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avoid prejudice concerning the nature of the features on the planet may regard as a mere
artifice to assist the memory and abbreviate the descriptions. After all, we speak in a similar
way of the maria of the Moon, knowing full well that they do not consist of liquid masses.
(translation by W. Sheehan).
Thus did the term canale – first used by Secchi (Secchi 1859c: 59) in the sense
Schiaparelli intended, meaning ‘channel’, but soon to be translated into English as
‘canal’, implying artificial construction, which would prove so fateful in the subsequent history of the planet – rather innocently started to circulate in the literature of
Mars. Secchi’s contribution to all this is little remembered today. It was significant.
Perhaps one might paraphrase the words supposedly spoken by Isaac Newton when
his favourite dog, by upsetting a candle, set fire to some of his manuscripts and say,
‘O Father Secchi, thou little knowest the mischief thou hast done’!
6.5 Observations of the Outer Planets
Secchi devoted less attention to the outer planets than he did to Mars. When he
watched Saturn during 1854–1856, Secchi concluded there was a slight ellipticity in
the ring. Its rotation period seemed to correspond with the theoretical period for a
particle in ring A, but this and other such impressions of asymmetry have since been
found to be illusory, due to the presence (except precisely at opposition) of a phase
effect. However, William F. Denning (1848–1931) found a similar rotation period in
November 1903 from his observations of brightness differences between the east
and west ansae (Alexander 1962a). The presence of such a brighter spot could suggest ellipticity in a series of micrometre measurements.
But Secchi could also be commendably cautious. When Otto W. Struve
(1819–1905) inferred a spreading of the ring system from past micrometrical data,
Secchi in 1856 wrote to the Astronomer Royal in England, George Biddell Airy
(1801–1892), and – while not challenging Struve directly – wisely drew attention to
the discrepancies between the historical and contemporary measurements. Later,
further observations caused Struve to abandon the idea (Alexander 1962b).
Secchi was able to observe the thin dark line of ‘Encke’s Division’ upon the open
south face of ring A in the winter of 1854–1855 and the step-like concentric bands
of shading on ring B, which we now know correspond to annuli of ring material of
different spatial densities. (Alexander 1962c).
When the rings were edgewise in 1862, Secchi observed, as have others at such
times, several fixed bright points upon the unilluminated ring face (Alexander
1962d; Secchi 1863b). These ‘lucid points’, in the words of the astronomers of the
day, were later found to be due to light filtering through (and being scattered by) the
more widely spread particles of such relatively transparent areas as the Cassini
Division and ring C. A recent review of the edgewise apparition of the rings in
1995–1996 (McKim 2015) offers more detailed modern drawings and images which
portray the evolution of this beautiful – but no longer enigmatic – effect.
6 Planetary Observations by Angelo Secchi
avoid prejudice concerning the nature of the features on the planet may regard as a mere
artifice to assist the memory and abbreviate the descriptions. After all, we speak in a similar
way of the maria of the Moon, knowing full well that they do not consist of liquid masses.
(translation by W. Sheehan).
Thus did the term canale – first used by Secchi (Secchi 1859c: 59) in the sense
Schiaparelli intended, meaning ‘channel’, but soon to be translated into English as
‘canal’, implying artificial construction, which would prove so fateful in the subsequent history of the planet – rather innocently started to circulate in the literature of
Mars. Secchi’s contribution to all this is little remembered today. It was significant.
Perhaps one might paraphrase the words supposedly spoken by Isaac Newton when
his favourite dog, by upsetting a candle, set fire to some of his manuscripts and say,
‘O Father Secchi, thou little knowest the mischief thou hast done’!
6.5 Observations of the Outer Planets
Secchi devoted less attention to the outer planets than he did to Mars. When he
watched Saturn during 1854–1856, Secchi concluded there was a slight ellipticity in
the ring. Its rotation period seemed to correspond with the theoretical period for a
particle in ring A, but this and other such impressions of asymmetry have since been
found to be illusory, due to the presence (except precisely at opposition) of a phase
effect. However, William F. Denning (1848–1931) found a similar rotation period in
November 1903 from his observations of brightness differences between the east
and west ansae (Alexander 1962a). The presence of such a brighter spot could suggest ellipticity in a series of micrometre measurements.
But Secchi could also be commendably cautious. When Otto W. Struve
(1819–1905) inferred a spreading of the ring system from past micrometrical data,
Secchi in 1856 wrote to the Astronomer Royal in England, George Biddell Airy
(1801–1892), and – while not challenging Struve directly – wisely drew attention to
the discrepancies between the historical and contemporary measurements. Later,
further observations caused Struve to abandon the idea (Alexander 1962b).
Secchi was able to observe the thin dark line of ‘Encke’s Division’ upon the open
south face of ring A in the winter of 1854–1855 and the step-like concentric bands
of shading on ring B, which we now know correspond to annuli of ring material of
different spatial densities. (Alexander 1962c).
When the rings were edgewise in 1862, Secchi observed, as have others at such
times, several fixed bright points upon the unilluminated ring face (Alexander
1962d; Secchi 1863b). These ‘lucid points’, in the words of the astronomers of the
day, were later found to be due to light filtering through (and being scattered by) the
more widely spread particles of such relatively transparent areas as the Cassini
Division and ring C. A recent review of the edgewise apparition of the rings in
1995–1996 (McKim 2015) offers more detailed modern drawings and images which
portray the evolution of this beautiful – but no longer enigmatic – effect.
6 Planetary Observations by Angelo Secchi