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| 30 | “scientia„ |
STAR-STREAMS
At most of the principal observatories of the world, at least one telescope is devoted continually night after night to the task of measuring the positions of stars on the celestial sphere. At the rate of from ten to twenty stars an hour for each instrument, this work of determining and redetermining the positions of the stars proceeds at stations distributed all over the world. After long reductions, the labour of which far exceeds the labour of the observations themselves, the results are embodied in the star catalogues of the various observatories. Finally from the comparison of observations separated by long intervals of time, data have been collected showing the minute changes of position of the stars among themselves. It is these data, so laboriously obtained, which form the foundation of the theories and deductions here described.
In 1718 it was first shown definitely by Halley that motions and displacements were occurring among the so-called « fixed stars »; and from that time the study of these motions has been one of the principal tasks of sidereal astronomy. I suppose that something like half a million stars are bright enough to be well within the reach of the ordinary meridian instruments. But at present it is only for samples of these that information as to the motions is available. Modern observations are abundant, though by no means exhaustive; but for the majority of stars the changes of position are so minute that it is necessary, before they can be detected, to extend the observations over a very long period.
