Altitude-based Sundials
Altitude dials measure the height of the sun in the sky, rather than its rotation about the celestial axis. They are not oriented towards true North, but rather towards the sun and generally held vertically. The sun's elevation is indicted by the position of a nodus, either the shadow-tip of a gnomon, or a spot of light. The time is read from where the nodus falls on a set of hour-curves that vary with the time of year. Since the sun's altitude is the same at times equally spaced about noon (e.g., 9am and 3pm), the user had to know whether it were morning or afternoon. Many of these dials are portable and simple to use, although they are not well-suited for travelers, since their hour-curves are specific for a given latitude.
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