What To Do On Cloudy Days
Today is a heavily overcast day in Austin. There couldn't possibly be anything related to astronomy to talk about. Or could there?
I was just remembering today the legend of how the Vikings would find their way around on the high seas using what has later been called a "sun stone" for navigation. The idea was that they had a rock that would change color depending on the angle of light from the sun hitting the rock, even on a cloudy day! They used this special stone to find the sun, and thus find to roughly find north and south! Historians believe that the stone they might have used is a calcite material called Icelandic Spar which reflects and filters polarized light.
Polarized light is light that behaves differently in different directions. The light from the sun becomes partially polarized when it hits the atmosphere and gets scattered. Since most of the sun light still comes from the direction of the sun and only some of it comes from every other direction (making the blue sky) we say that the light is polarized to the direction of the sun.
"Polarized" sunglasses use special lenses that filter only horizontally polarized light. They are made to filter light that gets polarized when it reflects off of horizontal surfaces like the surface of water. This helps not only fishermen and sailors, but also drivers who see a lot of this kind of light reflected off of cars in traffic.
If you have these kind of glasses you could probably use your own "sun stone" to find the sun on a cloudy day! Put the glasses on then look at the clouds. Tilt your head left and right until you notice that the light appears darkest. The spot above your head is roughly in the angle of the sun. Try turning around too to see if that changes the brightness of the light you see.
Historians and scientists are still unsure if the Vikings could really navigate on the open ocean on a heavily overcast day because the light would be so scattered as to make it difficult to measure the direction of the sunlight. It is more likely they could only use the sun stone on partially cloudy days, or only when there was a small opening of clear sky.
Experiment on your own to see if you can find the sun on a cloudy day!
To learn all about polarization and how maybe the Vikings used a sun stone to navigate on the ocean on cloudy days, visit:
Polarization website Vikings page
Polarization website explaining how polarized sunglasses work
Excerpt from 1997 Sky and Telescope sun stone article
Navigating Viking Ships
Good night!
I was just remembering today the legend of how the Vikings would find their way around on the high seas using what has later been called a "sun stone" for navigation. The idea was that they had a rock that would change color depending on the angle of light from the sun hitting the rock, even on a cloudy day! They used this special stone to find the sun, and thus find to roughly find north and south! Historians believe that the stone they might have used is a calcite material called Icelandic Spar which reflects and filters polarized light.
Polarized light is light that behaves differently in different directions. The light from the sun becomes partially polarized when it hits the atmosphere and gets scattered. Since most of the sun light still comes from the direction of the sun and only some of it comes from every other direction (making the blue sky) we say that the light is polarized to the direction of the sun.
"Polarized" sunglasses use special lenses that filter only horizontally polarized light. They are made to filter light that gets polarized when it reflects off of horizontal surfaces like the surface of water. This helps not only fishermen and sailors, but also drivers who see a lot of this kind of light reflected off of cars in traffic.
If you have these kind of glasses you could probably use your own "sun stone" to find the sun on a cloudy day! Put the glasses on then look at the clouds. Tilt your head left and right until you notice that the light appears darkest. The spot above your head is roughly in the angle of the sun. Try turning around too to see if that changes the brightness of the light you see.
Historians and scientists are still unsure if the Vikings could really navigate on the open ocean on a heavily overcast day because the light would be so scattered as to make it difficult to measure the direction of the sunlight. It is more likely they could only use the sun stone on partially cloudy days, or only when there was a small opening of clear sky.
Experiment on your own to see if you can find the sun on a cloudy day!
To learn all about polarization and how maybe the Vikings used a sun stone to navigate on the ocean on cloudy days, visit:
Polarization website Vikings page
Polarization website explaining how polarized sunglasses work
Excerpt from 1997 Sky and Telescope sun stone article
Navigating Viking Ships
Good night!
posted by Dan at 8:03 PM
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