249
some of the gnomonic instruments and some written traces of these historical studies can still be found in the Roman College.
13
15.4 Angelo Secchi’s Sundials
A scholar attentive to tradition, Angelo Secchi devoted himself to the history of time
systems and in particular to Italian time, which he saw as an evolution of the system
in use in ancient Rome, following the natural period of daylight but neglecting the
variability of the hours.
He wrote about it in L’Astronomia in Roma nel Pontificato di Pio IX (Astronomy
in Rome during the Pontificate of Pope Pius IX), detailing the reasons why the seasonal hours of solar time were ill-matched to the constant rhythms of a precision
clock. His interest in this subject is confirmed by five papers Sulla divisione del
Tempo (On the Divisions of Time) preserved in the archives of the Pontifical
Gregorian University, in which Secchi deals with the subject following a logical
method and with reference to documentary sources
14
:
(...) We must note two things about Italian Time: first, the day began in the evening; second,
the day was divided into twenty-four hours.
The first point dates back to the Romans; their civil day actually began at midnight but
in practice their hours were counted beginning in the evening, because they divided the day
into two periods, day and night, each of 12 hours (...) these hours were unequal in duration,
changing according to the seasons. Back then they numbered them from 1 to 12 like we do
now, but the difference was that the hours were not of the same duration, varying from day
to night with the seasons, so that the hours of the day were longer in the summer and shorter
in the winter and vice versa [for hours of the night]; and they began counting them at sunset
and at sunrise. For the second point, it is not possible to establish when this 12 and 12 division was replaced with 24 hours; I believe that it was a consequence of the introduction of
geared machines to count and strike the hours. The point is, these machines could not measure hours of unequal time(...). As a result, the machines had to abandon the division of 12
and 12, and go to a numbering ... up to 24 hours. In fact, we find that the old machines
struck 24 hours, indicating the first at one o’clock at night (...). Then it was decided to start
the day from the hour of sunset, which was very natural since everyone would be awake at
that time and able to see the actual moment of the phenomenon of sunset, and adjust the
clockworks accordingly; that is not always the case for sunrise. If one could specify when
such machines were introduced, it could be known within certain limits when the use of
twenty four hours began, but this will be hard to do.
13 Some sundials can still be seen on the facade; there is a darkroom meridian line in St. Luigi’s
rooms and another in the Calandrelli tower. Francesco Carlini in 1843 recalled: “There was no
shortage of transit instruments at the Roman College Observatory then; in order to adjust the
clocks, every day at midday we used to observe a small meridian line inscribed on the floor and the
wall of a room.”
14 APUG, Secchi Archive, II n, Sulla divisione del Tempo (Tuscano 2012a). Thanks are given to
Prof. Martin Maria Morales SJ and the officials of the Archive of the Pontifical Gregorian
University for their great courtesy in allowing access to consult the manuscripts.
15 Angelo Secchi and the Measurement of Time
some of the gnomonic instruments and some written traces of these historical studies can still be found in the Roman College.
13
15.4 Angelo Secchi’s Sundials
A scholar attentive to tradition, Angelo Secchi devoted himself to the history of time
systems and in particular to Italian time, which he saw as an evolution of the system
in use in ancient Rome, following the natural period of daylight but neglecting the
variability of the hours.
He wrote about it in L’Astronomia in Roma nel Pontificato di Pio IX (Astronomy
in Rome during the Pontificate of Pope Pius IX), detailing the reasons why the seasonal hours of solar time were ill-matched to the constant rhythms of a precision
clock. His interest in this subject is confirmed by five papers Sulla divisione del
Tempo (On the Divisions of Time) preserved in the archives of the Pontifical
Gregorian University, in which Secchi deals with the subject following a logical
method and with reference to documentary sources
14
:
(...) We must note two things about Italian Time: first, the day began in the evening; second,
the day was divided into twenty-four hours.
The first point dates back to the Romans; their civil day actually began at midnight but
in practice their hours were counted beginning in the evening, because they divided the day
into two periods, day and night, each of 12 hours (...) these hours were unequal in duration,
changing according to the seasons. Back then they numbered them from 1 to 12 like we do
now, but the difference was that the hours were not of the same duration, varying from day
to night with the seasons, so that the hours of the day were longer in the summer and shorter
in the winter and vice versa [for hours of the night]; and they began counting them at sunset
and at sunrise. For the second point, it is not possible to establish when this 12 and 12 division was replaced with 24 hours; I believe that it was a consequence of the introduction of
geared machines to count and strike the hours. The point is, these machines could not measure hours of unequal time(...). As a result, the machines had to abandon the division of 12
and 12, and go to a numbering ... up to 24 hours. In fact, we find that the old machines
struck 24 hours, indicating the first at one o’clock at night (...). Then it was decided to start
the day from the hour of sunset, which was very natural since everyone would be awake at
that time and able to see the actual moment of the phenomenon of sunset, and adjust the
clockworks accordingly; that is not always the case for sunrise. If one could specify when
such machines were introduced, it could be known within certain limits when the use of
twenty four hours began, but this will be hard to do.
13 Some sundials can still be seen on the facade; there is a darkroom meridian line in St. Luigi’s
rooms and another in the Calandrelli tower. Francesco Carlini in 1843 recalled: “There was no
shortage of transit instruments at the Roman College Observatory then; in order to adjust the
clocks, every day at midday we used to observe a small meridian line inscribed on the floor and the
wall of a room.”
14 APUG, Secchi Archive, II n, Sulla divisione del Tempo (Tuscano 2012a). Thanks are given to
Prof. Martin Maria Morales SJ and the officials of the Archive of the Pontifical Gregorian
University for their great courtesy in allowing access to consult the manuscripts.
15 Angelo Secchi and the Measurement of Time