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WHY THE SUNDIAL KEEPS TIME - Page 7It is the earth turning upon its axis that gives us our darkness and light. Hence, it is obvious that a sundial in Boston would not show the same time as one in New York if both sundials were read simultaneously. This is shown in Figure 12, where the real motion of the earth and the apparent motion of the sun are indicated. The two sundials mentioned would not show the same time, owing to the fact that when the sun is directly over Boston the sundial will read twelve o'clock. It will therefore take some time for the earth to turn sufficiently upon its axis so that the sun is directly over New York, at which time the New York sundial will read twelve o'clock and the Boston one will record some few minutes after twelve. Perhaps a few who read this will recall the days when watches acted the same way. That is, each locality had its own time called local mean time. Consequently if a New Yorker set his watch at twelve noon and traveled to Boston, he would find that his watch did not agree with that of a Bostonian by some twelve minutes. The Bostonian's watch would be twelve minutes faster, because he set his, say, when it was noon in Boston. Since Boston is east of New York, the sun will arrive overhead in Boston before it does in New York. The difference in time between the two watches is equal to the time it takes the earth to turn from that position where the sun is overhead in Boston to a similar position in New York as shown in Figure 12. Or stated in another way when it is noon at any given place it is noon at all other places on the same meridian (having the same longitude) ; and in places having different meridians it is forenoon if they are west and afternoon if they are east of the given place.
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