My Sundial

At work last year, the question arose of when, and on what days, the sun would be above a certain elevation in order to limit the length of shadows on aerial photography. In answering this question, I realized I had all the tools to make an accurate sundial, and so I designed the one you see here, which would be useable near St. Louis, MO.

Most sundials have a rod as the gnomon, or shadow caster, and have the hours marked as lines on the face of the dial. The time given by these dials can only be approximate, because the direction of the shadow varies at a given time of day throughout the year. The reason for this is the earth's varying speed through its orbit, as a function of its distance from the sun. Although the earth is rotating at a constant rate relative to the stars, the earth advances a little differently each day along its orbit, and so does the sun in moving along the ecliptic. The obliquity of the ecliptic with respect to the celestial equator also complicates the apparent motion of the sun. Anyway, twenty four hours is only the length of the mean solar day--the time between solar noons actually varies throughout the year. Sometimes the sun lags behind mean solar time, sometimes it is ahead of it.

Another, more noticeable, seasonal movement of the sun is its change in north/south position, or declination, with the seasons. The shadow cast by an object is shorter in the summer than it is in the winter, because the sun is higher in the sky in the summer. Together, these two motions cause the shadow of a point to trace out an assymmetric figure eight over the course of a year.

Finally, although the sundial can keep solar time, we operate by civil time. These two times are most in agreement at the longitudes corresponding to whole hour angles. These meridians are near the center of each time zone. Any distance from the meridian will tend to skew the each figure eight. The distance that Chicago is east of the 90th meridian causes the large three o'clock lobe. The lobes will have different sizes and orientations for each location.

On my sundials, I trace figures for hours between 9 am and 3 pm. The shadow cast by the point of the gnomon tells the time. The gnomon is wedge-shaped, as tall as one of the legs of the triangle shown in the picture. The wedge meets the face at the base of the triangle.