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58 to be less than usual. Such an occasion was the Transit of Venus. Perhaps my present audience has not heard so much about the Transit of Venus as their parents and grandparents did, because the last Transits occurred in 1874 and 1882, long before you were born: and the next will happen in 2004 and 2012, when all of you will be a good deal older. But the echo of the great sensations may not yet have died down, and so you may have heard of a Transit of Venus and wondered what it was.

What concerns us is that it is simply an occasion when the difficulties of measuring the Sun's distance are less than usual. The base remains the same, because we cannot get away from the Earth: the best we can do is to put our eyes or telescopes on opposite sides of it, and indeed in all parts of it. Astronomers were scattered for the Transits of Venus to such places as the Sandwich Islands, New Zealand, and Australia, Mexico, Kerguelen Island (a very desolate spot), Egypt, and so on: and they watched Venus cross (or "transit") the Sun, and noticed the exact moment when the transit began and ended. You will scarcely want me to explain fully how this told them the Sun's distance; but I think you can see in a general way how they worked it out if you