207
sites, one can determine how much the ground has changed or not in the last century
and a half.
The measurement consisted of placing a series of surveying telescopes along the
straight line connecting the Endpoints A and B. Next, the distance between the telescopes was measured with the tesa, making sure that the end point of a measure
matched the starting point of the next. For this reason, graduated silver tabs were
placed at the ends of the metal rods. The precision in positioning the tesa after moving forward was entrusted to the meroscope, a telescope positioned vertically on an
independent stand which allowed one to maintain the position of the ends during the
movement of the tesa itself. Graduated vertical rods placed on the tesa allowed one
to keep it horizontal.
By chance, Endpoint A was uncovered during excavations in front of Cecilia
Metella in July 1999 (Marcelli 2000). The manhole (which is visible today at the
level and at the center of the roadway) is the original one and requires permanent
maintenance given the continuous passage of vehicles.
Endpoint B was discovered in January 2013 using ground-penetrating radar
measurements (Aebischer 2013). It has been reburied in order to preserve it.
Once the two main endpoints were defined, at the end of each day of measurement,
the stopping points were determined for which partial termini or points were
inserted into the ground. The following day, measurement operations would start
again from the partial terminus. In total, 50 partial termini were built; however,
none of them have yet been found.
It is good to remember that this is one of the longest geodetic baselines ever
measured. It involved 6 months of measurements on the ground (with the repetition
of the first 2 km) and 3 years of calculations. Ultimately, taking into account all the
corrections of horizontality, alignment, and temperature, the length of the baseline
is 12043.139 m with an error that Secchi estimated to be less than 1 cm (Secchi 1858).
12.3 The Problem of the Earth’s Shape and the Prime
Meridian of Italy
The geodetic baseline established by Boscovich in 1751 and remeasured by Secchi
in the mid-nineteenth century was not an isolated scientific operation but grew out
of work that began in the late seventeenth century. Isaac Newton (1643–1727)’s
Philosophiae Naturalis Principia Mathematica (1687) had noted that, because of its
rotation, the Earth should assume an oblate ellipsoidal shape, and Newton tried to
calculate the flattening at the poles by supposing the Earth to be fluid and homogeneous. By contrast, J. D. Cassini (1625–1712) had proposed that the ellipsoid of the
Earth was prolate, i.e., lemon-shaped, narrower at the equator than at the poles. To
resolve this dispute, the French Academy of Sciences (Greenberg 1995) supported
two epic expeditions: one to Lapland organized by P. L. Moreau de Maupertuis
(1698–1759) and A.-C. Clairaut (1713–1765) and the other one to Peru (today
12 The Measurement of the Geodetic Baselines Along the Via Appia Antica
sites, one can determine how much the ground has changed or not in the last century
and a half.
The measurement consisted of placing a series of surveying telescopes along the
straight line connecting the Endpoints A and B. Next, the distance between the telescopes was measured with the tesa, making sure that the end point of a measure
matched the starting point of the next. For this reason, graduated silver tabs were
placed at the ends of the metal rods. The precision in positioning the tesa after moving forward was entrusted to the meroscope, a telescope positioned vertically on an
independent stand which allowed one to maintain the position of the ends during the
movement of the tesa itself. Graduated vertical rods placed on the tesa allowed one
to keep it horizontal.
By chance, Endpoint A was uncovered during excavations in front of Cecilia
Metella in July 1999 (Marcelli 2000). The manhole (which is visible today at the
level and at the center of the roadway) is the original one and requires permanent
maintenance given the continuous passage of vehicles.
Endpoint B was discovered in January 2013 using ground-penetrating radar
measurements (Aebischer 2013). It has been reburied in order to preserve it.
Once the two main endpoints were defined, at the end of each day of measurement,
the stopping points were determined for which partial termini or points were
inserted into the ground. The following day, measurement operations would start
again from the partial terminus. In total, 50 partial termini were built; however,
none of them have yet been found.
It is good to remember that this is one of the longest geodetic baselines ever
measured. It involved 6 months of measurements on the ground (with the repetition
of the first 2 km) and 3 years of calculations. Ultimately, taking into account all the
corrections of horizontality, alignment, and temperature, the length of the baseline
is 12043.139 m with an error that Secchi estimated to be less than 1 cm (Secchi 1858).
12.3 The Problem of the Earth’s Shape and the Prime
Meridian of Italy
The geodetic baseline established by Boscovich in 1751 and remeasured by Secchi
in the mid-nineteenth century was not an isolated scientific operation but grew out
of work that began in the late seventeenth century. Isaac Newton (1643–1727)’s
Philosophiae Naturalis Principia Mathematica (1687) had noted that, because of its
rotation, the Earth should assume an oblate ellipsoidal shape, and Newton tried to
calculate the flattening at the poles by supposing the Earth to be fluid and homogeneous. By contrast, J. D. Cassini (1625–1712) had proposed that the ellipsoid of the
Earth was prolate, i.e., lemon-shaped, narrower at the equator than at the poles. To
resolve this dispute, the French Academy of Sciences (Greenberg 1995) supported
two epic expeditions: one to Lapland organized by P. L. Moreau de Maupertuis
(1698–1759) and A.-C. Clairaut (1713–1765) and the other one to Peru (today
12 The Measurement of the Geodetic Baselines Along the Via Appia Antica