Ciao to everyone! Many years ago I published in "Astronomia" N.2 Apr-June 1987 (ISSN: 0392-2308) p.14-17 a short review on the methods for measuring the declination of a wall. Most of them were taken from the literature. One was original, as far as I know, and is based on finding the time when the sun is in the plane perpendicular to the wall. Its technique is rather rough but easy and essential.
A small mirror is fixed to the wall and a plumb-line is suspended in front of it at, say, 10 cm from the wall. Behind the line a sheet of paper is used to collect he reflected image of the sun. When the sun is in the plane perpendicular to the wall you will see on the paper two shadows of the plumb-line: one broader and more diffuse and another one narrower and more neat. The first one originates when the light hits the line before hitting the mirror and the second one when the light reflected by the mirror hits again the line. When the two shadows overlap the time is taken and the azimut is calculated therefrom. That's the declination of the wall. Instead of a plumb-line one can better use an element in form of a long vertical slit. For instance, a glass 70x3 cm on which two stipes of black adhesive tape are applied longitudinally spaced 1 mm from each other. In this case the sun light passes through the slit, is reflected by the mirror and again passes through the slit and finally is collected by the paper where two overlapping lights appears (instead of the shadows as above). The advantages of this method are that no geometric measure is required, but only time recording and that it is independent of atmospheric diffraction (other methods are). The disadwantages are that only one measure per day and at a precise time is possible and that it is applicable to walls not excessively declining. Due to the limited applicability in practice of this method I have not carried out extensive tests to determine its degree of precision. Ciao Angelo Brazzi --- 44 32' 10''N 11 32' 15''E ---
