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Journal of the Optical Society of America A

Journal of the Optical Society of America A

| OPTICS, IMAGE SCIENCE, AND VISION

  • Editor: Franco Gori
  • Vol. 31, Iss. 7 — Jul. 1, 2014
  • pp: 1645–1656
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Accuracy of sun localization in the second step of sky-polarimetric Viking navigation for north determination: a planetarium experiment

Alexandra Farkas, Dénes Száz, Ádám Egri, Miklós Blahó, András Barta, Dóra Nehéz, Balázs Bernáth, and Gábor Horváth  »View Author Affiliations


JOSA A, Vol. 31, Issue 7, pp. 1645-1656 (2014)
http://dx.doi.org/10.1364/JOSAA.31.001645


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Abstract

It is a widely discussed hypothesis that Viking seafarers might have been able to locate the position of the occluded sun by means of dichroic or birefringent crystals, the mysterious sunstones, with which they could analyze skylight polarization. Although the atmospheric optical prerequisites and certain aspects of the efficiency of this sky-polarimetric Viking navigation have been investigated, the accuracy of the main steps of this method has not been quantitatively examined. To fill in this gap, we present here the results of a planetarium experiment in which we measured the azimuth and elevation errors of localization of the invisible sun. In the planetarium sun localization was performed in two selected celestial points on the basis of the alignments of two small sections of two celestial great circles passing through the sun. In the second step of sky-polarimetric Viking navigation the navigator needed to determine the intersection of two such celestial circles. We found that the position of the sun (solar elevation θS, solar azimuth φS) was estimated with an average error of +0.6°Δθ+8.8° and 3.9°Δφ+2.0°. We also calculated the compass direction error when the estimated sun position is used for orienting with a Viking sun-compass. The northern direction (ωNorth) was determined with an error of 3.34°ΔωNorth+6.29°. The inaccuracy of the second step of this navigation method was high (ΔωNorth=16.3°) when the solar elevation was 5°θS25°, and the two selected celestial points were far from the sun (at angular distances 95°γ1, γ2115°) and each other (125°δ145°). Considering only this second step, the sky-polarimetric navigation could be more accurate in the mid-summer period (June and July), when in the daytime the sun is high above the horizon for long periods. In the spring (and autumn) equinoctial period, alternative methods (using a twilight board, for example) might be more appropriate. Since Viking navigators surely also committed further errors in the first and third steps, the orientation errors presented here underestimate the net error of the whole sky-polarimetric navigation.

© 2014 Optical Society of America

1. INTRODUCTION

  • Step 1 (Fig. 1A): Viking navigators are assumed to have determined the direction of skylight polarization in at least two celestial points with the use of two sunstones to estimate the position of the sun occluded by cloud/fog or being below the horizon. The alleged tools for this task are the mysterious sunstones, the composition and usage of which are unknown, but their high value is sure [7

    7. P. G. Foote, “Icelandic sólarsteinn and the Medieval background,” J. Scandinavian Folklore 12, 26–40 (1956).

    ]. It has been hypothesized that sunstones were birefringent (e.g., calcite) or dichroic (e.g., cordierite or tourmaline) crystals. A recent archaeological artifact raised the possibility that calcite crystals were used for navigation purposes even in the 16th century [8

    8. A. Le Floch, G. Ropars, J. Lucas, S. Wright, T. Davenport, M. Corfield, and M. Harrisson, “The sixteenth century Alderney crystal: a calcite as an efficient reference optical compass?” Proc. R. Soc. A 469, 20120651 (2013). [CrossRef]

    ].
  • Step 2 (Fig. 1B): A short scratch on each sunstone could help the navigator to set two celestial great circles across the two investigated sky points parallel to the scratches being perpendicular to the local direction of skylight polarization. Then the navigator determined the above-horizon intersection of these celestial circles. According to the Rayleigh theory of sky polarization [9

    9. K. L. Coulson, Polarization and Intensity of Light in the Atmosphere (A. Deepak, 1988).

    ], this intersection coincides with the position of the invisible sun.
  • Step 3 (Fig. 1C): Finally, from the estimated position of the invisible sun the navigator derived true compass directions. Since no astronomical charts are known from the Viking area before the 12th century, the method of the third step can only be speculated. Probably, it involved the use of a sun-compass, the fragment of which was found at Uunartoq, for example. The navigator might have determined the direction of the imaginary light rays originating from the invisible sun with a shadow-stick [10

    10. B. Bernáth, M. Blahó, Á. Egri, A. Barta, G. Kriska, and G. Horváth, “Orientation with a Viking sun-compass, a shadow-stick and calcite sunstones under various weather situations,” Appl. Opt. 52, 6185–6194 (2013). [CrossRef]

    ]. Then, he turned the horizontal disk of the sun-compass around its vertical axis of rotation, until the tip of the imaginary shadow of the vertical compass gnomon reached the actual gnomonic line engraved into the disk [5

    5. B. Bernáth, A. Farkas, D. Száz, M. Blahó, Á. Egri, A. Barta, S. Åkesson, and G. Horváth, “How could the Viking sun-compass be used with sunstones before and after sunset? Twilight board as a new interpretation of the Uunartoq artefact fragment,” Proc. R. Soc. A 470, 20130787 (2014). [CrossRef]

    ]. In this situation the mirror symmetry axis of the gnomonic line pointed toward the geographical north.
Fig. 1. Three main steps of sky-polarimetric Viking navigation. A, step 1: estimation of the direction of skylight polarization with sunstones. B, step 2: estimation of the intersection of the two celestial great circles across the selected two sky points and parallel to the scratches on the sunstones. C, step 3: estimation of the northern direction with the Viking sun-compass (left, side view of the sun-compass; right, sun-compass disk seen from above). Further details in the text.

2. MATERIALS AND METHODS

A. Planetarium Experiment

The test persons of our experiment were 11 city-dwelling male volunteers aged between 23 and 63 years. The experiment was performed in the digital planetarium of Eötvös University in Budapest (Hungary) in 2013. This planetarium has a dome diameter of 8 m and uses a fixed central single-lens Digitarium ε projector (Digitalis Education Solutions, Inc., Bremerton, USA) provided with Nightshade 11.12.1 software and a circumferential resolution of 2400 pixels.

The test persons sat in the immediate vicinity (30 cm) of the planetarium projector with their eye level about 5 cm below the projector lens in order to minimize parallax error and to avoid being dazzled by the projector. Two thin (0.6°) black bars with an angular length of 5° were projected in two different points of the white planetarium dome (Figs. 2 and 3). Since the white dome canvas was composed of several sectors, a pale radial pattern with a perpendicular circle formed by the sector junction lines was visible on it. The task of the test persons was (i) to elongate imaginarily (mentally) each bar to a spherical great circle, (ii) to locate the intersection of these two circles, and (iii) to mark the intersection with a green laser pointer on the planetarium dome, which represented the sky dome. The intersection represented the position of the invisible sun, while the black bars modeled the hypothesized short scratches engraved in Viking sunstones and pointing toward the sun along celestial great circles [8

8. A. Le Floch, G. Ropars, J. Lucas, S. Wright, T. Davenport, M. Corfield, and M. Harrisson, “The sixteenth century Alderney crystal: a calcite as an efficient reference optical compass?” Proc. R. Soc. A 469, 20120651 (2013). [CrossRef]

,10

10. B. Bernáth, M. Blahó, Á. Egri, A. Barta, G. Kriska, and G. Horváth, “Orientation with a Viking sun-compass, a shadow-stick and calcite sunstones under various weather situations,” Appl. Opt. 52, 6185–6194 (2013). [CrossRef]

,18

18. G. Ropars, G. Gorre, A. Le Floch, J. Enoch, and V. Lakshminarayanan, “A depolarizer as a possible precise sunstone for Viking navigation by polarized skylight,” Proc. R. Soc. A 468, 671–684 (2012). [CrossRef]

]. The horizontal circular bottom edge of the planetarium dome was by θdome=8° above the eye level of the test persons. Thus, the images were projected onto the dome with an appropriate slight zoom. Although the test persons had the possibility to point the green laser spot on the vertical cylindrical wall under the dome bottom, green spots projected below eye level were not accepted; i.e., we tested only situations in which the sun was above the horizon. After estimating the intersection of the two great circles (i.e., the sun position), the test persons did not see the correct solution (the correct position of the invisible sun). Thus, they were not influenced by any information feedback.

Fig. 2. Geometry of our planetarium experiment with the Digitarium ε projector in the center, the test person, the fisheye camera, and the two black bars projected onto the white planetarium dome with a diameter d=8m. The sun position to be guessed by the test person is at the above-horizon intersection of the two celestial great circles 1 and 2 passing through and parallel to the black bars 1 and 2. There are four free parameters: solar elevation angle θS, angular distances γ1 and γ2 of the center of bars 1 and 2 from the sun, and angle δ between the planes of circles 1 and 2. The test person marked the estimated sun position with a green laser spot. Δφ and Δθ are the azimuth error and the elevation error, respectively, between the real and estimated sun positions. The bottom of the dome was at θdome=8° above the real horizon being at the eye level of the test person.
Fig. 3. A, the test person sat immediately next to the planetarium projector almost at the center of the hemispherical dome. He was instructed by the experiment leader (i) to observe the two black bars projected onto the white dome, (ii) to estimate the position of the invisible sun (i.e., the intersection of the two great circles passing through and parallel to the bars), and (iii) to mark the position of the estimated intersection with a green laser spot. The instructor of the experiment photographed the dome by a fisheye camera with a vertical optical axis. The pale radial pattern (composed of a circular and 24 radial thin white lines) visible on the white dome originated from the structure of the canvas composed of several sectors. B, example for the image projected onto the planetarium dome showing a measurement situation with the two black bars, and the three (red, blue, violet) calibration spots at an elevation θ=10°. The number under the blue spot codes the image number of the script file. In this example, the values of the four free parameters of the black bars were θS=41°, γ1=84°, γ2=50°, and δ=129°. C, the azimuth error Δφ and the elevation error Δθ between the real sun position (white dot) and the estimated sun position (green dot) were calculated after the undistortion and appropriate rotation of the photographed calibration image (PCI). D, the sun is positioned at the intersection (φS=0, θ=θS). The arcs of the two great circles across and parallel to black bars 1 and 2 were not projected during the tests.

Besides the two black bars, a red, a blue, and a violet calibration spot were also projected on the planetarium dome (Fig. 3). They were used later as beacons in the evaluation process. These colored spots were projected at elevation θ=10° with φ=120° azimuth angle difference between the neighboring spots. The calibration spots could not provide assistance for the test persons, because they were projected onto the dome with a random azimuth rotation.

We classified the measurement situations according to the following four free parameters (Fig. 2): (1) solar elevation angle θS, (2), (3) angular distance γ1 and γ2 of the center of bar 1 and bar 2 from the sun (intersection of the two great circles going across and parallel to the two bars), and (4) angle δ between the planes of the two great circles. The values of these four parameters were divided into ranges of 30°. The parameter values were chosen from the middle 20° interval of each range to make the ranges more separable from each other. Only realistic range combinations were used, which could really occur at the geographical areas where Vikings lived (Table 1). We generated situations for the solar elevation angle θS from 5°–25° to 35°–55°, because θS>55° does not occur along the 61° northern latitude, where Vikings regularly sailed between Scandinavia and Greenland (the highest possible solar elevation angle at the 61° northern latitude is θS=52.5°, if refraction is neglected). Because of the Arago, Babinet, and Brewster neutral (unpolarized) points of the sky [9

9. K. L. Coulson, Polarization and Intensity of Light in the Atmosphere (A. Deepak, 1988).

,19

19. G. P. Können, Polarized Light in Nature (Cambridge University, 1985).

21

21. G. Horváth and D. Varjú, Polarized Light in Animal Vision—Polarization Patterns in Nature (Springer Verlag, 2004).

], the measurement of skylight polarization with sunstones is impossible at and around the sun and antisolar point. Therefore, we omitted situations with γ1, γ2<30° and γ1, γ2>150°. We also avoided situations with 120°<γ1, γ2<150°, because in the case of higher solar elevations the projected black bars would have been positioned below the horizon. By generating random settings of the four free parameters (θS, γ1, γ2, δ) within the ranges defined in Table 1, we obtained 48 parameter configurations. Using our custom-made computer program, 240 individual situations were created in stereographic bitmap images with 2048×2048 pixel resolution by generating five images for all 48 configurations. A circle with a radius of 1024 pixels represented the horizon, and the center of this circle corresponded to the zenith in the planetarium dome. The azimuth angle φ and the elevation angle θ of an arbitrary point with a position vector p on the dome were defined in the following way:
φ={arctan((o̲p̲)y(o̲p̲)x),if(o̲p̲)x>0arctan((o̲p̲)y(o̲p̲)x)+π,if(o̲p̲)y0,(o̲p̲)x<0arctan((o̲p̲)y(o̲p̲)x)π,  if(o̲p̲)y<0,(o̲p̲)x<0+π2,if(o̲p̲)y>0,(o̲p̲)x=0phiv2,if(o̲p̲)y<0,(o̲p̲)x=0undefined,if(o̲p̲)y=0,(o̲p̲)x=0,
(1)
θ=90°|o̲p̲|r·90°,
(2)
where o̲ is the position vector of the image center, and r is the image radius. During the image generation process we first defined the position of the sun with a given solar elevation angle θS. The images were projected onto the planetarium dome with a randomized azimuth rotation; therefore we defined the azimuth angle of the sun as φS=0. Then, two celestial great circles with an angle δ between their planes were considered, which intersected each other at the sun position to be guessed by the test persons (Figs. 1B, 2, and 3D). The positions of the black bars were calculated on the basis of their chosen angular distances γ1 and γ2 from the sun. Finally, the 5° segments of the two great circles were drawn at the centers of the two bars.

Table 1. Ranges of the Solar Elevation Angle θS, the Angular Distances γ1 and γ2 between the Sun and Two Selected Celestial Points, and the Angle δ Enclosed by the Planes of the Two Celestial Great Circles Passing through the Two Celestial Points Parallel to Two Black Bars Projected onto the White Planetarium Dome in Our Experimenta

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The generated images were projected onto the planetarium dome using the StratoScript 2.2 scripting language. We presented the same 240 situations to all 11 test persons in a randomized order and with a random azimuth rotation. This resulted in 2640 individual estimations of the position of the invisible sun. All test persons were provided with a total of six series of 40 situations, and were allowed to survey a maximum of two series on a single day with 20 min of intermission.

B. Data Registration and Evaluation

Fig. 4. Undistortion procedure of the 180° field-of-view photographs taken about the planetarium dome. A, Calibration image (CI) projected before each script with specific colored spots placed on the grid points. B, slightly off-axis and off-center photograph of the dome with the photographed calibration image (PCI). Recognition of the appropriate grid point pairs was performed with the help of the colored spots that mark the grid points. C, the black and gray nets represent the grid in the CI and PCI, respectively. The recognized vectors of the grid points were used to calculate the correct pixel colors for each pixel: the v̲ vector is interpolated from the bracketing u̲i,j, u̲i+1,j, u̲i,j+1, and u̲i+1,j+1 vectors. D, result of the undistortion procedure performed on the PCI.

Spots with a diameter of 9 pixels and specific colors were manually placed on the grid points of both CI and PCI. A self-developed software recognized these spots and calculated the coordinates of their centers. A so-called undistortion vector v̲ was calculated between the spots on the CI and their counterparts on the PCI for each pair by subtracting all of the spots’ center positions on the PCI from those of the counterparts on the CI. This produced an ui̲,j (i=0,,16 azimuth index; j=0,,18 elevation index) array with Δφ=22.5° and Δθ=5° azimuth and elevation resolution, respectively. For arbitrary (φ, θ) pairs the undistortion vector v̲ was calculated with linear interpolation:
v̲=(1φ˜)·(1θ˜)·u̲i,j+(1φ˜)·θ˜·u̲i,j+1+φ˜·(1θ˜)·u̲i+1,j+φ˜·θ˜·u̲i+1,j+1,φ˜=ϕi·ΔφΔφ,θ˜=θj·ΔθΔθ,
(3)
where u̲i,j, u̲i+1,j, u̲i,j+1, and u̲i+1,j+1 are the four vectors in the calibration array (Fig. 4C). Hence, we defined a map for the undistortion as a bilinear interpolation of translation vectors obtained in the nodes of the radial calibration pattern. Note that there are more accurate methods to perform such a calibration (e.g., through determining radial distortion of both the projector and camera and calculating a projection matrix that translates between the undistorted, projected sky hemisphere to the slightly decentered view seen by the camera), but the accuracy of our method is good enough compared to the average measure of inaccuracies regarding the inferred sun position and northern direction.

After obtaining the ui̲,j array, the same was performed for all fisheye photographs taken during a given experimental session in order to construct the undistorted images: if the current pixel distance from the image center was smaller than r=1024 (=image radius), the azimuth and elevation angles (φ, θ) of the given pixel were calculated from Eqs. (1) and (2). Then the v̲ vector was interpolated from the calibration array of the u̲i,j vectors [Eq. (3)]. Finally, the coordinates of the pixel, which had the correct color on the photograph, were calculated by summarizing the position of the given pixel and the v̲ vector of the interpolation. The red, green, and blue values were read out of there and were loaded into the new image at the given pixel position. The two lowest horizontal circles of the CI corresponding to θ=0° (j=0) and 5° (j=1) were out of the spherical dome and could not be photographed. For pixels with an elevation lower than 10°, extrapolation was used instead of interpolation. (Figure 4D shows the result of such an undistortion on the CI itself.) With this calibration method the result of the undistorted fisheye photograph of any arbitrary image was practically identical to the original (Figs. 4A and 4D) with an accuracy of maximum 2 pixels (2/2048×180°=0.18°).

All situations with the green laser spot positioned by the test person on the planetarium dome were photographed, and these photos were transformed to be undistorted as described above. Then, the red, violet, and blue calibration spots were detected both on the photograph and the original image. The average azimuth differences φcal between the azimuths of the calibration spots with the same colors on both images were calculated. The whole image was rotated by φcal to overlap with the originally generated image, which was projected in the corresponding situation. Thereafter the green laser spot was detected, and its (φ, θ) coordinates were calculated. Then the azimuth error Δφ=φφS and the elevation error Δθ=θθS of sun localization were obtained. For each test person and situation the four free parameters (θS, γ1, γ2, δ) and the values of Δφ and Δθ were saved. The distributions of Δφ and Δθ under various measurement situations were analyzed by circular statistics [22

22. E. Batschelet, Circular Statistics in Biology (Academic, 1981).

]. The direction and the length R of the average vector of the azimuth and elevation errors Δφ and Δθ were calculated. Their dispersion was defined as 1R [22

22. E. Batschelet, Circular Statistics in Biology (Academic, 1981).

].

C. Accuracy of Estimating the Compass Direction

Using the azimuth and elevation errors Δφ and Δθ of the estimated sun position (φS, θS), we calculated the error ΔωNorth of the estimated northern direction relative to the true north ωNorth. If there were no sun localization errors (Δφ=0, Δθ=0), the tip of the gnomon shadow would fall on the appropriate gnomonic line engraved in the disk of the Viking sun-compass, and the mirror symmetry axis of the gnomonic line would point toward the geographic north (Fig. 1C). Because of an inaccurately estimated sun position (Δφ0, Δθ0), the shadow tip does not fall on the gnomonic line. In this case the sun-compass disk should be rotated by angle ΔωNorth around its vertical axis in order that the shadow tip falls on the gnomonic line (Fig. 1C). ΔωNorth was calculated for all measurement situations and the 11 test persons for gnomonic lines valid at spring equinox (21 March) and summer solstice (21 June) at the 61° northern latitude. These gnomonic lines were calculated with the program developed by Bernáth et al. [4

4. B. Bernáth, M. Blahó, Á. Egri, A. Barta, and G. Horváth, “An alternative interpretation of the Viking sundial artefact: an instrument to determine latitude and local noon,” Proc. R. Soc. A 469, 20130021 (2013). [CrossRef]

]. The position of the tip of the gnomon shadow on the horizontal sun-compass disk was calculated with a self-developed program. Since a given solar elevation angle θS can occur twice a day (in the morning and in the afternoon), we calculated two different values of ΔωNorth for a given measurement situation. We characterized the ΔωNorth values under various measurement situations by circular statistics [22

22. E. Batschelet, Circular Statistics in Biology (Academic, 1981).

]. We did not calculate ΔωNorth for the equinox in measurement situations with high solar elevation angles θS, which never occur at the 61° latitude in this period.

3. RESULTS

Figure 5 shows the azimuth errors Δφ (Fig. 5A) and elevation errors Δθ (Fig. 5B) of 2640 measurements performed with the 11 test persons. The solar azimuth angle φS was estimated with an average error Δφaverage=0.13° (Fig. 5A). In some cases the test persons located the antisolar point instead of the sun; therefore there are also some data points around Δφ=±180° in Figs. 5A and 5C. According to Fig. 5B, the solar elevation angle θS was estimated with an average error Δθaverage=+4.47° with a clear tendency of overestimation.

Fig. 5. A, B, circular histograms (radial, number of cases N; azimuthal, angle, bin width=2°) of the azimuth errors Δφ and elevation errors Δθ of sun localization by 11 test persons in our planetarium experiment. Black arrows show the directions of the average error vectors, the length of which is R. C, plot of Δθ versus Δφ showing the scatter of both errors.

Figures 6A and 6B show the averages Δφaverage and Δθaverage of the azimuth errors Δφ and elevation errors Δθ of the 11 test persons studied. The directions and lengths R of the average vectors Δφaverage and Δθaverage are summarized in Table 2. The most accurate solar azimuth estimation was performed by test person 10 with Δφaverage=0.14°, but he detected the sun with a relatively high dispersion 1Razimuth=0.392. The least accurate solar azimuth estimation was performed by test person 6 with Δφaverage=3.86°, and test person 11 with Δθaverage=8.81° was the least accurate in the estimation of solar elevation. The smallest and largest azimuth dispersion 1Razimuth was 0.119 and 0.427, respectively. According to Fig. 6B and Table 2, the most accurate solar elevation estimation was performed with an error of Δθaverage=0.58°. The smallest and largest elevation dispersion 1Relevation was 0.010 and 0.028, respectively.

Fig. 6. A, Average vectors of the azimuth error Δφ and B, elevation error Δθ of the 11 test persons studied in our experiment. The identity numbers of the test persons are at the corresponding arrow heads. R, vector length.

Table 2. Average Azimuth Error Δφaverage, Average Elevation Error Δθaverage, Average Compass Direction Error ΔωNorth, and the Length R of These Error Vectors Measured for the 11 Test Persons in Our Experiment for Spring Equinox (21 March) and Summer Solstice (21 June)a

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The direction and length R of the compass direction error ΔωNorth of the 11 test persons for the spring equinox (21 March) and summer solstice (21 June) are also summarized in Table 2. We found that the values of ΔωNorth were smaller at solstice than at equinox. In other words, the test persons could determine the northern direction more accurately at solstice. In the estimation of the northern direction the smallest error was ΔωNorth=0.06° (with 1R=0.283) at solstice, and ΔωNorth=0.05° (with 1R=0.671) at equinox. Not surprisingly, the smallest dispersion was found at both solstice (1R=0.282) and equinox (1R=0.569) for that test person (No. 5) who also estimated the solar azimuth and elevation with the lowest dispersion.

Figure 7 shows the average vectors of the compass direction error ΔωNorth in all measurement situations for spring equinox and summer solstice calculated from the azimuth and elevation errors Δφ and Δθ of the 11 test persons. The direction and length R of the average compass direction errors are summarized in Table 3. At solstice, the most accurate north determination was performed for the following parameter configurations: 35°θS55°, 95°δ115°, 35°γ155°, and 35°γ255° with ΔωNorth=0.2°. At equinox, the most exact north determination was achieved for 5°θS25°, 65°δ85°, 35°γ155°, and 35°γ255° with ΔωNorth=0.1°. The measurement situation 35°θS55°, 95°δ115°, 35°γ155°, and 65°γ285° was characterized by the smallest dispersion 1R=0.16 at solstice. Generally, the dispersion values were the largest when the sun elevation was 5°θS25° at equinox. The length R of the average compass error vector was the highest (R=0.16) at equinox in the measurement situation 5°θS25°, 125°δ145°, 65°γ185°, and 95°γ2115°. The following are clearly visible in Fig. 7: (i) north determination was more accurate at solstice, (ii) north estimation was less accurate when γ1, γ2, and δ increased, and (iii) north determination was more precise when the solar elevation angle was 35oθS55°.

Fig. 7. Average vectors of the compass direction error ΔωNorth in all measurement situations at spring equinox (21 March) and summer solstice (21 June) at the 61° latitude calculated from the azimuth and elevation errors Δφ and Δθ of the 11 test persons studied. Black arrows show the directions of the average error vectors ΔωNorth. The radius of the semicircles is proportional to the length R of the average vectors. The average vectors of ΔωNorth belonging to spring equinox are rotated by 180° for the sake of a better visualization.

Table 3. Average Compass Direction Errors ΔωNorth and the Length R of these Error Vectors Measured in All Measurement Situations for Spring Equinox (21 March) and Summer Solstice (21 June)a

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4. DISCUSSION

The compass direction error ΔωNorth caused by the inaccurate estimation of the sun position depends on the method of deriving the geographical north. If this method is based on fitting the gnomon’s shadow tip to a gnomonic line on the Viking sun-compass (Fig. 1C), ΔωNorth is greatly influenced by the geometrical arrangement of the shadow and the gnomonic line (Fig. 8). Identical sun position errors result in different distances between the estimated shadow tip and the gnomonic line. At higher solar elevations the linear equinox line (valid on 21 March) is farther from the shadow tip than the hyperbolic solstice line (valid on 21 June); thus smaller ΔωNorth values occur in summer than in spring. The relationship is the opposite at lower solar elevations.

Fig. 8. On a precisely orientated Viking sun-compass the gnomon’s shadow tip Sn (n=1, 2, 3, 4, 5) falls on the gnomonic line (thick solid lines), the mirror symmetry axis of which (dashed line) marks the true north. The estimated shadow tip Sn (n=1, 2, 3, 4, 5) is off the gnomonic line; thus the compass should be rotated along its vertical axis by an angle ΔωNorthPnGSn (called the compass direction error) between radii GPn and GSn in order to move Sn to the gnomonic point Pn. If all the angles SnGSn are identical, angle PnGSn is proportional to the angles enclosed by the shadow GSn and the gnomonic line marked by single arcs for the spring equinox (A, 21 March) and by double arcs for the summer solstice (B, 21 June). At solstice, rotating the sun-compass by ΔωNorth valid for equinox (P1GS1 or P3GS3) moves the estimated shadow tip (S2 and S4) off the gnomonic line (P1 and P3). Identical solar elevation angle, azimuth error, and elevation error results in different ΔωNorth in the morning (P2GS2) and in the afternoon (P5GS5).

Our test persons estimated the solar elevation more precisely (with smaller Δθ) than the solar azimuth (with larger Δφ). However, even small solar elevation errors Δθ can cause great compass direction errors ΔωNorth. This is most explicit when the sun is close to the horizon, because then even small changes in the solar elevation θS can result in great changes in the shadow length; thus the estimated shadow tip falls far off the actual gnomonic line. Since in such situations our test persons estimated the sun position with great errors, the compass direction errors ΔωNorth can be extremely large.

Although the planetarium dome was homogeneous white, a pale radial pattern was visible on it (Figs. 3A3C), because the dome canvas was composed of several sectors. Unfortunately, we could not eliminate this faint grid, which thus was seen by the test persons when confronted with the task to mentally project great circles through the black bars. The appearance of such a dim white grid pattern could slightly (mis)lead test subjects in their task.

5. CONCLUSION

ACKNOWLEDGMENTS

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2.

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4.

B. Bernáth, M. Blahó, Á. Egri, A. Barta, and G. Horváth, “An alternative interpretation of the Viking sundial artefact: an instrument to determine latitude and local noon,” Proc. R. Soc. A 469, 20130021 (2013). [CrossRef]

5.

B. Bernáth, A. Farkas, D. Száz, M. Blahó, Á. Egri, A. Barta, S. Åkesson, and G. Horváth, “How could the Viking sun-compass be used with sunstones before and after sunset? Twilight board as a new interpretation of the Uunartoq artefact fragment,” Proc. R. Soc. A 470, 20130787 (2014). [CrossRef]

6.

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

7.

P. G. Foote, “Icelandic sólarsteinn and the Medieval background,” J. Scandinavian Folklore 12, 26–40 (1956).

8.

A. Le Floch, G. Ropars, J. Lucas, S. Wright, T. Davenport, M. Corfield, and M. Harrisson, “The sixteenth century Alderney crystal: a calcite as an efficient reference optical compass?” Proc. R. Soc. A 469, 20120651 (2013). [CrossRef]

9.

K. L. Coulson, Polarization and Intensity of Light in the Atmosphere (A. Deepak, 1988).

10.

B. Bernáth, M. Blahó, Á. Egri, A. Barta, G. Kriska, and G. Horváth, “Orientation with a Viking sun-compass, a shadow-stick and calcite sunstones under various weather situations,” Appl. Opt. 52, 6185–6194 (2013). [CrossRef]

11.

C. Roslund and C. Beckman, “Disputing Viking navigation by polarized skylight,” Appl. Opt. 33, 4754–4755 (1994). [CrossRef]

12.

I. Pomozi, G. Horváth, and R. Wehner, “How the clear-sky angle of polarization pattern continues underneath clouds: full-sky measurements and implications for animal orientation,” J. Exp. Biol. 204, 2933–2942 (2001).

13.

L. K. Karlsen, Secrets of the Viking Navigators (One Earth, 2003).

14.

B. Suhai and G. Horváth, “How well does the Rayleigh model describe the E-vector distribution of skylight in clear and cloudy conditions? A full-sky polarimetric study,” J. Opt. Soc. Am. A 21, 1669–1676 (2004). [CrossRef]

15.

A. Barta, G. Horváth, and V. B. Meyer-Rochow, “Psychophysical study of the visual sun location in pictures of cloudy and twilight skies inspired by Viking navigation,” J. Opt. Soc. Am. A 22, 1023–1034 (2005). [CrossRef]

16.

R. Hegedüs, S. Ǻkesson, R. Wehner, and G. Horváth, “Could Vikings have navigated under foggy and cloudy conditions by skylight polarization? On the atmospheric optical prerequisites of polarimetric Viking navigation under foggy and cloudy skies,” Proc. R. Soc. A 463, 1081–1095 (2007). [CrossRef]

17.

G. Horváth, A. Barta, I. Pomozi, B. Suhai, R. Hegedüs, S. Ǻkesson, V. B. Meyer-Rochow, and R. Wehner, “On the trail of Vikings with polarized skylight: experimental study of the atmospheric optical prerequisites allowing polarimetric navigation by Viking seafarers,” Phil. Trans. R. Soc. B 366, 772–782 (2011). [CrossRef]

18.

G. Ropars, G. Gorre, A. Le Floch, J. Enoch, and V. Lakshminarayanan, “A depolarizer as a possible precise sunstone for Viking navigation by polarized skylight,” Proc. R. Soc. A 468, 671–684 (2012). [CrossRef]

19.

G. P. Können, Polarized Light in Nature (Cambridge University, 1985).

20.

G. Horváth, B. Bernáth, B. Suhai, A. Barta, and R. Wehner, “First observation of the fourth neutral polarization point in the atmosphere,” J. Opt. Soc. Am. A 19, 2085–2099 (2002). [CrossRef]

21.

G. Horváth and D. Varjú, Polarized Light in Animal Vision—Polarization Patterns in Nature (Springer Verlag, 2004).

22.

E. Batschelet, Circular Statistics in Biology (Academic, 1981).

23.

A. E. J. Ogilvie, L. K. Barlow, and A. E. Jennings, “North Atlantic climate c. ad. 1000: millennial reflections on the Viking discoveries of Iceland, Greenland and North America,” Weather 55, 34–45 (2000). [CrossRef]

24.

R. Hegedüs, S. Ǻkesson, and G. Horváth, “Polarization patterns of thick clouds: overcast skies have distribution of the angle of polarization similar to that of clear skies,” J. Opt. Soc. Am. A 24, 2347–2356 (2007). [CrossRef]

25.

R. Kammann, “The overestimation of vertical distance and slope and its role in the moon illusion,” Perception Psychophys. 2, 585–589 (1967). [CrossRef]

26.

M. Hershenson, The Moon Illusion (Erlbaum, 1989).

OCIS Codes
(000.4930) General : Other topics of general interest
(010.1290) Atmospheric and oceanic optics : Atmospheric optics
(260.1180) Physical optics : Crystal optics
(260.5430) Physical optics : Polarization
(330.5510) Vision, color, and visual optics : Psychophysics
(330.7321) Vision, color, and visual optics : Vision coupled optical systems

ToC Category:
Vision, Color, and Visual Optics

History
Original Manuscript: April 17, 2014
Manuscript Accepted: May 13, 2014
Published: June 30, 2014

Virtual Issues
August 26, 2014 Spotlight on Optics

Citation
Alexandra Farkas, Dénes Száz, Ádám Egri, Miklós Blahó, András Barta, Dóra Nehéz, Balázs Bernáth, and Gábor Horváth, "Accuracy of sun localization in the second step of sky-polarimetric Viking navigation for north determination: a planetarium experiment," J. Opt. Soc. Am. A 31, 1645-1656 (2014)
http://www.opticsinfobase.org/josaa/abstract.cfm?URI=josaa-31-7-1645


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References

  1. T. H. McGovern, “The archeology of the Norse North Atlantic,” Annu. Rev. Anthropol. 19, 331–351 (1990).
  2. O. Olsen and O. Crumlin-Pedersen, Five Viking Ships from Roskilde Fjord (National Museum, 1978).
  3. S. Thirslund, Viking Navigation: Sun-Compass Guided Norsemen First to America (Gullanders Bogtrykkeri a-s, 2001).
  4. B. Bernáth, M. Blahó, Á. Egri, A. Barta, and G. Horváth, “An alternative interpretation of the Viking sundial artefact: an instrument to determine latitude and local noon,” Proc. R. Soc. A 469, 20130021 (2013). [CrossRef]
  5. B. Bernáth, A. Farkas, D. Száz, M. Blahó, Á. Egri, A. Barta, S. Åkesson, and G. Horváth, “How could the Viking sun-compass be used with sunstones before and after sunset? Twilight board as a new interpretation of the Uunartoq artefact fragment,” Proc. R. Soc. A 470, 20130787 (2014). [CrossRef]
  6. T. Ramskou, “Solstenen,” Skalk 2, 16–17 (1967).
  7. P. G. Foote, “Icelandic sólarsteinn and the Medieval background,” J. Scandinavian Folklore 12, 26–40 (1956).
  8. A. Le Floch, G. Ropars, J. Lucas, S. Wright, T. Davenport, M. Corfield, and M. Harrisson, “The sixteenth century Alderney crystal: a calcite as an efficient reference optical compass?” Proc. R. Soc. A 469, 20120651 (2013). [CrossRef]
  9. K. L. Coulson, Polarization and Intensity of Light in the Atmosphere (A. Deepak, 1988).
  10. B. Bernáth, M. Blahó, Á. Egri, A. Barta, G. Kriska, and G. Horváth, “Orientation with a Viking sun-compass, a shadow-stick and calcite sunstones under various weather situations,” Appl. Opt. 52, 6185–6194 (2013). [CrossRef]
  11. C. Roslund and C. Beckman, “Disputing Viking navigation by polarized skylight,” Appl. Opt. 33, 4754–4755 (1994). [CrossRef]
  12. I. Pomozi, G. Horváth, and R. Wehner, “How the clear-sky angle of polarization pattern continues underneath clouds: full-sky measurements and implications for animal orientation,” J. Exp. Biol. 204, 2933–2942 (2001).
  13. L. K. Karlsen, Secrets of the Viking Navigators (One Earth, 2003).
  14. B. Suhai and G. Horváth, “How well does the Rayleigh model describe the E-vector distribution of skylight in clear and cloudy conditions? A full-sky polarimetric study,” J. Opt. Soc. Am. A 21, 1669–1676 (2004). [CrossRef]
  15. A. Barta, G. Horváth, and V. B. Meyer-Rochow, “Psychophysical study of the visual sun location in pictures of cloudy and twilight skies inspired by Viking navigation,” J. Opt. Soc. Am. A 22, 1023–1034 (2005). [CrossRef]
  16. R. Hegedüs, S. Ǻkesson, R. Wehner, and G. Horváth, “Could Vikings have navigated under foggy and cloudy conditions by skylight polarization? On the atmospheric optical prerequisites of polarimetric Viking navigation under foggy and cloudy skies,” Proc. R. Soc. A 463, 1081–1095 (2007). [CrossRef]
  17. G. Horváth, A. Barta, I. Pomozi, B. Suhai, R. Hegedüs, S. Ǻkesson, V. B. Meyer-Rochow, and R. Wehner, “On the trail of Vikings with polarized skylight: experimental study of the atmospheric optical prerequisites allowing polarimetric navigation by Viking seafarers,” Phil. Trans. R. Soc. B 366, 772–782 (2011). [CrossRef]
  18. G. Ropars, G. Gorre, A. Le Floch, J. Enoch, and V. Lakshminarayanan, “A depolarizer as a possible precise sunstone for Viking navigation by polarized skylight,” Proc. R. Soc. A 468, 671–684 (2012). [CrossRef]
  19. G. P. Können, Polarized Light in Nature (Cambridge University, 1985).
  20. G. Horváth, B. Bernáth, B. Suhai, A. Barta, and R. Wehner, “First observation of the fourth neutral polarization point in the atmosphere,” J. Opt. Soc. Am. A 19, 2085–2099 (2002). [CrossRef]
  21. G. Horváth and D. Varjú, Polarized Light in Animal Vision—Polarization Patterns in Nature (Springer Verlag, 2004).
  22. E. Batschelet, Circular Statistics in Biology (Academic, 1981).
  23. A. E. J. Ogilvie, L. K. Barlow, and A. E. Jennings, “North Atlantic climate c. ad. 1000: millennial reflections on the Viking discoveries of Iceland, Greenland and North America,” Weather 55, 34–45 (2000). [CrossRef]
  24. R. Hegedüs, S. Ǻkesson, and G. Horváth, “Polarization patterns of thick clouds: overcast skies have distribution of the angle of polarization similar to that of clear skies,” J. Opt. Soc. Am. A 24, 2347–2356 (2007). [CrossRef]
  25. R. Kammann, “The overestimation of vertical distance and slope and its role in the moon illusion,” Perception Psychophys. 2, 585–589 (1967). [CrossRef]
  26. M. Hershenson, The Moon Illusion (Erlbaum, 1989).

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