Highlights of Past Exhibits

TELLING TIME BY THE SUN   Telling time by the sun is easy - just use a sundial. Right? Well, yes and no. A sundial will give you the approximate time. But if you want to know clock time accurately you have to do a little work. Read these instructions, gleaned from the METC exhibit Time Made Visible:

A sundial measures apparent solar time according to the position of the real sun. But our clocks display standard time based on an idealized sun. To tell time by a sundial, you must convert apparent solar time to standard time. The shadow cast by the gnomon on the face of the sundial shows apparent solar time, which is 10:30 AM in the picture. Because the longitude of Madison is 0.8° East of the 75th meridian, and because the sun appears to move 15° per hour, apparent solar time here is about 3 minutes ahead of the standard time on our clocks. In addition to this correction for longitude, the sun's irregular day-by-day motion due to the Earth's elliptical orbit around the sun and 23.5° tilt on its axis can be accounted for by the equation of time. This graph shows the equation of time as corrected for Madison, New Jersey at 74.2° W longitude. The picture was taken in late March. From the graph, apparent solar time in Madison on that day was about four minutes behind clock time, so the picture was taken at approximately 10:34 AM. Tom Judd

©2001 Museum of Early Trades & Crafts