Navigating the seas with polarized crystals
BUDAPEST, Hungary – You might think that seafaring Vikings
who from 900 to 1200 AD traveled thousands of kilometers in the rough waters would
have floated adrift on cloudy days. Without a magnetic compass, they often traveled
in the long days of summer at high latitudes that would hinder them from viewing
any stars for navigation.
But they didn’t lose their way, thanks to what was once
considered a mythical navigational tool: the sólarsteinn, or sunstone –
and scientists are finding new evidence to support its existence.
In 1967, Danish archaeologist Thorkild Ramskou suggested that
the sólarsteinn could have been a piece of Icelandic spar, which is calcite
in the form of a transparent polarizing crystal, commonly found in Iceland. Other
possible candidates for the sunstone include cordierite and tourmaline, both of
which are common in Scandinavia.
Polarizing crystals allow light polarized in only one direction
to pass through and, depending upon the orientation to the polarized light, they
will appear dark or light. Ramskou predicted that the Vikings rotated the sunstone
to check the polarization direction, deducing the position of the sun when it was
hidden by fog or when it was just beneath the horizon.
Knowing that our atmosphere can scatter sunlight and polarize
it, scientists from Eötvös University in Budapest and from Lund University
in Sweden decided to put the calcite to the test.
Using photographs of the cloudy sky, they tested how well volunteers
could estimate the sun’s position, and their conclusion was: not very. The
volunteers were highly inaccurate, making errors of up to 99°.
In 2005, the researchers traveled the Arctic Ocean measuring the
polarization patterns of the sky, finding that they were very similar on clear and
cloudy days. While the polarization appeared weaker on overcast days, their findings
suggested that the Vikings could have used this information if they were in possession
of polarized crystals. The scientists next plan to see whether the volunteers can
use polarization crystals to determine the sun’s location under a variety
of weather conditions.
The research was published in Philosophical Transactions of the
Royal Society B, March 2011 (doi: 10.1098/rstb.2010.0194).
- A doubly refracting mineral used to produce polarizing prisms. It is uniaxial negative and in the trigonal division of the hexagonal system of crystals. Its indices are e = 1.486, w = 1.658; its hardness is 3 Mohs; and its specific gravity is 2.711.
- With respect to light radiation, the restriction of the vibrations of the magnetic or electric field vector to a single plane. In a beam of electromagnetic radiation, the polarization direction is the direction of the electric field vector (with no distinction between positive and negative as the field oscillates back and forth). The polarization vector is always in the plane at right angles to the beam direction. Near some given stationary point in space the polarization direction in the beam...
- A naturally occurring crystalline mineral that has the property of polarizing transmitted light. It is little used now that the Polaroid sheet is available.
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