsuccessful. Copernicus ascribed the absence of any parallax to the immense distances of the stars as compared with the dimensions of the terrestrial orbit. Tycho Brahé, though possessing better appliances, and instruments of more perfect construction, was unable to perceive any annual displacement of the stars, and brought this forward as evidence against the Copernican theory.

Galileo suggested a method of obtaining the parallax of the fixed stars, by observing two stars of unequal magnitude apparently near to each other, though really far apart. Those, when observed from different points of the Earth’s orbit, would appear to change their positions relatively to each other. The smaller and more distant star would remain unaltered, whilst the larger and nearer star would have changed its position with respect to the other. By continuing to observe the larger star during the time that the Earth accomplished a revolution of her orbit, Galileo believed that its parallax might be successfully determined. Though he did not himself put this method into practice, it has been tried by others with successful results.

In 1669, Hooke made the first attempt to ascertain the parallax of a fixed star, and selected for this purpose γ Draconis, a bright star in the Head of the Dragon. This constellation passed near the zenith of London at the time that he made his observations, and was favourably situated, so as to avoid the effects of refraction. Hooke made four observations in the months of July, August, and October, and believed that he determined the parallax of the star; but it was afterwards discovered that he was in error, and that the apparent displacement of the star was mainly due to the aberration of light—a phenomenon which was not discovered at that time.

A few years later, Picard, a French astronomer, attempted to find the parallax of α Lyræ, but was unsuccessful. In 1692-93, Roemer, a Danish astronomer, observed irregularities in the declinations of the stars which could neither be ascribed to parallax or refraction, and which he imagined resulted from a changing position of the Earth’s axis.

One of the principal causes which baffled astronomers in their endeavours to determine the parallax of the fixed stars was a phenomenon called the ‘Aberration of Light,’ which was discovered and explained by Bradley in 1727. The peculiar effect of aberration was perceived by him when endeavouring to obtain the parallax of γ Draconis.

Owing to the progressive transmission of light, conjointly with the motion of the Earth in her orbit, there results an apparent slight displacement of a star from its true position. The extent of the displacement depends upon the ratio of the velocity of light as compared with the speed of the Earth in her orbit, which is as 10,000 to 1. As a consequence of this, each star describes a small ellipse in the course of a year, the central point of which would indicate the place occupied by the star if the Earth were at rest. The shifting position of the star is very slight, and at the end of a year it returns to its former place.

Prior to the discovery of aberration, astronomers ascribed the apparent displacement of the stars arising from this cause as being due to parallax—a conclusion which led to erroneous results; but after Bradley’s discovery this source of error was avoided, and it was found that the parallax of the stars had to be considerably reduced.

Bessel was the first astronomer who merited the high distinction of having determined the first reliable stellar parallax, and by this achievement he was enabled to fathom the profound abyss which separates our solar system from the stars.

Frederick William Bessel was born in 1764 at Minden, in Westphalia. It was his intention to pursue a mercantile career, and he commenced life by becoming apprenticed to a firm of merchants at Bremen. Soon afterwards he accompanied a trading expedition to China and the East Indies, and while on this voyage picked up a good deal of information with regard to many matters which came under his observation. He acquired a knowledge of Spanish and English, and made himself acquainted with the art of navigation. On his return home, Bessel endeavoured to determine the longitude of Bremen. The only appliances which he made use of were a sextant constructed by himself, and a common clock; and yet, with those rude instruments, he successfully accomplished his object. During the next two years he devoted all his spare time to the study of mathematics and astronomy, and, having obtained possession of Harriot’s observations of the celebrated comet of 1607—known as Halley’s comet—Bessel, after much diligent application and careful calculation, was enabled to deduce from them an orbit, which he assigned to that remarkable body. This meritorious achievement was the means of procuring for him a widely known reputation.

A vacancy for an assistant having occurred at Schröter’s Observatory at Lilienthal, the post was offered to Bessel and accepted by him. Here he remained for four years, and was afterwards appointed Director of the new Prussian Observatory at Königsberg, where he pursued his astronomical labours for a period of upwards of thirty years. Bessel directed his energies chiefly to the study of stellar astronomy, and made many observations in determining the number, the exact positions, and proper motions of the stars. He was remarkable for the precision with which he carried out his observations, and for the accuracy which characterised all his calculations.

In 1837 Bessel, by the exercise of his consummate skill, endeavoured to solve a problem which for many years baffled the efforts of the ablest astronomers, viz., the determination of the parallax of the fixed stars. This had been so frequently attempted, and without success, that the results of any new observations were received with incredulity before their value could be ascertained.

Bessel was ably assisted by Joseph Frauenhofer, an eminent optician of Munich, who constructed a magnificent heliometer for the Observatory at Königsberg, and in its design introduced a principle which admirably adapted it for micrometrical measurement.

The star selected by Bessel is a binary known as 61 Cygni, the components being of magnitudes 5·5 and 6 respectively. It has a large proper motion, which led him to conclude that its parallax must be considerable.

This star will always be an object of interest to astronomers, as it was the first of the stellar multitude that revealed to Bessel the secret of its distance.

Bessel commenced his observations in October 1837, and continued them until March 1840. During this time he made 402 measurements, and, before arriving at a conclusive result, carefully considered every imaginable cause of error, and rigorously calculated any inaccuracies that might arise therefrom. Finally, he determined the parallax of the star to be 0''·3483—a result equivalent to a distance about 600,000 times that of the Earth from the Sun. In 1842-43 M. Peters, of the Pulkova Observatory, arrived at an almost similar result, having obtained a parallax of 0''·349; but by more recent observations the parallax of the star has been increased to about half a second.

About the same time that Bessel was occupied with his observation of 61 Cygni, Professor Henderson, of Edinburgh, when in charge of the Observatory at the Cape of Good Hope, directed his attention to α Centauri, one of the brightest stars in the Southern Hemisphere. During 1832-33 he made a series of observations of the star, with the object of ascertaining its mean declination; and, having been informed afterwards of its large proper motion, he resolved to make an endeavour to determine its parallax. This he accomplished after his return to Scotland, having been appointed Astronomer Royal in that country. By an examination of the observations made by him at the