130 North Wells Street, Chicago, Ill.
In the first place, you see three broad, concentric circles, on the outside of which the rising and setting sun is depicted for both midsummer and midwinter day. The figures, 30°–50°, alongside of the sun represent degrees of north latitude, wherever you may happen to live, which, with the exception of most of Florida and southern Texas, cover the United States. The short arrows show the direction of the sun’s rays at sunrise and sunset.
Garden Side of Residence, Westbury, L. I.
Peabody, Wilson & Brown, Architects
The inner circle represents your horizon, and the degrees marked upon it show the points of sunrise and sunset, north or south of the direct east and west line. These angular distances, in terms of degrees, are called amplitudes, north or south, and must not be confused with the degrees of latitude on the earth’s surface, indicated by the numbers along side of the sun, though intimately dependent upon them. The amplitude of the horizon point, where the sun rises and sets from time to time during the year, always depends upon the latitude on the earth’s surface where you happen to live, as may be seen by following with your eye the direction of the arrows of latitude through the amplitude circle. Starting from the number indicating the latitude where you live, trace the arrow until it touches the amplitude circle. You can then read the degree on it which shows how far north or south of the east and west line the sun rises or sets. We are indebted to Professor Philip Fox, of the Dearborn Astronomical Observatory at Evanston, Illinois, for determining these points.
The two outer circles are sun-dials for midsummer and midwinter day at the 40th degree of north latitude; and, if you imagined them pivoted on their rising and setting points and tipped up from the south to represent the slanting path of the sun during the day, they show the direction from which the sun is shining during successive hours of the day (or night on the other side of the world). The shaded portions of these circles represent night, which for all northern latitudes is short in summer and long in winter, as the day is short in winter and long in summer. If you examine the hour spaces on the winter dial of your winter night, you will find them exactly like those on the summer dial of your summer day. So also your winter day hours are spaced like your summer night hours. South of the equator, people have precisely the same experiences only in the reverse order. New Zealanders, we fancy, wear straw hats in January and fur caps in July. If you liked summer well enough and cared to move, you could live in a perpetual summer on our little globe. It is probable, however, that, like most people, you rather prefer the change of seasons, in spite of occasional extremes.
The irregular hour spaces on the dials would make it appear that the sun moves around the earth in a sort of jerky way. On the contrary, it moves, or rather the earth rotates, at an absolutely uniform rate, but the tilt of the sun’s path to your horizon line gives you at certain points a fore-shortened view by which the sun seems to cover various distances at various stages of its course.
The sun-dial time is used on our chart as it more universally applies at different meridians on a given latitude, than does our standard time which, for the sake of timepiece uniformity, is a compromise based on mean solar time. As the earth’s orbit is such as to make the sun gain a little or lose a little in crossing the meridian each day during the year, the total annual time of the sun is averaged into uniform daily periods, which in turn are divided into 24 uniform hours and referred to certain meridians the world round, about an hour apart. To show the exact position of the sun in strict agreement with these standardized hours would require a special chart for every degree of longitude, and then be of no especial value for our present purpose; for while the astronomer must have exact time to the fraction of a second, the differences between watch and sun-dial are not enough at any time to affect essentially our problem. So that, if you do not find the hours marked on our summer and winter sun-dials, at any time, exactly agreeing with your watch, you may, nevertheless, trust them to show you accurately enough from what direction the sun is shining at different hours of the day.
Residence, St. Paul, Minnesota
James Alan MacLeod, Architect