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Originally published in the Historic Nantucket, Vol 44, no. 2 (Fall 1995), p. 87-89

Ever Wonder About Those Meridian Markers?
By Robert C. Orr

Among the objects of curiosity to be found in downtown Nantucket perhaps the most unusual are two stone markers — one on the curb of Main Street near the Pacific National Bank ("Northern Extremity of the Town Meridian Line") and the other ("Southern Extremity of the Town Meridian Line") about 350 feet up the west side of Fair Street in front of the Fair Street Museum. They are aligned on the north-south meridian line approximately 70 degrees 8 minutes west longitude. They were placed there in 1849 by William Mitchell, an accomplished amateur astronomer who was an expert at rating chronometers and, at that time, cashier of the Pacific National Bank.

The stone meridian markers on Main Street and Fair Street had nothing to do with the actual "rating" process. Mitchell probably installed them more as a matter of interest than of functional use. His "Astronomical Journal" indicates that he was rating chronometers long before the markers were erected.

The only practical purpose of the line would be to determine the angle of variation (now approximately 16 degrees west at Nantucket) between true north and the north magnetic pole toward which compasses point. This angle of variation varies widely in different parts of the ocean, and for any given point it changes slightly every year.

It has been mistakenly assumed that ships' captains would take their compasses to these markers to determine the accuracy of their instruments. The notion has no substance because a compass in a place where there are no metallic or magnetic elements will always point to magnetic north.

Mr. Mitchell was a man of many parts. In his autobiography, he summarizes his accomplishments this way:

A cooper, a soap boiler, an oil and candle manufacturer, a farmer, a schoolmaster, an Insurance Broker, a Surveyor, a chronometer rater, an astronomical observer for the Coast Survey, Justice of the Peace, Executor of Wills and Administrator of Estates, Writer of Wills, deeds and instruments, Cashier of a Bank, Treasurer of a Savings Bank, and without emolument a member and for some years President of the Board of Trustees of the Nantucket Atheneum — a member and for many years Chairman of the Board of Trustees of the Coffin School. For many years Chairman of the Committee for the Superintendence of the Observatory of Harvard College. Clerk at two different periods of the Nantucket Monthly Meeting of Friends, once for ten and once for five years.

This article concerns Mitchell's role as "a chronometer rater."

The calculation for longitude (east-west position) by celestial navigation for a particular location at a particular time requires determining the exact position of a celestial body such as the sun, the moon, a planet or a star, west of Greenwich, England, and north or south of the equator at that exact time. These coordinates can be obtained from the Nautical Almanac for every second of Greenwich Civil Time (GCT) throughout the year.

When navigators set out to determine the position of a ship by celestial navigation, they use a sextant to measure the angle (altitude) between the horizon and a celestial body and note the exact time as recorded on the ship's chronometer. Using the Nautical Almanac and other tables along with this estimated position, they can calculate what the true altitude and azimuth of the body would be if the ship were actually at the estimated position. The comparison of the observed altitude with the computed one gives the difference between the two, enabling navigators to plot a "line of position" from that sighting.

Navigators then repeat the process, using a different celestial body as a reference, and plot its "line of position" The ship's actual position will be at the intersection of the two lines.

Ships' chronometers are set to show Greenwich Civil Time (GCT), the mean solar time at the Greenwich, England, meridian as recorded at the British Naval Observatory there. Even the best chronometers lose or gain a few seconds or fractions of a second every day. The chronometer error, and therefore the "daily rate of gaining or losing," can be determined today by radio time signals from Washington DC, but in the 1840s the rating of a chronometer had to be done at a celestial observatory or by comparison with another chronometer whose daily rate was known.

When Mitchell was appointed cashier of the Pacific National Bank, he moved his family into quarters above the bank and set up a small observatory on the roof so that he could more efficiently pursue his astronomic avocation. Mitchell instructed his daughter Maria, who demonstrated an interest in, and aptitude for, the science at an early age. As we know, she later became an internationally respected astronomer in her own right.

A review of William Mitchell's Astronomical Journal indicates that he used observations of the moon and stars to ascertain exact Greenwich Civil Time at a given instant. By recording his chronometer reading at that instant he could determine its amount of error. By repeating the process over a period of days he could determine its "daily rate of gaining or losing."
Ship masters or navigators would bring their chronometers to Mitchell to have them "rated." He would determine error by observation or by comparing the time shown on their instruments to that on his own chronometer over a period of several days. This comparison would indicate the daily rate of gaining or losing. The navigator would apply this daily rate to the reading of his vessel's chronometer each time he took a celestial sight. It was important to ascertain the exact GCT of the sight because an error of just four seconds meant a discrepancy of one mile east or west of the vessel's actual position.

When two ships met at sea, they would often compare the time on their chronometers. It was agreed that one ship would signal, by means of a black ball dropped from the yardarm, at a prearranged moment. The other ship's master would check his chronometer to learn how nearly it agreed with that of his companion.

Ships at sea followed a standard procedure to assure that the vessel's chronometer was wound daily. At about ten minutes before noon (local time) the mate would report to the captain: "Sir, it is approaching eight bells and the chronometers have been wound." The captain would reply: "Strike the bells on time." This procedure was vitally important because the chronometers had to be wound every twenty-four hours in order for the daily rate to remain constant.

The records in William Mitchell's work-books indicate that his daughter Maria assisted him in calibrating chronometers and became so proficient at the technique that she was able to carry out the procedure by herself when she was only fourteen years old.

 

About the author. A lieutenant in the U. S. Coast Guard Reserve, Robert Orr commanded a 327-foot LST in the Pacific, served as an instructor in navigation at the USCG Academy for two and a half years during World War II, and was the professional compass adjuster for the Gloucester (MA) fishing fleet.
From Nathaniel Bowditch's The New American Practical Navigator (NY: E. & G. Blunt, 1837).