Fig. 4—12-Inch Modern Horizontal Sundial for Latitude 40°-43´
Fig. 5—The Earth, Showing Relation of Dial Styles to Axis
The reason that the table given here is convenient for setting clocks to mean time is that a minute is as close as a dial can be read, but if you wish for greater accuracy, then the almanac, which gives the “equation of time” to a second for each day, will be better. The reason that these noon-mark dials are better than ordinary commercial dials is that they are larger, and still further, noon is the only time that any dial is accurate to sun time. This is because the sun's rays are “refracted” in a variable manner by our atmosphere, but at noon this refraction takes place on a north and south line, and as that is our noon-mark line the dial reads correctly. So, for setting clocks, the corner of your house is far ahead of the most pretentious and expensive dial. In [Fig. 4] is shown a modern horizontal dial without the usual confusing “ornamentation,” and in [Fig. 5] it is shown set up on the latitude of New York City for which it is calculated. This shows clearly why the edge FG of the style which casts the shadow must be parallel to the earth's axis and why a horizontal dial must be made for the latitude of the place where it is set up. [Figure 6] is the same dial only the lines are laid out on a square dial plate, and it will give your young scientific readers a hint of how to set up a dial in the garden. In setting up a horizontal dial, consider only noon and set the style, or 12 o'clock line, north and south as described above for noon-mark dials.
Fig. 6—Modern Sundial Set Up in Garden