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294 instead of 200, firstly to make the table simpler by having all the figures the same, and secondly to remind us that when I said 200 millions just now it was little more than a guess. It was the best guess I could make, but it may be wildly wrong; perhaps 30 million will do just as well, and it has the advantage of making our table simple.

We on our Earth, then, are accompanying the Sun in these migrations—we are all making this great, not metaphorically by telescope, but actually and in reality. We are doing it in a leisurely manner, perhaps you will think, seeing that it takes so many million years to make even one "migration": we cannot, even the youngest of us, hope to see the end of a turn, or even to see the scenery change much: it takes us all our time and attention to detect that we are moving at all. Have you read how Mark Twain once boarded a glacier to travel downhill? He had learnt that a glacier was really a moving river of ice, and solemnly pretended to use it for a journey, and to "be astonished and bitterly disappointed when he learnt that it would take years to go a few yards. I hope you will not form any similar expectations about our journey. We are certainly making it, but as regards change of scenery in our short lives it is slower even than a glacier.

And I have used the word "migration" to denote the time taken by the Sun to swing from one end of its tether to the other. It seems possible that all of the stars in our cluster take about the same time to make their "migrations," just as the billiard balls did when hung by strings of the same