map of eastern united states |
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Is it possible for a garden sundial to tell perfect time? The chances are very good if you can make one yourself, or get one custom made for your location. But both these alternatives will take either time or money. The reason for this is that apart from adjustments for time zones and daylight saving , covered in another article, there are three things that must be done. Compensating for the Earth's Path Around the Sun If the earth followed a perfect circle as it revolved around the sun, life with sundials would be easy. But its path is elliptical or oval, and this causes errors of up to 16 minutes in sundial time at some times of the year. The corrections are straightforward, and can be made using a table, or from a figure 8 line called the Analemma, often seen on old globes and sundials. Correcting for Latitude Let's consider the components of a sundial for a moment. It consists of a dial on which the time divisions, and sometimes other information is marked, and a triangular piece which sits vertically on the dial. It is called the gnomon, pronounced nomon, and the part which casts the shadow onto the dial is called the style. For more information on how sundials work, including how to design your own, go to Google and type "how + sundial", without the quotes but with the +, in the search box. To be accurate, the angle between the triangular part of the gnomon and the horizontal must be the same as the latitude of the place it is to be placed in (You can find the latitude - and longitude - of your home from any topographic map or good atlas).The arrangement and distance between the hour markings on the dial must also be correct for the latitude. Hmmmm! This means that unless you are very lucky, that elegant sundial in your local garden supply shop will probably not show the time particularly well. It may be calibrated for an average latitude (commonly 45 degrees), which is good if your latitude is not too different. Or it may be purely ornamental and will really only be useful around noon. Now of course this doesn't matter at all if you are looking for something pleasing to the eye, and don't mind answering the inevitable question "Does it tell the time?" But if you'd like your sundial to be more useful, make sure you find out which latitude it is calibrated to. Once you know this, all you need to do to compensate is to work out the difference, and tilt the dial towards or away from due south depending on whether you need to add to or subtract from the latitude the sundial was designed for. There may be slight differences to the ideal spacing of the hour marks, but the apparent time will be reasonably close. Finding North The final essential in sundial installation is to make sure the gnomon is oriented north-south. Sounds easy and, with a little patience, it is. One way, suitable for the northern hemisphere, is to identify the pole star. This is very close to the projected position of the earth's axis, about which the sun and stars seem to revolve. You could mark the direction from your sundial's location to the pole star, but this method isn't quite accurate, and needs to be done in the dark. And the southern hemisphere doesn't have a pole star. Method 2 uses a compass. Sounds pretty simple, doesn't it. But you guessed it - there are some complications. Firstly, the needle on a compass points to magnetic north, not true north, which is what we want. The difference between the two is called the magnetic declination, and is usually shown on good topographic maps. And while a simple addition or subtraction of the difference between the two norths should give you the right direction, there may be some local magnetic effects which can't be compensated for. The third method goes back to the ancients - and there were some pretty smart operators around in the old days. You'll need a stick, some paper or board, a marker, a tape measure or long rule, a sunny day, and a bit of time on your hands. Set the stick up vertically at the location you have chosen for your sundial, so that the top of its shadow falls on the sheet of paper or board. If you stand with your back to the sun, behind the pole, set the paper up so that the morning shadow falls on its left hand side. Now mark the end of the shadow with a permanent marker. Come back through the day and mark the new positions of the tip of the shadow - the more often the better. As the day goes on, you'll notice the marks form a curve. Later in the afternoon - any time after three is OK - connect the marks you've made into the smoothest curve you can manage. Do this while the pole and pap ... Author: Graham McClung |
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