The equatorial, the armillary, and the horizontal dial all have 6 oclock (am or pm) at 90 degrees to the noon line (for non longitude adjusted dials). So, why does the azimuth dial not have 6 oclock at 90 degrees to noon?
The azimuth dial is horizontal and thus is a projection of the equatorial dial, and that skews the lines. On the equinox, yes 6am and 6pm are perpendicular to noon for the azimuth dial. However, the summer solstice ray from the sun to the gnomon will hit the equatorial dial at 90 degrees to noon, however if the ray continues down to the horizontal azimuth dial, it displaces south. So, for horizontal azimuth dials, 6 o’clock will be north or south of the 90 line from noon. But you say, how is it the horizontal hour angle dial has 6 o’clock at 90 degrees? the answer is because of the sloping gnomon. In fact the tip of the gnomon (nodus) on a horizontal dial is north of the 6 o’clock lines at all times of the year (for the northern hemisphere).
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