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Earth travels round the Sun; hence we can sometimes see the emergence and sometimes Jupiter blocks the way. You see the two kinds of blocking: he blocks the sunlight in one case, to make the eclipse; but he may also block the view from the Earth, if the Earth lies nearly in the same direction as the Sun.

There are two different cases in which the Earth may lie nearly in this direction; it may be on the near side of the Sun or on the further side; and because of the difference between the two cases a very important discovery was made, viz. that light takes time to travel. We know that sound takes time to travel because of echoes; we shout "Jack!" and after 'a perceptible interval a faint shout "Jack!" comes back to us from a distant wall or hill. If we measure the interval between shout and echo, and also measure the distance of the wall from us, we can find how quickly sound travels—about 1100 feet per second. Perhaps you have recognized this fact in another way; when there is a thunderstorm, we first see the lightning and we hear the thunder later. If we count the number of seconds between the flash and the first clap of thunder, and multiply by 1100 we can find how many feet away the flash took place. (As regards the rest of the thunder I suppose it is made up of echoes, partly at any rate.)

Now, has it ever occurred to you that the strike of a church clock never gives you the exact time unless you are close to it? If you are 1100 feet away you will not hear the strike until a whole second after it has occurred; if you are a mile away, you will hear