Calais, Maine

Drawing the Line
Harold E. Nelson
14 Hill Avenue,
Newport, ME 04953

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Edwin DansonDear Harold,

Thanks for the notes. First, Eugene O'Neill’s interest. Sequence of the processes:

The key starting point was ‘the southernmost point of the city of Philadelphia’. This was fixed by the city corporation to be a house on the south side of Cedar Street (now South Street) known as the Plumstead-Hoddle House.

Mason and Dixon established the LATITUDE for this location from a portable observatory.

The West Line (separating Pennsylvania from Maryland) had to lie exactly 15 statute miles south of this point.

Mason and Dixon then headed west to a farm owned by John Harlan and where it was understood a point from the 1736 survey still stood. They found the point and fixed the latitude. They were 10.5" too far south (this they took into account). In my mind, this is the famous Stargazer’s Stone.

They then measured 15 due south and (adjusting for the 10.5" = 15.92 chains) set up ‘the Post mark’s West’. This was the starting point for the West Line (so-called Mason-Dixon Line).

They then left off to set out the eastern boundary of the Tangent Line.

From the Post mark’s West they set out the line of constant latitude.

They almost certainly had a compass, probably the so-called ‘Jeremiah’s Theodolite’ (a circumferentor). This was not used to run the line per se but may have been used on occasion to keep on line.

TheoditeJeremiah Dixon’s ‘theodolite’.

Now, it is not possible to run a line of constant latitude because it makes a slight curve on the surface of the Earth, i.e. its direction constantly changes - it would have been necessary to calculate changes of direction every mile or so AND have a knowledge of the size and shape of the Earth which then was not known sufficiently accurately. In fact, Mason and Dixon finished their time in America measuring a degree of latitude and this helps scientist better refine the size of the Earth.

Instead, Mason resolved to run 10’ segments of great circle. The true latitude curved south of the ‘straight’ line then north to rejoin at the end of the segment.

Mason used the stars to determine the direction westwards (the so-called Secant method). By calculating the angle from true North it was possible to calculate the precise LOCAL SIDEREAL TIME when the selected star was due west.

At the end of the great circle they reobserved the latitude and compared this with the latitude observed at the Post mark’s West. They then corrected their position and calculated the offsets from the great circle to the true latitude.

So, how did they stay on line? The chief instrument was the ‘transit and equal altitude instrument’ made by John Bird. This was the 18th century equivalent of the geodetic theodolite with the import difference that it did not measure (accurately) horizontal angles. Mason instead used the stars alone as his ‘horizontal’ circle. By observing the chosen stars at the correct times (he had tables of their right ascensions and declinations), he could maintain the correct direction. Where the land was difficult they may have used the circumferentor (it is not mentioned) as they could easily calculate the magnetic variation from observations. But, Mason was an astronomer first, surveyor second. Dixon was the land surveyor.

tangentsSo, recapping, a line of latitude could not be set out, only tangents to the line could be observed (this is a bit like setting out a highway curve, but in Mason & Dixon’s case, they didn’t know the rate of curvature). So, they ran the only straight line possible; a great circle. By keeping the distances short (10’ or about 11 miles), errors were kept to a minimum.

How did they measure things?

Latitude: a John Bird 6 foot Zenith Sector, measured the zenith distances of stars of known declination. Declination ± zenith distance = co-latitude.

Bird zenith sector (12 feet)  Quadrant Quadrant

Latitude: a John Bird ‘Hadley’ quadrant of 18 inch radius, nowhere near as accurate as the sector but useful.

Longitude: by solar and lunar eclipses observed by telescope and the Transit.

Time: all time was kept in local SIDEREAL time. Local Sidereal Time = right ascension of any star as it crosses the local Meridian. Mason & Dixon had a ‘stop watch’, not terribly accurate but sufficient for short spells of say 3 or 4 hours. Also had a pendulum clocks (strictly an astronomical regulator) made by Jackson of Philadelphia. For the degree of latitude, they also had a Shelton regulator.

Regulator John Shelton’s astronomical regulator used by M&D.

Meridian: north-south line fixed by observing the transit of a star as it crossed from east to west. The chief method was ‘equal altitudes’; the transit was fixed on a rising star and the vertical axis clamped (or angle observed). The time was noted by the watch or clock. The star was tracked until it had crossed the meridian and was setting – at the precise moment the star reached the fixed altitude (or angle), the time was noted. Hence, the mean of the two times was the precise moment the star crossed the meridian. Knowing this time and the right ascension of the star, the clock could be calibrated. Now, knowing their local sidereal time, more stars were chosen and observed. At the instant their right ascension = clock time, the horizontal circle was clamped and the telescope depressed to the ground level and pegs set out marking the meridian. These were then check next night by repeating the original observations.Transit

The quadrant could also be used but not as accurate as the Transit and was probably limited to setting out local meridians.

M&D’s transit instrument.

To speed up the work later on, Mason & Dixon ‘changed’ direction every other great circle segment by setting out a very large triangle on the ground and setting out the angle as a short distance (N-M).

Zenith Sector

Mounting the clock.

When the astronomical regulator was used, it was necessary to fix this to a heavy piece of timber sunk several feet into the ground. This would have been tedious and inconvenient so the great majority of times were measured with their watch (this was not a chronometer, far from it).

There is a picture of the Shelton regulator installed in a typical observing tent. This is from Cook’s expedition of 1770 but almost certainly exactly the same as used by Mason & Dixon.

Shelton Regulator

Regarding the Hardy Astronomical Clock, I have no information. There is a reference to such a clock used in Australia at

Thanks for the information about the USCS Calais Observatory. This is very interesting and the picture of the pillar certainly seems to be the clock’s support. These timepieces had to be kept perfectly firm and upright to run at a constant rate. Almost certainly the Hardy kept sidereal time. They ‘clanked’ loudly so the astronomer could keep count of the seconds while he made the observations.

Have you any details about the Astronomical Transit – was it fixed in the meridian?

There is a picture of a John Shelton regulator (ca. 1769) fixed to the observatory wall in Madras. Apparently, it’s still in use! See

I hope this helps. I’d appreciate being kept informed of your progress in restoring the old site, this is very interesting stuff and very definitely a worthwhile project representing as it does a key moment in US history. I don’t suppose there is anything I can do to help, but if you think of something, let me know.

Best regards,

Ed Danson.

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