Abstract

In the late 1960s it was hypothesized that Vikings had been able to navigate the open seas, even when the sun was occluded by clouds or below the sea horizon, by using the angle of polarization of skylight. To detect the direction of skylight polarization, they were thought to have made use of birefringent crystals, called “sunstones,” and a large part of the scientific community still firmly believe that Vikings were capable of polarimetric navigation. However, there are some critics who treat the usefulness of skylight polarization for orientation under partly cloudy or twilight conditions with extreme skepticism. One of their counterarguments has been the assumption that solar positions or solar azimuth directions could be estimated quite accurately by the naked eye, even if the sun was behind clouds or below the sea horizon. Thus under partly cloudy or twilight conditions there might have been no serious need for a polarimetric method to determine the position of the sun. The aim of our study was to test quantitatively the validity of this qualitative counterargument. In our psychophysical laboratory experiments, test subjects were confronted with numerous 180° field-of-view color photographs of partly cloudy skies with the sun occluded by clouds or of twilight skies with the sun below the horizon. The task of the subjects was to guess the position or the azimuth direction of the invisible sun with the naked eye. We calculated means and standard deviations of the estimated solar positions and azimuth angles to characterize the accuracy of the visual sun location. Our data do not support the common belief that the invisible sun can be located quite accurately from the celestial brightness and/or color patterns under cloudy or twilight conditions. Although our results underestimate the accuracy of visual sun location by experienced Viking navigators, the mentioned counterargument cannot be taken seriously as a valid criticism of the theory of the alleged polarimetric Viking navigation. Our results, however, do not bear on the polarimetric theory itself.

© 2005 Optical Society of America

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References

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  1. T. Ramskou, “Solstenen,” Skalk 2, 16–17 (1967).
  2. T. Ramskou, Solstenen—Primitiv Navigation I Norden for Kompasset (Rhodos, Kobenhavn, 1969).
  3. J. H. P. Barfod, “Navigation,” Kulturhistoriskt Lexikon Nordisk Medeltid 12, 260–263 (1967).
  4. H. LaFay, “The Vikings,” Natl. Geogr. 137, 492–541 (1970).
  5. A. L. Binns, “Sun navigation in the viking age, and the Canterbury portable sundial,” Act. Archaeol. 42, 23–34 (1971).
  6. W. Britton, “The Britton Viking sun-stone expedition,” Nutrition Today, May/June 1972, pp. 14–23.
  7. U. Schnall, “Navigation der Wikinger,” Schr. Deutsch. Schiffahrtsmuseums 6, 92–115 (1975).
  8. R. Wehner, “Polarized-light navigation by insects,” Sci. Am. 235(7), 106–115 (1976).
    [CrossRef] [PubMed]
  9. J. Walker, “More about polarizers and how to use them, particularly for studying polarized sky light,” Sci. Am. 238(1), 132–136 (1978).
    [CrossRef]
  10. G. P. Können, Polarized Light in Nature (Cambridge U. Press, Cambridge, UK, 1985).
  11. W. H. McGrath, “The stars look down,” Navig. News 3, May/June 1991, pp. 14–15.
  12. S. Thirslund, Viking Navigation: Sun-Compass Guided Norsemen First to America (Hummelbaek, Denmark, 1997).
  13. C. Roslund, C. Beckman, “Disputing Viking navigation by polarized skylight,” Appl. Opt. 33, 4754–4755 (1994).
    [CrossRef] [PubMed]
  14. B. E. Schaefer, “Vikings and polarization sundials,” Sky Telesc., May 1997, pp. 91–94.
  15. J. Gál, G. Horváth, V. B. Meyer-Rochow, R. Wehner, “Polarization patterns of the summer sky and its neutral points measured by full-sky imaging polarimetry in Finnish Lapland north of the Arctic Circle,” Proc. R. Soc. London Ser. A 457, 1385–1399 (2001).
    [CrossRef]
  16. M. Minnaert, Light and Color in the Open Air (Bell, London, 1940).

2001 (1)

J. Gál, G. Horváth, V. B. Meyer-Rochow, R. Wehner, “Polarization patterns of the summer sky and its neutral points measured by full-sky imaging polarimetry in Finnish Lapland north of the Arctic Circle,” Proc. R. Soc. London Ser. A 457, 1385–1399 (2001).
[CrossRef]

1994 (1)

1978 (1)

J. Walker, “More about polarizers and how to use them, particularly for studying polarized sky light,” Sci. Am. 238(1), 132–136 (1978).
[CrossRef]

1976 (1)

R. Wehner, “Polarized-light navigation by insects,” Sci. Am. 235(7), 106–115 (1976).
[CrossRef] [PubMed]

1975 (1)

U. Schnall, “Navigation der Wikinger,” Schr. Deutsch. Schiffahrtsmuseums 6, 92–115 (1975).

1971 (1)

A. L. Binns, “Sun navigation in the viking age, and the Canterbury portable sundial,” Act. Archaeol. 42, 23–34 (1971).

1970 (1)

H. LaFay, “The Vikings,” Natl. Geogr. 137, 492–541 (1970).

1967 (2)

T. Ramskou, “Solstenen,” Skalk 2, 16–17 (1967).

J. H. P. Barfod, “Navigation,” Kulturhistoriskt Lexikon Nordisk Medeltid 12, 260–263 (1967).

Barfod, J. H. P.

J. H. P. Barfod, “Navigation,” Kulturhistoriskt Lexikon Nordisk Medeltid 12, 260–263 (1967).

Beckman, C.

Binns, A. L.

A. L. Binns, “Sun navigation in the viking age, and the Canterbury portable sundial,” Act. Archaeol. 42, 23–34 (1971).

Britton, W.

W. Britton, “The Britton Viking sun-stone expedition,” Nutrition Today, May/June 1972, pp. 14–23.

Gál, J.

J. Gál, G. Horváth, V. B. Meyer-Rochow, R. Wehner, “Polarization patterns of the summer sky and its neutral points measured by full-sky imaging polarimetry in Finnish Lapland north of the Arctic Circle,” Proc. R. Soc. London Ser. A 457, 1385–1399 (2001).
[CrossRef]

Horváth, G.

J. Gál, G. Horváth, V. B. Meyer-Rochow, R. Wehner, “Polarization patterns of the summer sky and its neutral points measured by full-sky imaging polarimetry in Finnish Lapland north of the Arctic Circle,” Proc. R. Soc. London Ser. A 457, 1385–1399 (2001).
[CrossRef]

Können, G. P.

G. P. Können, Polarized Light in Nature (Cambridge U. Press, Cambridge, UK, 1985).

LaFay, H.

H. LaFay, “The Vikings,” Natl. Geogr. 137, 492–541 (1970).

McGrath, W. H.

W. H. McGrath, “The stars look down,” Navig. News 3, May/June 1991, pp. 14–15.

Meyer-Rochow, V. B.

J. Gál, G. Horváth, V. B. Meyer-Rochow, R. Wehner, “Polarization patterns of the summer sky and its neutral points measured by full-sky imaging polarimetry in Finnish Lapland north of the Arctic Circle,” Proc. R. Soc. London Ser. A 457, 1385–1399 (2001).
[CrossRef]

Minnaert, M.

M. Minnaert, Light and Color in the Open Air (Bell, London, 1940).

Ramskou, T.

T. Ramskou, “Solstenen,” Skalk 2, 16–17 (1967).

T. Ramskou, Solstenen—Primitiv Navigation I Norden for Kompasset (Rhodos, Kobenhavn, 1969).

Roslund, C.

Schaefer, B. E.

B. E. Schaefer, “Vikings and polarization sundials,” Sky Telesc., May 1997, pp. 91–94.

Schnall, U.

U. Schnall, “Navigation der Wikinger,” Schr. Deutsch. Schiffahrtsmuseums 6, 92–115 (1975).

Thirslund, S.

S. Thirslund, Viking Navigation: Sun-Compass Guided Norsemen First to America (Hummelbaek, Denmark, 1997).

Walker, J.

J. Walker, “More about polarizers and how to use them, particularly for studying polarized sky light,” Sci. Am. 238(1), 132–136 (1978).
[CrossRef]

Wehner, R.

J. Gál, G. Horváth, V. B. Meyer-Rochow, R. Wehner, “Polarization patterns of the summer sky and its neutral points measured by full-sky imaging polarimetry in Finnish Lapland north of the Arctic Circle,” Proc. R. Soc. London Ser. A 457, 1385–1399 (2001).
[CrossRef]

R. Wehner, “Polarized-light navigation by insects,” Sci. Am. 235(7), 106–115 (1976).
[CrossRef] [PubMed]

Act. Archaeol. (1)

A. L. Binns, “Sun navigation in the viking age, and the Canterbury portable sundial,” Act. Archaeol. 42, 23–34 (1971).

Appl. Opt. (1)

Kulturhistoriskt Lexikon Nordisk Medeltid (1)

J. H. P. Barfod, “Navigation,” Kulturhistoriskt Lexikon Nordisk Medeltid 12, 260–263 (1967).

Natl. Geogr. (1)

H. LaFay, “The Vikings,” Natl. Geogr. 137, 492–541 (1970).

Proc. R. Soc. London Ser. A (1)

J. Gál, G. Horváth, V. B. Meyer-Rochow, R. Wehner, “Polarization patterns of the summer sky and its neutral points measured by full-sky imaging polarimetry in Finnish Lapland north of the Arctic Circle,” Proc. R. Soc. London Ser. A 457, 1385–1399 (2001).
[CrossRef]

Schr. Deutsch. Schiffahrtsmuseums (1)

U. Schnall, “Navigation der Wikinger,” Schr. Deutsch. Schiffahrtsmuseums 6, 92–115 (1975).

Sci. Am. (2)

R. Wehner, “Polarized-light navigation by insects,” Sci. Am. 235(7), 106–115 (1976).
[CrossRef] [PubMed]

J. Walker, “More about polarizers and how to use them, particularly for studying polarized sky light,” Sci. Am. 238(1), 132–136 (1978).
[CrossRef]

Skalk (1)

T. Ramskou, “Solstenen,” Skalk 2, 16–17 (1967).

Other (7)

T. Ramskou, Solstenen—Primitiv Navigation I Norden for Kompasset (Rhodos, Kobenhavn, 1969).

G. P. Können, Polarized Light in Nature (Cambridge U. Press, Cambridge, UK, 1985).

W. H. McGrath, “The stars look down,” Navig. News 3, May/June 1991, pp. 14–15.

S. Thirslund, Viking Navigation: Sun-Compass Guided Norsemen First to America (Hummelbaek, Denmark, 1997).

B. E. Schaefer, “Vikings and polarization sundials,” Sky Telesc., May 1997, pp. 91–94.

M. Minnaert, Light and Color in the Open Air (Bell, London, 1940).

W. Britton, “The Britton Viking sun-stone expedition,” Nutrition Today, May/June 1972, pp. 14–23.

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