252
15.4.3 Alatri Sundial (1867–1875)
Of all the gnomonic instruments devised by Secchi, the Alatri (Frosinone) sundial
in solar and mean time is the best known,
21
both for its privileged position and for
its graphic significance.
The dial is of considerable size (4.60 × 4.80 meters), and is well highlighted on
the facade of a noble palace of medieval origin, home since 1870 of the ContiGentili High School and, before that, of the Collegio delle Scuole Pie, founded in
1729 by the will of the couple Giuseppe Conti and Innocenza Gentili.
22
The dial face, exposed to the southwest, is made on plaster, marking the shadows
of the sun over the hours ranging from 10 am to 4 pm. The system adopted is the
European one, and all time lines are accompanied by an analemma for the calculation of mean time. There are also calendar lines related to the passage of the sun
through the twelve zodiac signs, each depicted with its own graphic symbol. The
gnomon is a 42 cm iron style, perpendicular to the wall (“orthostylus”), and at its
node is a disk pierced by a hole in the shape of a star. Sunlight entering the hole
generates a small spot of light that touches the inscriptions of the disk at the point
indicating the time of day, similar to what happens in darkroom sundials (Fig. 15.1).
23
There are three inscriptions: the first two indicate the geographical coordinates,
parameters necessary for determining the local solar time, with reference to the bell
tower of the Cathedral of Alatri; the third is Long. Roma (Oss° Collegio Romano)
which indicates that the mean time has been calculated based on the time at the
Roman College. Finally, we read the signatures “P. A. Secchi,” as designer, and
“Olivieri pince 1875,” for the graphic execution of the project. The engineer
Giuseppe Olivieri, from Rome, had previously collaborated with Secchi in 1865
during the construction of the new Alatri aqueduct (Secchi 1865).
24
Although the instrument is dated 1875, there is a record of it in historical documents, unfortunately now scattered, in which it was reported that Secchi worked on
the calculations of this dial as early as 1867. The sundial was restored in 1938
(P. Carinci) and 1989 (by the Sovrintendenza alle Gallerie del Lazio [Superintendent
of Museums of Lazio]).
25
21 Nicola Severino (www.nicolaseverino.it), Renzo Righi (Luci e Ombre, sulle ali del Tempo),
Tuscano M.L. 2012b and 2020.
22 G. Baptist, Mantovani, “Il Liceo Conti Gentili, Introduzione storica” in Il filo del Tempo: l’antico
lab fisico Instrumenta selecta. (Mantovani R.ed.). Alatri 1996 (II rist.). We would like to thank
Prof. Roberto Mantovani, Scientific Curator of Physics Cabinet: Urbinate Museum of Science and
Technology, University of Urbino, for the courtesy of the providing this text.
23 Andrewes,W. J. H. , Mantovani R., “La meridiana del Palazzo Conti Gentili,” in Il filo del
Tempo: l’antico lab fisico instrumenta selecta. (Roberto Mantovani, ed.), op. cit.
24 Analemmas refer to time lines that are not drawn but calculated for the longitude of Rome.
25 Ibid.
M. L. Tuscano
15.4.3 Alatri Sundial (1867–1875)
Of all the gnomonic instruments devised by Secchi, the Alatri (Frosinone) sundial
in solar and mean time is the best known,
21
both for its privileged position and for
its graphic significance.
The dial is of considerable size (4.60 × 4.80 meters), and is well highlighted on
the facade of a noble palace of medieval origin, home since 1870 of the ContiGentili High School and, before that, of the Collegio delle Scuole Pie, founded in
1729 by the will of the couple Giuseppe Conti and Innocenza Gentili.
22
The dial face, exposed to the southwest, is made on plaster, marking the shadows
of the sun over the hours ranging from 10 am to 4 pm. The system adopted is the
European one, and all time lines are accompanied by an analemma for the calculation of mean time. There are also calendar lines related to the passage of the sun
through the twelve zodiac signs, each depicted with its own graphic symbol. The
gnomon is a 42 cm iron style, perpendicular to the wall (“orthostylus”), and at its
node is a disk pierced by a hole in the shape of a star. Sunlight entering the hole
generates a small spot of light that touches the inscriptions of the disk at the point
indicating the time of day, similar to what happens in darkroom sundials (Fig. 15.1).
23
There are three inscriptions: the first two indicate the geographical coordinates,
parameters necessary for determining the local solar time, with reference to the bell
tower of the Cathedral of Alatri; the third is Long. Roma (Oss° Collegio Romano)
which indicates that the mean time has been calculated based on the time at the
Roman College. Finally, we read the signatures “P. A. Secchi,” as designer, and
“Olivieri pince 1875,” for the graphic execution of the project. The engineer
Giuseppe Olivieri, from Rome, had previously collaborated with Secchi in 1865
during the construction of the new Alatri aqueduct (Secchi 1865).
24
Although the instrument is dated 1875, there is a record of it in historical documents, unfortunately now scattered, in which it was reported that Secchi worked on
the calculations of this dial as early as 1867. The sundial was restored in 1938
(P. Carinci) and 1989 (by the Sovrintendenza alle Gallerie del Lazio [Superintendent
of Museums of Lazio]).
25
21 Nicola Severino (www.nicolaseverino.it), Renzo Righi (Luci e Ombre, sulle ali del Tempo),
Tuscano M.L. 2012b and 2020.
22 G. Baptist, Mantovani, “Il Liceo Conti Gentili, Introduzione storica” in Il filo del Tempo: l’antico
lab fisico Instrumenta selecta. (Mantovani R.ed.). Alatri 1996 (II rist.). We would like to thank
Prof. Roberto Mantovani, Scientific Curator of Physics Cabinet: Urbinate Museum of Science and
Technology, University of Urbino, for the courtesy of the providing this text.
23 Andrewes,W. J. H. , Mantovani R., “La meridiana del Palazzo Conti Gentili,” in Il filo del
Tempo: l’antico lab fisico instrumenta selecta. (Roberto Mantovani, ed.), op. cit.
24 Analemmas refer to time lines that are not drawn but calculated for the longitude of Rome.
25 Ibid.
M. L. Tuscano