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18 Sir Robert Ball (who several times gave these Christmas lectures) had mentioned this fact that the Sun occupies one of the foci of the ellipse, he was earnestly implored by a lady after the lecture to tell her which of the foci was the one. I am afraid I should have tried to explain that it did not really matter, and the explanation might have taken a long time. But Sir Robert was cleverer than that: raising his right hand, he said, "Madam! the right

one" (which of course was right in any case), and the lady thanked him warmly and went away quite satisfied. But whichever focus we choose it will be nearer one end of the ellipse than the other. Now Kepler's second law tells us that the planet moves quicker when it is near the Sun than when far away. He put it into more precise form, but that will be enough for us to remember—that when a planet or comet is near the Sun, it bustles along at a great pace, when it is far away it moves more soberly,