Abstract
Skylight polarisation pattern in general and the positions of the polarisation neutral points in particular are important parameters in atmospheric science. The construction and operation of a low cost partially automated polarimeter for studying the polarisation pattern of skylight in the principal plane of the sun, and some results obtained for the skylight polarisation pattern using the polarimeter are presented in this paper. The polarimeter mainly consists of a collimating tube with a linear polariser and a detector consisting of a light dependent resistor controlled by a microprocessor. The degree of polarisation for a given angular distance from the sun in the principal plane of the sun was measured using the maximum and minimum currents passing through the light dependant resistor (LDR) computed through voltage measurements across the LDR. Positions of the polarisation neutral points and the maximum degree of polarisation were determined graphically using plots of the degree of polarisation as functions of the angular solar distance. The error in the neutral point positions was estimated to be 2°, while that of the degree of polarisation was estimated to be a maximum of 18 %. The measurements taken at different times on relatively clear sky days during a period of over one year provided consistent results in fairly good agreement with similar studies carried out using more sophisticated instruments as well as theoretical predictions for a molecular atmosphere. An attempt has also been made to account for any deviations of the quantitative results from those of other studies.
Highlights
Observations on the polarisation of skylight have been carried out as far back as the early years of the nineteenth century
The microcontroller was programmed to rotate the polariser by 360° in 52 steps and record the light dependant resistor (LDR) voltage at each step
The LDR voltage was recorded after a 50 ms delay to allow for the LDR response time
Summary
Observations on the polarisation of skylight have been carried out as far back as the early years of the nineteenth century. Arago’s neutral point is located a few degrees (20° ‒ 30°) above the anti-solar point. This discovery opened up the path to a sequence of investigations on the polarisation of skylight. Babinet’s and Brewster’s neutral points were discovered in 1839 and 1846, respectively following the observations of Jacques Babinet and David Brewster (Konnen, 1985; Hovarth et al, 1998). Babinet’s and Brewster’s neutral points are respectively located a few degrees (25° – 30°) above and below the sun for a clear molecular atmosphere. Later in 2001, a fourth neutral point was discovered beside the anti-sun in the direction directly opposite to the Babinet’s neutral point (Hovarth et al, 2002).
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