Sun Dial: Scientific

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Site Contents

by Ernest Thompson Seton 

The spacing can best be done by actually registering the course of the sun's shadow during the entire day, making sure that the pointer is constantly pointed to the north.

More scientific ways of ascertaining the angles between the lines may be used if desired; for instance, such as are given on page 81 of Professor Jacoby's Astronomy or in the Encyclopedia Britannica.

However, there is no necessity for those living in the latitude of New York City to use any of the above methods, as the following angles have been worked out and tested and will be found to be approximately correct:

The angle between hrs. 6 and 7 P. M. (likewise between 5 and 6 A.M.) is also 22 1/2 degrees, as the night hour-lines are prolongations of the day hour-lines.

For boys and girls who might have difficulty in determining angles by degrees, the following thumb rule for the locality of New York City will be found useful:

Have the quarter circle from 12 to 6 o'clock divided into six unequal parts, the smallest sector being next to 12 o'clock and the other sectors uniformly increasing in size so that the largest is next to 6 o'clock. The smallest angle should be 1/9 of the quarter circle, and the largest angle should be about 1/4 the quarter circle.

Bear in mind that the entire dial must be symmetrical in two ways; that is, the A. M. and P. M. halves of the dial are symmetrical, also the day-time and night-time halves are symmetrical (insofar as the sun shores before 6 A. M. and after 6 P. M.). This double symmetry is true in any latitude or longitude; hence the importance of affixing the pointer at the center of the dial, its upper edge meeting the surface of the dial on the straight line extending across from 6 A. M. to 6 P. M.

When the dial has been constructed by any of the above methods, put it in the sun and do not be surprised if it does not register the same time as common clock time. It will probably be a few minutes ahead or behind it even if accurately made, as for astronomical reasons too complex for explanation here the sun is irregular in its apparent movements. To ascertain our railroad time there would have to be convenient to hand the following memorandum:

Dec. 23d Dial Correct. 
Feb. 15th Dial Slow 15 minutes. 
April 15th Dial Correct. 
May 15th Dial Fast 4 minutes. 
June 15th Dial Correct.
Aug. 1st Dial Slow 6 minutes.
Sept. 1st Dial Correct. 
Nov. 1st Dial Fast 16 minutes.

A further allowance would have to be made if the location does not happen to be on the standard meridian from which clock or railroad time is taken. For instance, New York City lies one degree east of the standard meridian for Eastern time, hence a further adjustment would have to be made due to the fact that the sun is thus 4 minutes faster than it would be on the exact meridian. The variation of any other locality can be calculated similarly, allowing 4 minutes fast for each degree the place lies east of its standard meridian; similarly the dial will be slow if its locality is west of its meridian.

If the hour-lines are originally laid out on the dial by the practical process of noting the movement of the sun's shadow hour by hour, then it is preferable either that this be done at one of the times during the year when sun-time and clock time are approximately identical or that proper allowance computed from the above table be made; otherwise it is easy to see that a variation from clock time rising at times to a maximum of half an hour might result.

Another point which is added for the sake of completeness rather than its importance, is the variation between morning and afternoon time caused by the width of the sun itself, as it covers half a degree in the sky. The light from preceding edge of the sun is in the morning a minute ahead of what it would be were the sun but a point in the sky. Likewise the light lags a minute behind in the afternoon. These trivial variations can of course be disregarded.

Caution: Do not attempt to locate the north by following rally a magnetic needle. In New York City the needle points ten degrees to the west of the real north. Moreover, a compass is apt to be affected by surrounding iron or steel objects such as a steam radiator. The north pole is a much more accurate guide although it at times varies a degree from the true north. For the information of those living in New York City it is convenient to know that the up and down avenues of the city (such as Fifth Avenue and Amsterdam Avenue) point east of north at an angle of about 30 degrees. 

It is better to have the pointer as thin as possible, as if has any substantial thickness the eastern and western halves of the dial markings will have to be separated by that width. This is because the sun registers with one edge in the morning and the other edge in the afternoon. Also do not forget that it is always the side of the shadow cast by the upper side of the pointer which is to be regarded in noting the time.

Sun dials are an ornament to any garden. New York residents can find one quietly at work in the flower gardens at Van Cortlandt Park. Chronic pessimists would do well to ponder the inscription on King Edward's sun dial:

"Let others tell of rains and showers, 
I only record the sunny hours."

CLARKE G. DAILEY.

To make a more scientifically accurate Sundial, see Collins Book of the Stars.

See Also:

Simple Indian Clock, Shadow Clock, or Sundial 

The Birch Bark Roll