Samuel Molyneux was a gentleman of fortune much attached to science, and particularly to astronomy, who was living about this time at Kew. He was one of the few, moreover, who are not content merely to amuse themselves with a telescope, but had the ambition to do some real earnest work, and the courage to choose a problem which had baffled the human race for more than a century. The theory of Copernicus, that the earth moved round the sun, necessitated a corresponding apparent change in the places of the stars, one relatively to another; and it was a standing difficulty in the way of accepting this theory that no such change could be detected. In the old days before the telescope it was perhaps easy to understand that the change might be too small to be noticed, but the telescope had made it possible to measure changes of position at least a hundred times as small as before, and still no “parallax,” as the astronomical term goes, could be found for the stars. The observations of Galileo, and the measures of Tycho Brahé, as reduced to systematic laws by Kepler, and finally by the great Newton, made it clear that the Copernican theory was true: but no one had succeeded in proving its truth in this particular way.Attempts to find stellar parallax. Samuel Molyneux must have been a man of great courage to set himself to try to crack this hard nut; and we can understand the attraction which his enterprise must have had for Bradley, who had just lost the beloved colleague of many courageous astronomical undertakings. His co-operation seems to have been welcomed from the first; his help was invited and freely given in setting up the instrument, and he fortunately had the leisure to spend considerable time at Kew making the observations with Molyneux, just as he had been wont to observe with his uncle.
I must now briefly explain what these observations were. There is a bright star γ Draconis, which passes almost directly overhead in the latitude of London. Its position is slowly changing owing to the precession of the equinoxes, but for two centuries it has been, and is still, under constant observation by London astronomers owing to this circumstance, that it passes directly overhead, and so its position is practically undisturbed by the refraction of our atmosphere.
It was therefore thought at the time that, there being no disturbance from refraction, the disturbance from precession being accurately known, and there being nothing else to disturb the position but “parallax” (the apparent shift due to the earth’s motion which it was desirable to find), this star ought to be a specially favourable object for the determination of parallax. Indeed it had been announced many years before by Hooke that its parallax had been found; but his observations were not altogether satisfactory, and it was with a view of either confirming them or seeing what was wrong with them that Molyneux and Bradley started their search. They set up a much more delicate piece of apparatus than Hooke had employed.The instrument. It was a telescope 24 feet long pointed upwards to the star, and firmly attached to a large stack of brick chimneys within the house. The telescope was not absolutely fixed, for the lower end could be moved by a screw so as to make it point accurately to the star, and a plumb-line showed how far it was from the vertical when so pointing. Hence if the star changed its position, however slightly, the reading of this screw would show the change.Expected results. Now, before setting out on the observations, the observers knew what to expect if the star had a real parallax; that is to say, they knew that the star would seem to be farthest south in December, farthest north in June, and at intermediate positions in March and September; though they did not know how much farther south it would appear in December than in June—this was exactly the point to be decided.
Fig. 2.
The reason of this will be clear from [Fig. 2]. [Remark, however, that this figure and the corresponding figure 4 do not represent the case of Bradley’s star, γ Draconis: another star has been chosen which simplifies the diagram, though the principle is essentially the same.] Let A B C D represent the earth’s orbit, the earth being at A in June, at B in September, and so on, and let K represent the position of the star on the line D B. Then in March and September it will be seen from the earth in the same direction, namely, D B K; but the directions in which it is seen in June and December, viz. A K and C K, are inclined in opposite ways to this line. The farther away the star is, the less will this inclination or “parallax” be; and the star is actually so far away that the inclination can only be detected with the utmost difficulty: the lines C K and A K are sensibly parallel to D B K. But Bradley did not know this; it was just this point which he was to examine, and he expected the greatest inclination in one direction to be in December. Accordingly when a few observations had been made on December 3, 5, 11, and 12 it was thought that the star had been caught at its most southerly apparent position, and might be expected thereafter to move northwards, if at all.Unexpected results. But when Bradley repeated the observation on December 17, he found to his great surprise that the star was still moving southwards. Here was something quite new and unexpected, and such a keen observer as Bradley was at once on the alert. He soon found that the changes in the position of the star were of a totally unexpected character. Instead of the extreme positions being occupied in June and December, they were occupied in March and September, just midway between these. And the range in position was quite large, about 40″—not a quantity which could have been detected in the days before telescopes, but one which was unmistakable with an instrument of the most moderate measuring capacity.
Tentative explanations.
What, then, was the cause of this quite unforeseen behaviour on the part of the star? The first thought of the observers was that something might be wrong with their instrument, and it was carefully examined, but without result. The next was that the apparent movement was in the plumb-line, the line of reference. If the whole earth, instead of carrying its axis round the sun in a constant direction, were to be executing an oscillation, then all our plumb-lines would oscillate, and when the direction of a star like γ Draconis was compared with that of the plumb-line it would seem to vary, owing actually to the variation in the plumb-line. The earth might have a motion of this kind in two ways, which it will be necessary for us to distinguish, and the adopted names for them are “nutation of the axis” and “variation of latitude” respectively. In the case of nutation the North Pole remains in the same geographical position, but points to a different part of the heavens. The “variation of latitude,” on the other hand, means that the North Pole wanders about on the earth itself. We shall refer to the second phenomenon more particularly in the sixth chapter.
Nutation?