You remember how we said in the last lecture that it takes us only eight minutes to get to the Sun on the wings of light, whereas it takes years to get to the nearest star. Another way to think of it is this: suppose our path round the Sun were represented by a wedding-ring, then on the same scale the nearest star would be some miles away. Hence we can regard all the planets and satellites as a tiny little bunch travelling with the Sun, and we can talk of the Sun alone as travelling, carrying us with him in his pocket so to speak. Now how is our Sun behaving in this matter of migrations? Is he one of the stars that pass to and fro through the centre, or does he remain at a distance from it? The answer seems to be, from the best evidence we can get at the moment, that he travels to and fro, and at the present moment he has just been through or near the centre—only about a million years ago. Perhaps you think that is so long ago that we must by this time have come very far away; but a million years is probably only a small fraction of the time for a whole swing, which may be estimated roughly at about 200 million years. And that brings us back to that little table which we made at the beginning of the lecture; we had to leave it incomplete, but now we can complete it.
30 million light vibrations make one wireless vibration.
30 million wireless vibrations make one second.
30 million seconds make one year.
30 million years make one "migration."
I have now put 30 million years for a migration
