396 



AMERICAN HOMES AND GARDENS 



October, 19 to 



A sun dial on a plain wooden column 



variation from day to day is but slight. However, it 

 would confuse us, greatly, to have to set our watches so 

 that they would run slower at certain times of the year 

 than at others. Therefore, astronomers have theoretically 

 constructed an ideal sun which keeps perfect time, and 

 when this sun crosses our meridian, we call it 12 o'clock 

 local mean solar time. In the middle of February, and at 

 the end of October of each year, the real sun's time, which 

 astronomers call apparent solar time, varies about fifteen 

 minutes from mean solar time, but on four days of the 

 year, the sun is on time and agrees with the theoretical 

 or mean sun. These days are the 15th of April, the 15th 

 of June, the ist of September, and the 25th of December. 



Before we can use our watch to lay out the sun dial, 

 there is still one more complication that must be considered. 

 New York lies on the seventy-fourth meridian west of 

 Greenwich. When the mean sun crosses this meridian 

 it is not 12 o'clock by our watches, but four minutes of 12, 

 because standard time in eastern United States is taken 

 from the seventy-fifth meridian, and it takes the sun four 

 minutes to pass from the seventy-fourth to the seventy-fifth 

 meridian. If a town were situated on the seventy-eighth 

 meridian, for instance, the clocks of that town, if correct, 

 would point to twelve minutes after 12, when the mean 

 sun crossed the meridian of that town. 



Bearing this fact in mind, one can lay out the dial face 

 of a sun dial by means of a good watch on the four days 

 of the year mentioned, when apparent solar time and mean 

 solar time agree, by setting his watch fast or slow so that 

 it will indicate local time rather than solar time. The Cen- 

 tral States take their time from the ninetieth meridian, the 

 Rocky Mountain States from the one hundred and fifth 

 meridian, and the Pacific States from the one hundredth 

 and twentieth meridian. On Christmas day, if a man in 

 New York sets his watch four minutes slow, he can mark 

 the shadow cast by the gnomon on his sun dial face at the 

 end of each hour, as indicated by the watch, and there- 

 after the sun dial will give him correct apparent solar time. 



If the day should prove cloudy, he would have to wait until 

 the middle of April for the next favorable opportunity. 

 Of course, by consulting a good almanac, he could set his 

 watch to give apparent solar time on any day of the 

 year. 



Fortunately, there is a very simple way of laying out a 

 sun dial without the aid of a timepiece, which does not in- 

 volve any mathematics, and which requires only a slight 

 knowledge of drawing; and after all this method will prob- 

 ably be found simpler than that of using the watch, particu- 

 larly as it does not depend upon good weather and certainly 

 is not an all day task. 



The accompanying diagram. Figure i, illustrates the 

 method as applied to a square dial face. The gnomon is 

 indicated as rising from the center of the dial face at 

 O and extending to the upper edge at A, where the mark 

 for XII o'clock should be placed. On all dials, two lines 

 should be drawn for XII o'clock, spaced apart a distance 

 equal to the thickness of the gnomon. If the dial is to be 

 used in latitude 40 degrees north, the gnomon should, of 

 course, point to the north and rise at an angle of 40 degrees 

 from the face of the dial, as explained above. Taking the 

 afternoon hours to start with, draw a line OB at an angle 

 of 40 degrees to the right-hand side OA of the gnomon. 

 From A as a center, draw an arc tangent to OB from C 

 to E, where it meets an extension of the line OA. Extend 

 the line AB, which is the northern edge of the dial face, 

 and at right angles to OA, to H, and draw a line EG 

 parallel to AH. Then with E as a center, draw the arc 

 AFG. Divide this arc into six equal parts and through 

 the points thus found, draw radial lines, extending them 

 until they intersect the line AH. The points of intersection 

 give us the positions of the hour marks of the dial, and 

 lines from the center O should be drawn to these points of 

 intersection and marked I, II, III, IIII and V. The line 

 for VI o'clock will be parallel to the line AH. The hours 

 of VII and VIII may be found by drawing lines below the 



Sun dial pedestal in the form of an urn 



