Evaluating visibility at sea: Instrumental data and historical nautical records. Mount Etna from the Calabrian Ionian coast (Italy)

Abstract

Visibility has an encompassing importance in humans’ perception of the landscape, since the first encounter with a new environment normally occurs through sight. In historical and archaeological studies, two main methods (i.e., the geometric method and the Geographical Information System [GIS] computation) have been employed to determine the distance from which an object can be recognized. However, neither is exhaustive when applied to a maritime context, where the main factor affecting the visibility radius is weather. To establish how far at sea an object can be seen, and how its visibility would have changed in different weather conditions, we adopted a method from Aerosol Optics based on a well-established mathematical model of the light scattering phenomena. We applied this method to compute the visibility radius in historical studies. To demonstrate its application, we choose to examine the visibility of a key point in both historical and current seafaring, namely Mount Etna (Sicily, Italy), from the Ionian coast of Calabria (Italy). The results obtained by the application of this method have been validated by comparing them with mentions of Mount Etna in both written sources and on-the-ground records.

KEYWORDS:

• Visibility
• aerosols optics
• seafaring
• ethnoarchaeology/experimental
• coastal
• Western Europe

Acknowledgements

We would like to thank Stefano Corradini and Stefano Pignatti for their efforts in establishing and maintaining ETNA and MESSINA observatories used in this investigation. Furthermore, we have been fortunate to receive, at an earlier stage of this project, Marcos Llobera’s helpful comments and input; his suggestions have been greatly appreciated and contributed in a significant way to the improvement of this manuscript. We would like to express our appreciation to the Editor, Scott M. Fitzpatrick, and the referees for their thorough reading of the paper, helpful commentaries and suggestions, and for pointing out relevant typos and mistakes; and to Rocio Pérez-Campaña (UCM) for helping us with the viewshed analysis. Lastly, our gratitude goes to all the local fishermen from Lazzaro di Motta San Giovanni (Antonino Arecchio and his daughter Giada Arecchio) and Palizzi (“Siso” Polimeno, Domenico and Franco Rosato) who have actively contributed to this study.

Notes

1 Especially in comparison to larger glacial-interglacial changes (Wanner et al. 2008). Despite that, it is necessary to underline that some climate fluctuations did occur within the Holocene (Di Rita et al. 2018, and references therein) and that they should probably be connected to Holocene cold events over the North Atlantic (Brayshaw, Rambeau, and Smith 2011; Sabatier et al. 2012).

2 Analogous considerations were independently reached by Brugge (2017), who has preliminarily presented a different model based on (not open access) measures obtained with a transmissometer instead than with a solarimeter, as our proposal does.

3 With “direct visibility” we refer to the possibility of recognizing Mount Etna’s outline against the background. This concept differs from “indirect visibility,” which refers to the possibility of identifying features (in this case: peak covered with snow, sheets of lava, smoke) that can be used to assume the presence of an object (in this case, Mount Etna).

4 $d\approx 3.57\sqrt{h}$ . It is necessary to stress that this formula does not consider any correction due to imperfect roundness of the Earth (Arnaud 1993), nor the effect of atmospheric refraction. Currently, several GIS software (e.g., QGIS, ArcGIS, and GRASS GIS) incorporate corrections for the curvature of the Earth and for the refractivity coefficient of light (see below).

5 The default value for the Rrefr in ArcGis is 0.13, which is considered suitable to represent standard atmospheric pressure during daytime conditions with a limpid sky for locations whose height ranges between 40 and 100 m (Yoeli 1985, 93). To factor different atmospheric conditions on visibility, other Rrefr values can be used (comprised between 0.0 and 1.0).

6 E.g., see Portulano del mar Mediterráneo o vero rottero alla spagnola, 16th century, Biblioteca Nacional de España, Mss/1072, 37; Derrotero general del Mediterráneo Tomo II, 1858, Biblioteca Nacional de España, GMM/2660, 137–138; Derrotero y navegación del Mar Mediterráneo, 18th century, Biblioteca Nacional de España, Mss/9023, 34–35.

7 Capo Spartivento: lighthouse built in 1952. ARLHS ITA-043; EF-3384; Admiralty E1782; NGA 10580. See also Mediterranean Pilot (1978, 246). Capo dell’Armi: lighthouse built in 1933. ARLHS ITA-014; EF-3380; Admiralty E1780; NGA 9744. See also Mediterranean Pilot (1978, 245).

8 Strab. 4.1.7: “Then comes Heracleium, which is the last cape of Italy and inclines towards the south.”

9 See note 8 and Mediterranean Pilot (1978, 246). On easily recognizable headlands due to their color, see Mauro (2019, 127) and (Morton 2001, 189–190).

10 Strab. 6.1.7: “As one sails from Rhegium towards the east, and at a distance of fifty stadia, one comes to Cape Leucopetra—so called from its color—in which, it is said, the Apennine Mountain terminates.” See also Mediterranean Pilot (1978, 245).

11 Mediterranean Pilot (1978, 229).

12 Theoretical attempts aimed at reconstructing the original height of ancient freestanding towers based on their diameters could be found in Thielemans (1982) and Young (1956). According to them, the height of a freestanding tower was roughly between 2 and 2 ½ times its outer diameter; however, these attempts are based on chronologically and topographically limited studies and, in our opinion, cannot be universally extended.

13 Interestingly, he states that Etna could be seen after the Scylaceum’s shore, a point probably corresponding to Capo Spartivento. Cf. Strab. 6.2.8 (“Etna dominates especially the seaboard in the region of the Strait”).

14 E.g. Liber de Existencia Riveriarum et Forma Maris Nostri Mediterranei, c. 1200 AD (see Mons Gibellus, 1.2222); Viaggio dalla Sicilia a Madrid, 1579, Biblioteca Nazionale Vittorio Emanuele III di Napoli, MS XII.D.43; New Chart of the Mediterranean Sea, 1797, by W. Heather; Carta Esférica que comprende las Islas de Sicilia y Malta construida en la Dirección Hidrográfica, 1835, Biblioteca Nacional de España, MA00090138.

15 Strabo states that the crater is visible on windless days.

16 Ibn Jobayr specifies that the volcano could be seen for more than 100 miles exclusively in "fine weather." His estimation is then more restricted than what the geometric method should imply (see “Area Description”).

17 Bell states: “From Malta Etna is not visible but in the most favourable conditions.”

18 In his letters from Malta and Sicily, Waring states (1843, 104): “I am surprised to find that Mount Etna, though at a distance of about one hundred and ten miles, is distinctly visible from the island in clear weather.” On the other hand, during a sea-travel that he carried out on March from Augusta towards Catania, he writes (1843, 222–223): “The atmosphere was so thick and hazy, that Mount Etna, the grand object of interest, was entirely obscured nearly the whole of the day.”

19 “Sicily, in clear winter days, is visible from Malta, from whence the eruptions of Etna have been seen.” (Leith Adams 1847, 240).

20 A Sirocco’s duration may be as short as half a day or may last several days.

21 Even Theophr. Signs, 20, 36 describes the Sirocco as a dry wind bringing rain and clouds.