(This article originally appeared in the Q3 1995 STAR newsletter and was written by Greg Gittings. -I)
Polarization of the Sky – A Daytime Test for Transparency
A Short History
by Greg Gittings
Looking at the northern or southern polar section of the sky with an analyzer (a pair of Polaroid sunglasses will do) can tell you if the atmosphere is dry or humid in the upper levels. Dry air is highly polarized and it becomes almost blue-black in the winter. As it becomes mixed with more water vapor in the summer, the sky remains very bright as the water molecules scatter the light and destroy the plane of polarization. Heavy haze can’t be darkened by a crossed Polaroid sheet.
The polarization of the daytime sky is an interesting but little appreciated optical phenomenon. It was first discovered accidentally by Arago while trying to measure the reflection polarization angles of Mica. Looking through a prism of Iceland spar (CaCO3) he saw bright bands of color moving across the Mica. He reported this to the French Academy of Science as evidence that the Mica was somehow “Depolarizing” the light prior to its passing through the “Calcarious Spar”(1811).
You can simulate what Arago saw by placing a clear acrylic box, (a cheap cassette tape box) in an area illuminated by the northern sky and shielded from all other sources of light. Put on your Polaroid sunglasses and prismatic colors appear, revealing the molding stresses in the plastic. These will not appear on an overcast day. If you cannot get the colors to appear, try looking directly through the cassette box at arms length while holding it up to the polar section of the sky. Be sure to rotate your head, or glasses until the sky darkens. Shortly thereafter, Arago was looking through the crystal directly at the daytime sky. While rotating it, he found two positions 180 degrees apart where the darkening was the greatest. Measuring carefully he noticed that the angle of the greatest obscuration changed as the day went on.
Evidently, the air itself was polarizing the light in a wide band that moved daily, standing north to south in the morning and evening, it rotated to become east-west at Noon. This band also moved seasonally, always situated 90 degrees to the plane of the Sun. “Depolarizing” did not exist as a property of Mica, much to Arago’s embarrassment.
Sir David Brewster, along with Sir John Herschel realized that the 90 degree angle was in fact the sun’s light being reflected, polarized, and scattered by the atmosphere. This was used to verify Brewster’s fledgling hypothesis, that “the refractive index of a substance was the tangent of the angle of polarization” (1815). In other words, if you construct a right triangle with the vertical leg “one” unit long, and the angle of polarization is used at the vertex of the triangle, the length of the horizontal leg is proportional to the index of refraction of the material measured. Air’s index is one, at 45 degrees.
With the invention of the Knuckle prism in 1829, the birefringent Calspar was turned into a singly polarizing optical element and could then be used to analyze and to polarize light in a single plane. This was accomplished by sawing the crystal diagonally down its length and then gluing the parts back together with Canada balsam. The balsam changed the refractive index of the path traveled by one of the rays and reflected it completely out of the crystal. the result of this was to make two positions 180 degrees apart that could define a plane of polarization. Arago used one to obscure the skyglow due to moonlight while searching for a comet; he also was the first to discover that the light from a comet’s tail is polarized.
Charles Wheatstone invented what he termed a “Polar clock”, that took advantage of the daytime sky being highly polarized at 90 degrees to the position of the Sun. If a Knuckle prism was attached to a pre-divided scale and it was turned to extinguish the light from the north polar area of the sky, it would tell the time (local apparent) to within a minute. Later versions had geared hands and Equation of Time cams; they were easily portable and unlike the ordinary sundial, the “Polar clock” could also work on partly cloudy and hazy days as well (1843).
On a trip to see the total solar eclipse of 1870, John Tyndall carried a Knuckle prism with him. While in the Mediterranean the sky was so clear and dry that when he looked to the north through his prism he could see stars during the daytime. In the nineteenth century, checking the transparency of the sky during the day for that evening was standard procedure.
With a decent analyzer the polarization properties of all of the substances on Earth opened up for examination. In 1853, Dr. Herapath, in cooking a mixture of quinine (yes, the tonic in gin and tonic, for malaria, still used today to control leg cramps), acetic acid, and tincture of iodine, stumbled upon one of the most strongly polarizing crystals ever found. He was able to grow crystals large enough to replace the Knuckle prisms on his microscope. This was much easier than making a prism from Calspar, but the handling of small crystals was difficult enough to delay their use for another 80 years.
With the invention of Polaroid sheets (1934) by Land, this has been accomplished chemically by suspending microscopic crystals of herapathite (iodoquinine sulfate) in a plastic base and stretching the plastic to align the crystals along the axis. This brought the price of polarizing materials down into the range where they could be applied more widely.
In these days of cheap sunglasses, there is no reason why amateur astronomers can’t go out hours in advance of sunset, and by checking the “Depth” of polarization, have a firm idea as to whether or not they should get out their equipment. Hopefully this could save a lot of lost work and lost opportunities. It would be possible to set up a system of measuring the darkening of the pole and compare it to a section of the sky not in the polarizing band. The sky not in the band could be darkened by two rotated Polaroid sheets and the result could be given in the degrees from 0, the start point. This would give us a crude index that would express the transparency for that night, and a way to compare the transparency on different nights. Checking the polarization of the sky could give us a prognostical edge on Nature using one natural phenomenon to undo another — BAD TRANSPARENCY.
