THE SUN'S DISTANCE TO BE AGAIN ALTERED ?
RATHER more than twenty years ago, the news went abroad that the estimate of the Sun's distance which had then been accepted for more than half-a-century must be replaced by a greatly reduced estimate. From more than ninety-five millions of miles, the Sun's distance was diminished, according to the new estimate, to about ninety-one and a half millions of miles, or by considerably more than a thirtieth part. With this change came most important modifications in our ideas respecting the Solar system, Some, indeed, ignorant of the relation which really subsists between the distance of the Sun and the motions of our Earth, imagined that a change of the most momentous kind had really taken place. They supposed that our earth had drawn nearer to the sun by nearly four millions of miles since 1762, when the observations giving the greater distance had been made. Of course, if this had been the case, if in about ninety years the earth had drawn in by more than a thirtieth of her former distance, then in about 2,500 years the earth would fall into the sun, and long before that period had passed, the increase of heat resulting from her approach towards him would destroy all races now living on the earth. Fortunately, we have the most satisfactory evidence that no real change had taken place. It is absolutely certain that a reduction of the earth's distance by about a thirtieth would result in a reduction of the year by about a twentieth, or by some eighteen days, whereas we know that the year has not changed one minute in length in the last 2,000 years. Yet the changes resulting from our reduced estimate of the sun's distance were in truth suffi- ciently important. Our estimate of the sun's volume was re- duced from about 1,400,000 times the earth's to about 1,250,000 times, or by a volume exceeding his 150,000 times. Our estimate of his mass was reduced from about 355,000 times to about 316,000 times the earth's, or by about 29,000 times the mass of this great globe on which we live. Our ideas of hie energy and stability as ruler, fire, light, and life of the Solar system were correspondingly reduced, for his power as a ruler depends on the quantity of matter he contains, while his vitality as a source of light and heat depends both upon the attractive energy he exerts on his own mass and matter around him, and upon the extent of space occupied by such matter and his own material. In fact, the whole of the Solar system was not only reduced in dimensions by our changed estimate of the sun's distance, but the duration of the system as a scheme of sunlit, sun-warmed worlds was also reduced.
The reduced estimate of the Sun's distance, maintained its ground. Several new methods of determining that import- ant element have been employed during the last twenty years. Many new investigations by old methods have been pursued, with all the refinements in instrumental and analytical means which belong to the astronomy of recent years. Even the Transit observations of 1769 — though unfortunately they could not be made over again with our im- proved means and methods—were re-examined and more consist- ently interpreted. When the sun's distance seemed likely to be determined by the best modern observations at about 91 millions of miles, Powalky, re-examining the records of last century's transits, found they could be so interpreted as to give a distance not exceeding 92 millions of miles. Stone, of Greenwich, seemed more fortunate, for he reconciled those records with a distance of Olf millions of miles. And receiving that official support which Greenwich astronomers seldom look for in vain, Mr. Stone was able to secure the approval of the Astronomical Society for his success in thus re- conciling the observations of the transits of 1761 and 1769 with a solar distance of 91,500,000 miles, This circumstance, in the presence of the new estimate of the sun's distance recently an- nounced from Greenwich, deserves to be most attentively noted, as will presently appear.
But the thorough investigation which the subject now received, by every method except the transit-of-Venus method, showed that in reality the distance of the Sun bad not to be quite so greatly reduced. On a careful examination, it was found that without a single exception all the best methods pointed to a dis- tance of about 92,350,000 miles. From a masterly analysis of observations by six different methods, Professor Newcomb, of Washington, who stands facile prineeps in this particular depart-
ment, arrived at six results, ranging between a minimum distance of 92,200,000 miles and a maximum distance of 92,800,000 miles ; but if one, confessedly the least valuable method of the six, is ex- cluded, the other five range only between 92,200,000 miles and 92,480,000 miles. The mean value of these five is not very different from the mean value of the six results, just weight being given to each, and is set by Newcomb at 92,393,000 miles. Then came Leverrier's most interesting determination of the sun's distance from the observed planetary motions, giving a distance of 92,200,000 mike. It is from combining this result, duly weighted, with Newcomb's, that the estimate above mentioned, 92,350,000 miles, is obtained.
But now we learn that the observations made by the British observing parties during the transit of December, 1874, point to a distance more than one million miles greater. " The conclusion which we have brought out," said the Astronomer-Royal, at a recent meeting of the Astronomical Society, " is that the mean distance of the Sun is 93,375,000 miles." "Considering," he added, " that the number of observers was eighteen, and that they made fifty-four observations, and considering also the degree of training they have had, and their zeal and the extreme care that was taken in the choice of stations, I think that there will not be anything to compete with the value which has been deduced." Although this estimate of the value of a result depending on operations for which Sir G. Airy was chiefly responsible might, perhaps, have come with somewhat better grace from others, yet one can understand and recognise as perfectly natural the tendency to regard as pre-eminently trust- worthy the result of operations long since planned, of preparations long and carefully conducted, and of the expenditure of a hand- some sum of the nation's money. In the same way, and quite as naturally, when the Heston Colliery experiment for determining the density and mass of the earth had been brought to a close, and the result was found to exceed all former results, and the mean of former results, in the startlingly large pro- portion of six to five, making the earth's mass greater by about twelve hundred millions of millions of millions of tons than former estimates, the Astronomer-Royal expressed the opinion that his result was "comparable on at least equal terms with those before obtained." The fact that no other astronomers (outside official circles, or in truth, outside the circle of Sir G. Airy's subordinates) have adopted this opinion, either in this country, on the Continent, or in America, the estimate of the earth's mass before obtained remaining everywhere in use, though twenty-three years have passed since the Hasten experiment was made, may suggest doubts as to the ready acceptance of the new measure of the sun's distance by Continental and American astro- nomers (and most probably by the leading English astronomers, outside Greenwich). This estimate is not only far removed from the mean value to which former researches had pointed, but lies even far outside the widest limits of probable error before regarded as admissible. The point to be decided is a very simple one. It is whether the most probable interpretation of the wide dis- crepancy between the lately-published result and the values obtained by Newcomb and Leverrier indicates the inaccuracy of former valuations, or the inadequacy of the method on which the British transit operations were conducted. The opinion of the Astronomer-Royal (above quoted) means noither more nor less than this,—that the weight of the British observations (by a method which many astronomers have for a long time regarded as open to grave exceptions) is greater some tenfold than the combined weight of many sets of observations by each of seven other methods, three of which have been described severally by Adams, Leverrier, and Newcomb, as the best available. Such a proposition savours of considerable confidence. Strangely enough, the observations thus confidently relied upon, instead of being closely aecordant inter se, may be divided into two sets, of which one gives to the sun a distance of 93,500,000 miles, the other a distance of only 92,400,000 miles.
But fortunately the matter will very soon be brought to a satisfactory test. The British observations thus far utilised are only those which can be combined together to give a result. They are, in point of fact, the Delislean observations about which so much controversy arose before the transit occurred. The British Halleyan observations, which were undertaken in compliance with the strongly expressed feeling of many English astronomers, will show their value only when combined with other Halleyan observations by America, Russia, and Germany. Their fruits have still to be gathered in. We venture to express our confi- dence that when this has been done, the transit results will be found in far more satisfactory agreement with results before obtained than the estimate which has recently been published by Sir G. Airy. If not, the transit method generally will fall under considerable discredit. At present discredit falls only on the Delisle= method,—a result long since predicted.
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