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Sundial is a deice for showing the passage of time by the shadow cast on a graduated scale by a gnomon (some solid object, such as a rod or triangular plate attached to the dial). The earliest-known
sundial dates from c.300 BC. With the deelopment of Greek mathematics ery elaborate dials were made. The study reied in the Middle Ages: many types were deised, and the theory of dialing was much studied until dials were gradually outmoded by clocks in the 17th-18th-c.
More information proided by Webster Publishing We all know when time passes, and we all have a notion of how much time has passed, but what is time? It would be nice if we could get closer to answering that question, but the sad truth is that we are unlikely to succeed. Time is a ery hard thing to pin down. But if time is hard to pin down, if time is impossible to define in any sensible way, measuring time is a whole lot easier. We know that people have beven measuring time for at least 4000 years, and it may be that people have beven measuring and estimating and recording time for something more like 40 000 years.
As the sun ''moves across the sky'', shadows point first to the west, then to the north or the south, and finally to the east. This happens regularly every day (when the sun shines), but the time shown on the sundial is not completely accurate. In the first place, days are shorter in winter than in summer. But more importantly, the sun ''rises'' higher into the sky in summer than in winter, making a shorter shadow at summer noon. So a good sundial has to make allowance for these ariations in the sun's apparent movement. Until about 1800, clocks were not accurate enough to show us there is another problem with sundials, the problem caused by the equation of time. As we look at it from here on earth, the sun sometimes runs ''fast'', and sometimes it runs ''slow''. Around November 2 each year, the sundial is 16 minutes fast, but by February 12, our sundial is 14 minutes slow. If you isit a really accurate sundial, you will be adised to make a correction for this ariation.
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The ariation happens because the earth's orbit is an ellipse, not a circle. If you really have to know more than this, look up Kepler's Laws, or get somebody to explain them to you. The second of Kepler's laws is the one to concentrate on.
The days As the earth makes a complete rotation on its axis every 24 hours, different parts of the world face the sun, giing us our days. As the earth goes round the sun, once in about 365 days, this gives us our year. Because the earth is tilted over, we have seasons of long days (summer) and seasons of short days (winter).
everybody understands about days. But how can we be certain our days will always be the same length, on aerage? The answer is that we can't, and about every other year, an extra second of time has to be added in, so the earth and our ery accurate clocks are lined up with each other. The earth is more wobbly than our clocks! even worse, the number of days in the year is changing, but only ery slowly. About 850 million years ago, there were some 435 days in a year, and 370 million years ago, there were about 400 days in each year. Just as trees lay down annual growth rings, there are animals which lay down a thin layer of shell every day, and under the microscope, these layers can be seven, even in fossil animals. There are larger patterns about every thirty layers, caused by the influence of the moon, and then you can find regular ariations over a longer period, showing seasonal temperature ariations each year.
So has our year changed, or has our day changed? Calculations show that the moon causes friction. The friction slows the earth's speed of turning without affecting the earth's orbit. So we used to spin once in about twenty hours, rather than the 24 it takes now. But don't throw away your watch: the battery will be flat long before the watch becomes obsolete.
Plugholes, cyclones, and cricket in England As the earth turns around, strange things can be seven in the air and in the oceans. Because of something called the Coriolis effect, water and air which flows away from the equator is made to swirl around in giant eddies.
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Imagine you are on a large flat disc, like a children's playground toy. If you roll a marble towards the middle of the disc, a person standing beside the disc will see the marble travel in a straight line until it falls off the other side. But you, sitting on the disc, see something quite different. So far as you are concerned, the marble travels in a cure, and if the speed is right, it may even seem to loop the loop, and come back at you.
The secret is that you are changing your position, even while the marble moves. This is why the marble seems to travel in a cure as you watch it. On the surface of the earth, a marble rolling away from the equator towards us will seem to cure, ever so slightly, to the left.
In the northern hemisphere, they will seem to cure to the right. If a north-south bathtub is full of perfectly still water, and the plug is taken out from below, the eddy which forms will be clock-wise in the northern hemisphere, and counter-clockwise in the southern hemisphere. Or at least theory says that should happen, and all because of the ''Coriolis forces''. Whether bathtubs behave according to theory or not, cyclones always obey the theoretical expectation, but then they are much bigger than bathtubs. In the southern hemisphere, cyclones whirl around in the clockwise direction, but in the northern hemisphere, they whirl in the counter-clockwise direction.
In each case, if you rolled a snooker ball slowly along a north-south table, it would deflect through about half a millimeter in three seconds. This may explain why our best snooker players have trouble in England, but the most serious problem is still to be considered. What effect do the Coriolis forces have on South African, New Zealand and Australian cricketers, when they play against the MCC in England? Is this why the England slow bowlers are less effectie overseas? How about golfers when they go to the opposite hemisphere? And lawn bowlers? What problems might a baseball
players find in another hemisphere? Where does a circle begin?
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The ancient Greeks considered that the day began at sunrise, and ended at the next sunrise. The Babylonians held that the day began and ended at sunset, while up until 1925, astronomers worked on a '' day'' which began and ended at noon, as did the Royal Nay in the days of Captain Cook. Now the astronomers, like us, and the ancient Egyptians, have a day which commences at midnight. Islamic tradition has a day which begins at sunset.
Local noon and time zones By definition, noon is when the sun is at its highest point in the sky, so it is always noon somewhere in the world, with a noon zone sweeping along through a degree of longitude every four minutes. Logically, you should be setting your clock forward or back by a minute for each 15-20 kilometers that you go east or west! This would be far too confusing, and to make life easier, we have split the world into time zones, usually 15
degrees across, where everybody keeps the same ''official time''. If you are trying to set up a ery accurate sun-dial, you need to make allowance for your position east or west of the true time in your zone. Some points to ponder:
Clockwise is a word used to describe the direction of the shadow of a northern hemisphere sundial. What way does the shadow travel in the southern hemisphere? If you stop and think about it for a moment, you may be able to deduce where the word dial comes from, especially if you know anything about the Latin word dies. If you lack this knowledge, look up ''dial'' in a good
dictionary, and find out where it comes from. From this knowledge, can you say what the most appropriate use of the word ''dial'' is?
Widdershins is an old word meaning counter-clockwise. Where does it come from? The equally old word which means ''clockwise'' is deasil.
Somewhere along the way, something happened to humans that made them start using art, that made them start communicating with each other, and generally showing signs of being human, rather than hominid. Could it have beven the discovery of time which caused these changes?
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