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The Sundials of Mystic Seaport

Mystic Seaport is "The Museum of America and the Sea." Established in 1929, it's the largest maritime museum in the United States. Four of its exhibits are National Historic Landmarks. The museum has earned worldwide renown for maritime preservation, restoration, research, and education. With a little imagination, the visitor feels transported to a 19th-century fishing town, strolling among period buildings or boarding the only surviving whaler in the U.S., Charles W. Morgan. You can train on the Joseph Conrad (seen here), still an active sailing vessel after more than 130 years. So how do sundials fit in?

Well, one thing leads to another. Sailors have used the stars to navigate for thousands of years, so the Seaport includes an outstanding planetarium. This is the back of that building. This side faces south, so its vertical sundial tells the time all day if it's not too cloudy.

If you take a critical look at a sundial, you'll notice that it usually doesn't match your watch. In part, this is because the Sun moves through the sky at different apparent speeds during the year. If the Earth were upright in its orbit, and if that orbit were a perfect circle, the Sun and a well-adjusted clock would always agree. But the elliptical orbit introduces a yearly variation; and the tilt of the Earth's axis (obliquity) adds another fluctuation that repeats twice a year. This graph shows how Sun time varies throughout the year.

These two offsets must be subtracted from Local Apparent Time ("Sun time") to find Local Mean Time. They're combined into the Equation of Time. It's easier to add a compensation to Sun time, so we've inverted the sign to plot the red line on the following graph. There are still two corrections to be made.

Near each sundial at Mystic, these tables show how much time to add or subtract to get Eastern Standard Time — the same information as the graph, but easier to use. Let's take a closer look at that vertical dial. It reads 11:02, on the 10th of July. The table says to subtract seven minutes from Sun time, for 10:55 EST. Add an hour for daylight time and it's 11:55 — exactly when this photo was taken.

This dial originally had a drawing of the Sun at its center, like the logo at the beginning of this article. It also had a simple gnomon. When it was restored a few years ago, the museum decided to use an anchor for the gnomon, and they painted over the Sun.

This vertical dial is on the planetarium's east wall. It's called a morning dial because it only tells time before noon. After that, it's in shadow.

10:33 AM EDT

With all that discussion about how these dials can show time to the nearest minute, I was surprised to see this one so far off. Using the right corrections for the date (10 July), the dial should read 9:40 Local Apparent Time. What happened?

A conversation with a man at the planetarium cleared this up. The morning dial was also repainted, and the artist painted "10" on the 9:30 line! I just happened to show up at the time when this mistake was glaringly obvious. The real 10:00 line is the next one along (here's the original design). I was told that virtually nobody picks up on this discrepancy, so there's probably not much motivation to fix it.

Albert Waugh designed the two vertical dials. After retiring as provost of the University of Connecticut, he wrote the definitive text Sundials: Their Theory and Construction.

The showpiece of this collection is this armillary sphere on the lawn in front of the planetarium. Armilla is the Latin word for bracelet, which is a good description of this thing. If the center of the sphere represents the Earth's position, the "bracelets" are the projection of several important markers onto the celestial sphere. The gnomon is aimed at the celestial North Pole; its shadow marks Sun Time on the inside of the Equator. On the equinoxes, the Equator's own shadow covers the time marks. On the solstices, the Tropics of Cancer and Capricorn cast their shadows on the time band. The Arctic and Antarctic Circles are sometimes represented, too.

This is a maritime museum, don't forget. There's also a wind vane.

When the Sun is as high as it will get for the day, the meridian and the gnomon line up on the noon mark. For an hour or so either side of noon, there are a lot of shadows in the same area. I've put an arrow on the gnomon's shadow, here. After correcting for the date, longitude, and Daylight Time, you can tell that it's 12:28 EDT. But you have to know which shadow to use.

The dots on the Equator are the builder's marks for longitude on the celestial sphere. The 75-degree mark should be three degrees from the local meridian, but it looks like the spacing is only 2°. No matter, the longitude correction is done in the table, not by shifting the time marks as some dials do.

10 July, 10:39 AM EDT

Earlier in the day, it's much easier to find the correct shadow for the gnomon, between the half-hour "+" and the Roman numeral "X". The tabulated corrections place the time at 10:39 EDT, which agrees nicely with the church clock in the background.

All three of these sundials were built by Edwin Pugsley, a superb gunsmith and a collector of fine firearms and timepieces. Pugsley also designed Mystic's armillary sphere. During World War I, Colt was building John Browning's legendary Automatic Rifle on a piecework basis. This wasn't fast enough for the Army, who persuaded Colt to loan their only prototype to Winchester for one weekend. Mr. Pugsley was the engineer who took custody of that model, and made complete production drawings from it, under that incredible time constraint. From September 1917 until Armistice Day, Winchester produced over 47,000 BARs.

Later, he designed the Winchester AT-50, an anti-tank rifle that wasn't produced because it was peacetime. Edwin Pugsley eventually became president of Winchester Arms. His gun collection became the core of the Cody Firearms Museum in Wyoming, part of the Buffalo Bill Historical Center.

Pugsley was friendly with another enthusiastic gun collector, cartoonist Charles Addams. For many years, Addams' New Yorker cartoon family were unnamed, but they needed names when they became stars of a new TV show in 1964. Addams named the family's grisly, nerdy scion for his friend from New Haven. Pugsley was amused, but he had more pride in the award-winning armillary sphere that he designed and built for Mystic Seaport.

A man on the planetarium staff noticed my interest in the sundials and invited me to see some others in his office. This place could be a timekeeping exhibit as well as a planetarium. There were a few "garden variety" horizontal dials, and also a few oddballs. The Shepherd's Dial is one of the oldest instruments for reading the time. It only works at one latitude, but if you're a shepherd that's not important. Its scales are calibrated for the Sun's declination (angle above the horizon) at different times of year. The user turns the pointer to match the current date and points it toward the Sun, keeping the instrument vertical by hanging it on a string. The pointer's shadow then tells the time. The name comes from the fact that shepherds originally scribed the scales right onto their staves. This is not a terribly accurate timekeeper, but it serves its purpose well.

He also had a few astrolabes. This is an instrument I won't even pretend to understand. It's a precursor to the sextant, and can be used to determine latitude or time (you need one to find the other). It's meant to be used with an almanac and a star chart.

To understand the most interesting sundial of Mystic's private collection requires a bit of explanation. We've seen how a correction table can use the Equation of Time to transform local Sun time to standard time, ignoring longitude correction for the moment. What if a sundial could be made to apply the Equation of Time automatically?

It turns out that's a fairly easy task, especially with an equatorial sundial. This is a special case of the armillary sphere, with a lot of clutter removed. The analemma is a shape that is generated by plotting the Equation of Time against the seasonal declination of the Sun. If you took a picture of the Sun every day at noon from the same spot, this is the path it would follow in the course of one year. If you could make a gnomon with the same shape, its shadow would mark the correct local time without the need to apply any correction tables.

© Kenneth Freeman

This is exactly how the sundial in the Ann Morrison Park (Boise, Idaho) works. The analemma is slightly asymmetrical, so it's important to know which edge of the shadow to use. This dial has instructions for how to do that without your head exploding. The result is a sundial that claims accuracy to fifteen seconds, ostensibly one of the most accurate timepieces in the world. The time scale on the Equator is offset to correct for longitude, so all the user must do is read the shadow that matches the date, which is engraved on the gnomon.

If you shrink this design to tabletop size and add a couple of small refinements, you have the basis for Mystic's Sunquest sundial, made from plans Richard Schmoyer published in a 1959 letter to the Scientific American (transcribed here). His design doesn't present both sides of the analemma at once, but instructs the user to turn it one way or the other depending on the season. That removes the ambiguity of the analemma's asymmetry. The meridian can be adjusted to match the installation's latitude, and the Equator moves to compensate for longitude. There is a company in Vermont that will build one for you, for a few thousand dollars. If you have a 3-D printer, you can get plans to make one yourself.

There's one very nice thing about the location. If things at the planetarium start to feel a bit too technical, you can always take a break, and go watch the Dyer Dhows racing.


The three outdoor dials are all registered with the North American Sundial Society: