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| The air scatters the sun rays in all directions, creating the joy of the blue sky. At the same time, it provides the means of determining the compass bearings if one knows the time of the day, or the time of the day if one knows the compass bearings. The reason is that the direction of the skylight polarization (as well as its strength) depends on the relative position of the sun and the patch of "sky" doing the scattering. |
A sky clock?For a given sky direction, the skylight polarization will change as the sun moves in the sky dome, thus providing the basis for a polarization clock. The best direction to use as reference is the celestial north (south) as the skylight polarization in that direction is always relatively high and it rotates with the sun, always remaining perpendicular to the sun-north-observer plane. This is the principle of the "polar clocks" invented by Sir Charles Wheastone[1]. At the time no polaroid film existed, so he used a Nicol prism as the analyzer. To increase the contrast he added a star formed by selenite crystals aligned at 45º to the Nicol to obtain changing colors as he rotated the assembly in front of the a background of polarized sky. When the selenite-Nicol assembly is rotated there are two positions where the rich colors of the selenite disappear (except for the center of the star which he rotated by an extra 22.5 degrees). A dial plate marked with hours and subdivisions of 1/5 or 1/10 of an hour gives the time. Recently, Bradley Schaefer of Yale, unaware of Wheastone work, re-invented the "polarization sundial" and built one using common inexpensive materials [2]. He calculated an accuracy of up to 20 seconds! |
A light compass?In the 1950's Scandinavian Airlines (SAS) used a polarization compass during transpolar flights, perhaps recreating what the Vikings had done one thousand years before. Bees routinely orient themselves with respect to the sun with their built-in polarization compass, even in the shadow of mountains. The Skylight Compass(TM) Card lets you experiment with this mode of navigation. |
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Polarization.com designed the Skylight Compass to let everyone find these invisible markers in the sky. Of course, it also works as a simple Polariscope Card. |
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Next Page: How to use the Skylight Compass =>
[1] William Spottiswoode, "Polarisation of Light," London 1874, Macmillan and Co, Ch. VII.
[2] Bradley E. Schaefer, "Vikings and Polarization Sundials", Sky & Telescope, May 1997, p. 91
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