~ by Katherine Brooks, Maria Mitchell Association ~
There is a commonly held misconception that the sun attains its highest point during the day at noon. This assertion is wrong (unless of course you happen to be looking at the time from the direct center of a time-zone). The sundial sitting in front of the Vestal Street Observatory at the Maria Mitchell Association was designed to create an accurate reading using mathematics and astronomy.
This sundial is a clock that can read both the date and time using the sun and shadows. Unlike other decorative sundials that may be approximate to the hour, this sundial is exceptionally accurate. Handmade of DuPont Corian and brass, the sundial has a gnomon or a triangular pointer, which is an instrument cut so that the angle makes 41 degrees and 5 minutes above the horizontal. diCurcio designed the sund ial this way to make sure that t he angle is equal to the latitude of the sundial’s location (Nantucket). The sundial includes both hour-lines and date-lines which were established by the height of the triangular gnomon. The gnomon must also sit parallel to the rotational axis of the earth: th e imaginary line passing through both poles and the center of the earth. As the earth rotates on its axis, the sun appears to rotate around the gnomon.
The sun crosses the meridian line (the imaginary north-south arc) at different times. The Earth has been divided up into twenty-four time zones (about 15 degrees of longitude each), causing time zones to differ by one hour from the time zone on either side. The U.S. includes four time zones: Eastern, Central, Mountain, and Pacific. Clearly, Nantucket is within the Eastern timezone which is centered about the 75th line of west longitude. Locations east of longitude 75 degrees see the sun cross the meridian earlier than noon, while the locations on the western side of the line will have the sun cross their meridian later. Nantucket is east of the 75th line of longitude by about 5 degrees and it sees the sun cross the meridian line about 20 minutes earlier than those on the west side of the 75th line. Approximately one degree of longitude equals about 4 minutes of clock-time.
When sundials were the most accurate and abundant tools to measure time, the terms A.M. and P.M. were created. A.M. means ante-meridian (before the meridian) and P.M. meant post meridian, which was meant to describe the sun’s position in the sky. Now, we use A.M. and P.M. to signify a time before or after noon.
diCurcio set up the sundial perfectly level and oriented it so that the meridian-line is exactly north-south using a compass. Once the dial is aligned magnetically, the gnomon is lined up to cast a shadow on the “figure of eights” which shows the time (synched to a correct watch). Installed by diCurcio, the sundial at the Maria Mitchell Association is accurate to this day. Below are the instructions on how to read the Sun d’Isle. We hope you stop by the Maria Mitchell Vestal Street Observatory on Vestal Street to see the 1908 observatory built by the MMA and to test out the Sun d’Isle on the lawn in front. If reading shadows interests you, we encourage you to attend our Summer Speaker Series this Tuesday, August 30, to learn about the “Effects of Light at Night” with Dr. Mario E. Motta.
TO READ CLOCK TIME: Read the tip: the point of the triangular shadow. When the tip just touches a “Figure of Eight,” then the sundial indicates clock time ON-THE-HOUR! But: take note of the month! The little figure-of-eight at the bottom tells where to read! And: daylight saving time? If “YES,” use the ROMAN NUMERALS top circle.
TO READ THE DATE: Please read the tip: the point of the triangular shadow. When the tip just touches one of the curves running from side-to-side, read the DATE shown at the end of the curve.
The MMA was founded in 1902 to preserve the legacy of Maria Mitchell and to promote her belief in learning-by-doing. An astronomer and natural scientist, as well as an educator, Maria Mitchell shot to worldwide fame when she discovered a comet in 1847. For her discovery, she was awarded a gold medal from the King of Denmark – the first American and first woman to receive the honor. She served as the Professor of Astronomy and Mathematics at Vassar College from 1865 until 1888. Today, the MMA operates two observatories, a natural science museum, an aquarium, and the birthplace of Maria Mitchell. The MMA conducts scientific research, leads classes and workshops for people of all ages year-round, and welcomes thousands of visitors to its museums and observatories. For details, visit mariamitchell.org.
