Gift sundials with anniversary line
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Once a year, for example a birthday or a wedding day, the shadow goes along a specially designed unique line called the anniversary line.
In order to register a birthday or a wedding day the sundial should have the nodus on a gnomon. Nodus (in Greek — node) is a point on the gnomon, the shadow of which corresponds to the sun declination.
Declination is the height of the sun above the ecliptic. Each day of the year corresponds to a certain value of the sun declination. As soon as the sun moves from east to west the nodus shadow moves from west to east. The trajectory of the nodus shadow is unique for each day of the year and does not change significantly over the centuries. This trajectory is called a declination line.
Declination line on a dial can be calculated for any date. The declination line calculated for the days especially important in one's personal life is called the anniversary line.