128
Secchi’s contributions to the advancement of solar physics and knowledge of the
Sun–Earth connection include the development and daily operation of a thermoheliometer. Secchi’s thermoheliometer (Fig. 7.4a, b) was designed to estimate the
energy radiated by the Sun by means of differential temperature measurements. It
represents one among the first attempts to measure the total solar irradiance, which
is the flux of radiative emission from the Sun reaching the Earth’s atmosphere per
unit time and area, integrated over the whole solar spectrum, at an average distance
of one astronomical unit. This was achieved about 20 years before the first modern
bolometer, designed by Samuel P. Langley (1834–1906) (Langley 1881).
The instrument developed by Secchi resembles modern devices (Fig. 7.4c) monitoring solar irradiance from space (Kopp et al. 2016; Kopp 2020). In Secchi’s instrument, sunlight enters the instrument through a small circular aperture of 25 mm
diameter to heat water collected in a small container, which is maintained thermally
insulated from the surrounding environment. Through two thermometers, one
exposed to the sunlight and the other immersed into the water container, Secchi
obtained an estimate of the solar irradiance by the change in temperature of the
water over a given time interval after its exposure to solar radiation. Modern devices
monitoring the total radiative emission of our star consist of double active cavity
radiometers, which are constructed by taking advantage of new materials and modern electronics in order to obtain high accuracy and high precision measurements of
total solar irradiance. Like the Secchi thermoheliometer, they estimate this amount
from the relative changes of the ambient temperature of the two active cavity
radiometers.
The solar irradiance measurements performed by Secchi with the thermoheliometer were aimed at improving knowledge of solar radiative emission, providing data
for meteorological studies, and searching for the relation between solar phenomena
and the Earth’s atmosphere phenomena. The latter constituted a specific thread of
Secchi’s scientific work.
7.4 Atmosphere Coupling
Starting in 1869, Angelo Secchi made regular observations of the solar chromosphere and prominences to study their nature and their relationship with sunspots
and faculae appearing on the solar disk. His drawings (Fig. 7.5) were mainly carried
out in the wavelength of the H-alpha spectral line using the equatorial Merz telescope and observational methods based on projection and spectroscopy. The Merz
telescope, equipped with a 25 cm aperture lens of 450 cm focal length, an objective
prism, an equatorial mount, and a solar projection board, was one among the best
telescopes available to Secchi for his solar spectroscopic studies.
From his observations of the solar prominences, Secchi deduced that “in their
infinitely bizarre forms” they are “yet susceptible to some classification.” Adopting
a classification scheme proposed in 1871 by Pietro Tacchini (1838–1905) (another
active observer of the Sun), he denoted three distinct classes: “those with active,
I. Ermolli and M. Ferrucci
Secchi’s contributions to the advancement of solar physics and knowledge of the
Sun–Earth connection include the development and daily operation of a thermoheliometer. Secchi’s thermoheliometer (Fig. 7.4a, b) was designed to estimate the
energy radiated by the Sun by means of differential temperature measurements. It
represents one among the first attempts to measure the total solar irradiance, which
is the flux of radiative emission from the Sun reaching the Earth’s atmosphere per
unit time and area, integrated over the whole solar spectrum, at an average distance
of one astronomical unit. This was achieved about 20 years before the first modern
bolometer, designed by Samuel P. Langley (1834–1906) (Langley 1881).
The instrument developed by Secchi resembles modern devices (Fig. 7.4c) monitoring solar irradiance from space (Kopp et al. 2016; Kopp 2020). In Secchi’s instrument, sunlight enters the instrument through a small circular aperture of 25 mm
diameter to heat water collected in a small container, which is maintained thermally
insulated from the surrounding environment. Through two thermometers, one
exposed to the sunlight and the other immersed into the water container, Secchi
obtained an estimate of the solar irradiance by the change in temperature of the
water over a given time interval after its exposure to solar radiation. Modern devices
monitoring the total radiative emission of our star consist of double active cavity
radiometers, which are constructed by taking advantage of new materials and modern electronics in order to obtain high accuracy and high precision measurements of
total solar irradiance. Like the Secchi thermoheliometer, they estimate this amount
from the relative changes of the ambient temperature of the two active cavity
radiometers.
The solar irradiance measurements performed by Secchi with the thermoheliometer were aimed at improving knowledge of solar radiative emission, providing data
for meteorological studies, and searching for the relation between solar phenomena
and the Earth’s atmosphere phenomena. The latter constituted a specific thread of
Secchi’s scientific work.
7.4 Atmosphere Coupling
Starting in 1869, Angelo Secchi made regular observations of the solar chromosphere and prominences to study their nature and their relationship with sunspots
and faculae appearing on the solar disk. His drawings (Fig. 7.5) were mainly carried
out in the wavelength of the H-alpha spectral line using the equatorial Merz telescope and observational methods based on projection and spectroscopy. The Merz
telescope, equipped with a 25 cm aperture lens of 450 cm focal length, an objective
prism, an equatorial mount, and a solar projection board, was one among the best
telescopes available to Secchi for his solar spectroscopic studies.
From his observations of the solar prominences, Secchi deduced that “in their
infinitely bizarre forms” they are “yet susceptible to some classification.” Adopting
a classification scheme proposed in 1871 by Pietro Tacchini (1838–1905) (another
active observer of the Sun), he denoted three distinct classes: “those with active,
I. Ermolli and M. Ferrucci