road_less_trvled
Stargazer's Stone
Mason had brought along state-of-the-art equipment for the survey. This included a “transit and equal altitude instrument,” a telescope with cross-hairs, mounted with precision adjustment screws, to sight exact horizontal points using a mounted spirit level, and also to determine true north by tracking stars to their maximum heights in the sky where they crossed the meridian. The famous “zenith sector,” built by London instrument-maker John Bird, was a six-foot telescope mounted on a six-foot radius protractor scale, with fine tangent screws to adjust its position; it was used to measure the angles of reference stars from the zenith of the sky as they crossed the meridian. These measurements could be compared against published measurements of the same stars’ angles of declination at the equator to determine latitude. These were more reliable than measurements of azimuth against a plumb bob, which were already known to be subject to local gravitational anomalies. The zenith sector traveled on a mattress laid on a cart with a spring suspension.
Mason and Dixon also brought a Hadley quadrant, used to measure angular distances; high-quality survey telescopes; 66-foot long Gunter chains comprised of 100 links each (1 chain = 4 rods; 1 chain ×10 chains = 1 acre; 80 chains = 1 mile), along with a precision brass measure to calibrate the chain lengths; and wood measuring rods or “levels” to measure level distances across sloping ground. A large wooden chest contained a collection of star almanacs, seven-figure logarithm tables, trigonometric tables and other reference materials; Mason was skilled at spherical trigonometry.
Mason had acquired a precision clock so that the local times of predicted astronomical events could be compared against published Greenwich times. Each one-minute local time difference implies a 15-second longitude difference. John Harrison’s “H4” chronometer had sailed to Jamaica and back in 1761, losing only 39 seconds on the round trip; the longitude calculations in Jamaica based on his clock were well within the accuracy standards Parliament had set for the £20,000 longitude prize. But Nevil Maskelyne, who had succeeded Bradley as royal astronomer, and the Royal Society remained skeptical about the reliability of chronometers in complementing astronomical calculations of longitude. Maskelyne insisted on the superiority of a purely astronomical approach, a computationally complex “lunar distance” method based on angular distances between the moon and various reference stars. Harrison wouldn’t collect his entire prize until 1773. Mason and Dixon would test the reliability of chronometric positioning, although Mason was skeptical of it.
The southernmost part of Philadelphia was determined by the survey commissioners to be the north wall of a house on the south side of Cedar Street (the address is now 30 South Street) near Second Street. Mason and Dixon had a temporary observatory erected 55 yards northwest of the house, and after detailed celestial observations and calculations, they determined the latitude of the house wall to be 39o56’29.1”N.
Since going straight south would take them through the Delaware River, they then surveyed and measured an arbitrary distance (31 miles) west to a farm owned by John Harland, in Embreeville, Pennsylvania, at the “Forks of the Brandywine.” They negotiated with Harland to set up an observatory, and set a reference stone, now known as the Stargazers' Stone, at the same latitude. They spent the winter at Harland’s farm making astronomical observations on clear nights and enjoying local taverns on cloudy nights. The Harland house still stands at the intersection of Embreeville and Stargazer Roads, and the Stargazers' Stone is in a stone enclosure just up Stargazer Road on the right. Its latitude is 39o56’18 9” N which they calculated to be 356.8 yards to south of the parallel determined in Philadelphia.
At Harland’s they observed and timed predicted transits of Jupiter’s moons, as well as a lunar eclipse on March 17th 1764. The average (sun) time of these events at the Stargazers’ Stone was 5 hours 12 minutes and 54 seconds earlier than published predicted times for the Paris observatory (longitude 2o20’14”E). So they were able to estimate their longitude as (5:12:54)/(24:00:00) x 360o = 78o13’30 west of Paris, and thus 78o13’30” - 2o20’14” = 75o53’6” west of Greenwich. They published these findings in the Royal Society’s Philosophical Transactions in 1769.
Stargazer's Stone
Mason had brought along state-of-the-art equipment for the survey. This included a “transit and equal altitude instrument,” a telescope with cross-hairs, mounted with precision adjustment screws, to sight exact horizontal points using a mounted spirit level, and also to determine true north by tracking stars to their maximum heights in the sky where they crossed the meridian. The famous “zenith sector,” built by London instrument-maker John Bird, was a six-foot telescope mounted on a six-foot radius protractor scale, with fine tangent screws to adjust its position; it was used to measure the angles of reference stars from the zenith of the sky as they crossed the meridian. These measurements could be compared against published measurements of the same stars’ angles of declination at the equator to determine latitude. These were more reliable than measurements of azimuth against a plumb bob, which were already known to be subject to local gravitational anomalies. The zenith sector traveled on a mattress laid on a cart with a spring suspension.
Mason and Dixon also brought a Hadley quadrant, used to measure angular distances; high-quality survey telescopes; 66-foot long Gunter chains comprised of 100 links each (1 chain = 4 rods; 1 chain ×10 chains = 1 acre; 80 chains = 1 mile), along with a precision brass measure to calibrate the chain lengths; and wood measuring rods or “levels” to measure level distances across sloping ground. A large wooden chest contained a collection of star almanacs, seven-figure logarithm tables, trigonometric tables and other reference materials; Mason was skilled at spherical trigonometry.
Mason had acquired a precision clock so that the local times of predicted astronomical events could be compared against published Greenwich times. Each one-minute local time difference implies a 15-second longitude difference. John Harrison’s “H4” chronometer had sailed to Jamaica and back in 1761, losing only 39 seconds on the round trip; the longitude calculations in Jamaica based on his clock were well within the accuracy standards Parliament had set for the £20,000 longitude prize. But Nevil Maskelyne, who had succeeded Bradley as royal astronomer, and the Royal Society remained skeptical about the reliability of chronometers in complementing astronomical calculations of longitude. Maskelyne insisted on the superiority of a purely astronomical approach, a computationally complex “lunar distance” method based on angular distances between the moon and various reference stars. Harrison wouldn’t collect his entire prize until 1773. Mason and Dixon would test the reliability of chronometric positioning, although Mason was skeptical of it.
The southernmost part of Philadelphia was determined by the survey commissioners to be the north wall of a house on the south side of Cedar Street (the address is now 30 South Street) near Second Street. Mason and Dixon had a temporary observatory erected 55 yards northwest of the house, and after detailed celestial observations and calculations, they determined the latitude of the house wall to be 39o56’29.1”N.
Since going straight south would take them through the Delaware River, they then surveyed and measured an arbitrary distance (31 miles) west to a farm owned by John Harland, in Embreeville, Pennsylvania, at the “Forks of the Brandywine.” They negotiated with Harland to set up an observatory, and set a reference stone, now known as the Stargazers' Stone, at the same latitude. They spent the winter at Harland’s farm making astronomical observations on clear nights and enjoying local taverns on cloudy nights. The Harland house still stands at the intersection of Embreeville and Stargazer Roads, and the Stargazers' Stone is in a stone enclosure just up Stargazer Road on the right. Its latitude is 39o56’18 9” N which they calculated to be 356.8 yards to south of the parallel determined in Philadelphia.
At Harland’s they observed and timed predicted transits of Jupiter’s moons, as well as a lunar eclipse on March 17th 1764. The average (sun) time of these events at the Stargazers’ Stone was 5 hours 12 minutes and 54 seconds earlier than published predicted times for the Paris observatory (longitude 2o20’14”E). So they were able to estimate their longitude as (5:12:54)/(24:00:00) x 360o = 78o13’30 west of Paris, and thus 78o13’30” - 2o20’14” = 75o53’6” west of Greenwich. They published these findings in the Royal Society’s Philosophical Transactions in 1769.