A vocabulary of landscape — a landscape of words
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sundial
A sundial tells time by the position of a shadow. The blade casting the shadow is the gnomon, and it has to be set parallel to the earth’s axis, which means angled to the local latitude, or the dial will not keep pace with the sun through the year. Marked correctly, it reads solar time at that exact spot, which is not the same as clock time and can differ by a quarter of an hour either way.
Etymology
Dial came into English from Medieval Latin, from the word for a day, so a dial was originally a device for telling the time of day and only later meant any circular scale. The gnomon is Greek for one who knows or an indicator.
Notes
The difference between sundial time and clock time is the equation of time, and it arises from two things: The earth’s orbit is elliptical so it moves faster in January, and its axis is tilted. Plotted through a year at the same clock time, the sun traces a long figure-eight in the sky called an analemma, which is printed on many dials as the correction table.