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closer to a neutral grey or reddish grey. The bluish tint is always more pronounced
with refractors. Furthermore, the degree of blueness in the Syrtis Major is seasonally increased by a scattering effect from the belt of equatorial white cloud that
persists from early northern spring till midsummer, from Ls ~ 0 to 145°. It is often
claimed that Secchi was the first to make out this feature. However, his observing
period was seasonally too late to have received this short-wavelength bonus.
In July, the drawings reveal some obscuration of albedo markings as well as a
yellowness (or lack of redness) of the disk. In discussing the work of Secchi, French
astronomer Camille Flammarion wrote (Flammarion 1892b):
‘On a remarqué que Mars paraissait moins rouge à l’oeil nu lorsque, dans la
lunette, on ne lui voyait aucune tache azurée notable’.
This association of the faintness of the albedo markings with the degree of yellowness of the disk has been noticed ever since, either telescopically or even with
the naked eye. To one of us engaged in cataloguing dust storms, there is no doubt
that this was an early example of activity over Mare Cimmerium and Eridania, as
shown, for example, in the published versions of the drawings in Fig. 6.3. In his
catalogue of telescopic storms, McKim wrote (McKim 1999):
On 1858 June 18 (CML = 262°) he first saw a shadowy vertical streak near the p. limb, this
being a manifestation of the Cyclopia development: in its neighbourhood the planet was
yellow in contrast to the normal redness elsewhere …. On June 20 (CML = 249°) a dark
marking that was present towards the Np. limb on the 18th, and which should then have
been nearer the CM, was covered by a light cloud whose W. end was approximately upon
the meridian. Further, the dark f. tip of Mare Cimmerium that had been visible on the 18th
was apparently obscured. On the 24th, with the CML at 215°, the Cyclopia extension is
recognisable near the CM, but Mare Cimmerium is extremely obscure. Although Secchi
notes that this latter observation was made through a thick terrestrial atmosphere, the
strangeness of the drawing is significant (drawings for June 18-20-24 are reproduced
here). …. On June 27 he writes of a yellow patch preceding the isthmus: presumably the
Cyclopia development. If this interpretation is correct, dust activity was still apparent. On
July 1–2 Mars was redder than usual to the naked eye but showed few markings. These later
drawings are not reproduced in Secchi’s Memoir, but the writer’s examination of colour
photographs of the original pastels ……. has shown that the markings in the south were
very pale during June 29–July 5 (CML = 103–162 °), before the Mare Erythraeum-Aurorae
Sinus regions were placed centrally at the hour of observation, from July 7. But the
conditions then were not very good, and Secchi’s delicate (and slightly faded) portrayals of
the foreshortened Mare Sirenum-Solis Lacus region seem quite conventional under the
circumstances.
During the next presentation of the longitudes of the possible dust storm, the dark f. end of
M. Cimmerium was sketched clearly (July 25–27)…….
One has to conclude that there is strong evidence for a regional dust storm during 1858 June
18–27 over Zephyria/Aeolis which affected the longitudes of Mare Cimmerium at least, and
probably also the desert regions to the north. Ls read 169° on June 18.
There have been later examples of Martian dust storms that affected the same
area of the planet. Secchi noticed that the surface markings were not at all welldefined towards the borders of the disk and correctly attributed this to the presence
6 Planetary Observations by Angelo Secchi
closer to a neutral grey or reddish grey. The bluish tint is always more pronounced
with refractors. Furthermore, the degree of blueness in the Syrtis Major is seasonally increased by a scattering effect from the belt of equatorial white cloud that
persists from early northern spring till midsummer, from Ls ~ 0 to 145°. It is often
claimed that Secchi was the first to make out this feature. However, his observing
period was seasonally too late to have received this short-wavelength bonus.
In July, the drawings reveal some obscuration of albedo markings as well as a
yellowness (or lack of redness) of the disk. In discussing the work of Secchi, French
astronomer Camille Flammarion wrote (Flammarion 1892b):
‘On a remarqué que Mars paraissait moins rouge à l’oeil nu lorsque, dans la
lunette, on ne lui voyait aucune tache azurée notable’.
This association of the faintness of the albedo markings with the degree of yellowness of the disk has been noticed ever since, either telescopically or even with
the naked eye. To one of us engaged in cataloguing dust storms, there is no doubt
that this was an early example of activity over Mare Cimmerium and Eridania, as
shown, for example, in the published versions of the drawings in Fig. 6.3. In his
catalogue of telescopic storms, McKim wrote (McKim 1999):
On 1858 June 18 (CML = 262°) he first saw a shadowy vertical streak near the p. limb, this
being a manifestation of the Cyclopia development: in its neighbourhood the planet was
yellow in contrast to the normal redness elsewhere …. On June 20 (CML = 249°) a dark
marking that was present towards the Np. limb on the 18th, and which should then have
been nearer the CM, was covered by a light cloud whose W. end was approximately upon
the meridian. Further, the dark f. tip of Mare Cimmerium that had been visible on the 18th
was apparently obscured. On the 24th, with the CML at 215°, the Cyclopia extension is
recognisable near the CM, but Mare Cimmerium is extremely obscure. Although Secchi
notes that this latter observation was made through a thick terrestrial atmosphere, the
strangeness of the drawing is significant (drawings for June 18-20-24 are reproduced
here). …. On June 27 he writes of a yellow patch preceding the isthmus: presumably the
Cyclopia development. If this interpretation is correct, dust activity was still apparent. On
July 1–2 Mars was redder than usual to the naked eye but showed few markings. These later
drawings are not reproduced in Secchi’s Memoir, but the writer’s examination of colour
photographs of the original pastels ……. has shown that the markings in the south were
very pale during June 29–July 5 (CML = 103–162 °), before the Mare Erythraeum-Aurorae
Sinus regions were placed centrally at the hour of observation, from July 7. But the
conditions then were not very good, and Secchi’s delicate (and slightly faded) portrayals of
the foreshortened Mare Sirenum-Solis Lacus region seem quite conventional under the
circumstances.
During the next presentation of the longitudes of the possible dust storm, the dark f. end of
M. Cimmerium was sketched clearly (July 25–27)…….
One has to conclude that there is strong evidence for a regional dust storm during 1858 June
18–27 over Zephyria/Aeolis which affected the longitudes of Mare Cimmerium at least, and
probably also the desert regions to the north. Ls read 169° on June 18.
There have been later examples of Martian dust storms that affected the same
area of the planet. Secchi noticed that the surface markings were not at all welldefined towards the borders of the disk and correctly attributed this to the presence
6 Planetary Observations by Angelo Secchi