I was fortunate enough last week to be taken on a tour by Bill Harvey of some of the engineering sights of western England. I’ll be putting up a few of the photos I took between now and the end of next week…starting with the picture above, of the front of the Corn Exchange in Bristol, a nicely preserved mid-1700s building. The clock came much later – it was installed in 1822.
If you click on the picture to enlarge it, you can just make out two minute hands, one red and one black. Unfortunately we were there at the wrong time to get a clear picture, as the red minute hand is almost parallel to the hour hand. (The red minute hand runs about ten minutes ahead of the black minute hand.) This is a good example of the way modernity, in the form of transportation technology, can challenge our basic assumptions.
Anyone who knows the earth is round, and fortunately that’s pretty much everyone today, can understand that the stated time will be different in different places. The fact that the sun will appear to be in different locations in the sky based on where you are was used to measure the earth’s circumference over 2200 years ago. But traditional forms of transportation were too slow, and ancient forms of time measurement too crude, for the differences in time from one place to another to be seen. The introduction of steam trains changed this. It became possible to travel hundreds of miles reasonably quickly, and train companies had fixed schedules that made time-keeping central to their operations.
Bristol is roughly 120 miles west of London, and both cities are at approximately 51 degrees north latitude. London is at 0°7′ west longitude, and Bristol is at 2°35′ west. The difference, 2°28′ is 0.69% of a full circle. Since the full circle is equivalent to a day, the difference is 0.69% of a day or just about ten minutes. London gets to a given time that ten minutes or so ahead of Bristol. So the red minute hand is telling people in Bristol the real local time in London and the black minute hand is telling local Bristol time. Since UK train timetables were based on London time, the red minute hand, which had the wrong time for Bristol, was the way to know when trains would actually leave.
There’s an easy, if non-intuitive, solution to this problem: time zones. If we standardize time in 24 strips of the globe (each 15°, or 1/24 of a full circle, wide and therefore representing one hour) then local time is, by definition, the same within each strip. We then have a one-hour jump at each boundary between time zones, but that’s easier to deal with than every single city and town having its own time.
Time zones are incredibly artificial – we are ignoring the sun, which is where our measurement of time began, in favor of a construct that we know to be wrong – but useful given that we live in a world of fast transportation and instantaneous (or, given the speed of light, close enough to instantaneous) communication. Without time zones, there would be a lot more clocks with multiple minutes hands, or the digital equivalent of them.
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