IN these days of accurate clocks and watches no one thinks of using a sun dial to tell the time of day. At best such an instrument can be right but four times a year, and in many places it can never agree with standard time. Yet the demand for sun dials was probably never greater than it is now, merely because, as an ornament, it adds a quaint and picturesque touch to the lawn or garden, or else it harmonizes well with the architectural treatment of a building. As standard time is.obtained primarily from the sun, with due allowances for its variations from noon to noon, there is no reason why a sun dial should not give us accurate time if we make similar allow ances. A novel sun dial in which such regulation can be made is illustrated on this page. This dial differs radically from the common type. Instead of showing the time of day by a shadow cast by the sun, it employs a spot of direct sunlight which' is brought to bear upon a hair line on a white screen. The dial of the instrument is tilted according to the latitude, so that it wiII lie parallel to the plane of the celestial equator. The photograph shows the instrument set for the latitude of New York. The base must be oriented accur1tely, and the dial face tipped to the latitude of New York (which is about 41 degrees), the upper angle being found by means of the graduated sector on which the dial face is supported. The dial proper is divided into equal divisions to indicate the hours of the day, and opposite the 6 A. M. and P . M. maIks are two upright brackets, one of which serves as a screen (at the left in the illustration), while the other contains the perforations through which the sun shines, making a spot of light that falls on the screen. As the face of the dial is parallel with the celestial equator, its edge forms an arUfcial horizon, along which the sun apparently travels. Because the sun mounts above the celestial equator in summer time, and falls below it in winter, two holes are formed in the bracket at the rigbt, one above tho SUN-DIAL THAT GIVES CORRECT STANDARD TIME other, so that the spot of light will be sure to strile the screen through one or the other of these holes at any time of the year. In order to allow for variations of the sun, the bracket containing the two apertures is made adjustable in the following manner: On the face of the dial will be noticed a smaller dial marked with the months of the year, while adjacent to it is a. scale divided into thirty-one equal parts', representing the days of the month. If the reading is to be taken on, say, February 6th (as shown in the photograph), this small months' dial must be moved to bring the February mark in line with the sixth mark on the scale. An elliptical cam is connected to the months' dial, and engages an arm to which the bracket is connected, thereby shifting the bracket laterally, according to the position of the dial. In February, the sun time is nearly a quarter of an hour slow, consequently the bracket is moved upward or eastward a corresponding amount. This done, the main dial is turned until the spot of light passing through the lower Ulperture is bisected by the hair line of the screen. The reading of the time is then taken upon a small scale shown at the right in the engraving. This is divided to indicate minutes, and in our illustration the time is shown to be 4: 20. When the instrument is first set up the imalinutes scale must be adjusted so as to harmonize mean local time with stapdard time. As the sun passes over New York (on the 74th meridian) four minutes before it reaches the 75th meridian, on which New York standard time is based. the minutes sClle must be set to read four minutes slow in order that the time as shown by the dial will agree with standard time. We who live in regions where there are plenty of clocks and timepieces of precision hardly realize th< practical value of such a dial. In sparsely settled localities, for instance, on remote plantations, an instrument that will give correct time to the minute, obtainin g this time directly from the sun, should be almost indispensable.
This article was originally published with the title "A Sun-dial as an Accurate Time-piece" in Scientific American 105, 5, 97 (July 1911)