115
Though Secchi could not resist offering a new value for the period, his interest was
mainly in the physical conditions on the planet – what would later be referred to as
the ‘physics’ of the planet Mars.
Secchi missed the 1860 opposition (as mentioned earlier), but other drawings
would follow in 1862 (Secchi 1863a), 1864–1865, and later apparitions. Again
observing with Cappeletti, Secchi in 1862 had noticed that the tail of his ‘Blue
Scorpion’ was absent, not (as he had concluded) due to the different perspective but
due to real change upon the surface. He had his clearest views of the so-called Eye
of Mars, the Solis Lacus, on October 18, but the drawing looks rather inaccurate.
Indeed, his remark that it resembled a cyclone was wide of the mark. Flammarion
handily presents a perfectly normal drawing of it at the same hour by Norman
Lockyer, who observed from England, pointing out the discrepancy. Clearly, the
1858 series had been his best Mars work.
At the opposition of 1862, with Mars farther north of the equator than it had been
in 1860, Secchi succeeded in getting excellent views of the South Polar Cap, which
was now tilted towards the Earth. From his observations of its perennial summer
retreat, he concluded:
the variations can be explained only by a melting of the snow or a disappearance of the
clouds covering the polar regions…. These aspects … prove that liquid water and seas exist
on Mars; this is a natural result of the behaviour of the snows. This conclusion is confirmed
by the fact that the blue markings which we see in the equatorial regions do not change
sensibly in form, whereas the white fields in the neighbourhood of the poles are adjacent to
reddish fields which can only be continents. Thus, the existence of seas and continents, and
even the alternations of the seasons and the atmospheric variations, have been today conclusively proved. [italics added.]
The idea that the dark areas on Mars were seas and the light areas continents
would continue to be endorsed by most later astronomers, at least until the 1890s,
informing ideas about Mars’s Earth-like nature, with which Secchi certainly agreed,
as well as influencing the nomenclature introduced on maps.
The archives of the Astronomical Observatory of Rome contain a notebook with
sketches for 1871, and though but rough sketches (one of which we reproduce in
Fig. 6.5), they accurately show the markings in the longitude of Sinus Sabaeus in
one case and the longitude of Mare Acidalium in the other. The blues and yellows
are too strongly marked.
The most noteworthy heir to Secchi’s Mars work was another Italian astronomer,
Giovanni Virginio Schiaparelli of the Brera Observatory in Milan. The two men
were close friends for many years, even though their friendship was almost entirely
carried out through correspondence, and they met only once, in Milan sometime
before September 1870 (Maffeo 2011). For instance, in the short period August
1866–February 1867, Schiaparelli had written to Secchi the famous letters showing
the connection between comets and meteors, which Secchi published in the
Meteorological Bulletin of the Roman College.
However, despite the fact that Schiaparelli devoted so much attention to the
planet, there is no mention of Mars in their vast correspondence. The reason for this
6 Planetary Observations by Angelo Secchi
Though Secchi could not resist offering a new value for the period, his interest was
mainly in the physical conditions on the planet – what would later be referred to as
the ‘physics’ of the planet Mars.
Secchi missed the 1860 opposition (as mentioned earlier), but other drawings
would follow in 1862 (Secchi 1863a), 1864–1865, and later apparitions. Again
observing with Cappeletti, Secchi in 1862 had noticed that the tail of his ‘Blue
Scorpion’ was absent, not (as he had concluded) due to the different perspective but
due to real change upon the surface. He had his clearest views of the so-called Eye
of Mars, the Solis Lacus, on October 18, but the drawing looks rather inaccurate.
Indeed, his remark that it resembled a cyclone was wide of the mark. Flammarion
handily presents a perfectly normal drawing of it at the same hour by Norman
Lockyer, who observed from England, pointing out the discrepancy. Clearly, the
1858 series had been his best Mars work.
At the opposition of 1862, with Mars farther north of the equator than it had been
in 1860, Secchi succeeded in getting excellent views of the South Polar Cap, which
was now tilted towards the Earth. From his observations of its perennial summer
retreat, he concluded:
the variations can be explained only by a melting of the snow or a disappearance of the
clouds covering the polar regions…. These aspects … prove that liquid water and seas exist
on Mars; this is a natural result of the behaviour of the snows. This conclusion is confirmed
by the fact that the blue markings which we see in the equatorial regions do not change
sensibly in form, whereas the white fields in the neighbourhood of the poles are adjacent to
reddish fields which can only be continents. Thus, the existence of seas and continents, and
even the alternations of the seasons and the atmospheric variations, have been today conclusively proved. [italics added.]
The idea that the dark areas on Mars were seas and the light areas continents
would continue to be endorsed by most later astronomers, at least until the 1890s,
informing ideas about Mars’s Earth-like nature, with which Secchi certainly agreed,
as well as influencing the nomenclature introduced on maps.
The archives of the Astronomical Observatory of Rome contain a notebook with
sketches for 1871, and though but rough sketches (one of which we reproduce in
Fig. 6.5), they accurately show the markings in the longitude of Sinus Sabaeus in
one case and the longitude of Mare Acidalium in the other. The blues and yellows
are too strongly marked.
The most noteworthy heir to Secchi’s Mars work was another Italian astronomer,
Giovanni Virginio Schiaparelli of the Brera Observatory in Milan. The two men
were close friends for many years, even though their friendship was almost entirely
carried out through correspondence, and they met only once, in Milan sometime
before September 1870 (Maffeo 2011). For instance, in the short period August
1866–February 1867, Schiaparelli had written to Secchi the famous letters showing
the connection between comets and meteors, which Secchi published in the
Meteorological Bulletin of the Roman College.
However, despite the fact that Schiaparelli devoted so much attention to the
planet, there is no mention of Mars in their vast correspondence. The reason for this
6 Planetary Observations by Angelo Secchi