At http://phys.org/print315047622.html …. a fresh look at Viking methods of navigating without a magnetic compass. They were able to reach Greenland, Newfoundland and Baffin Island on a regular basis, long distances that appear impossible with their rudimentary gear. The discovery of a sun compass fragment some time ago led to several theories being developed. We now have another one. They were able to use the sun to find north with the sun compass – but how did they navigate in the twilight (or in moonlight). In this research it is suggested they used a crystal which they called a sun stone (mentioned in various documents but as yet not found in an archaeological context). The sun compass operated in a similar fashion to a sun dial, using the position of the Sun to determine direction. The sun stone, on the other hand, it is suggested, was used when the Sun was low on the horizon – even below the horizon.