The Journal

How Does a Sundial Compass Work?

October 5, 2026

A sundial compass combines two very old instruments into one small object: a magnetic compass, which points to a fixed direction, and a sundial, which tells time using nothing but light and geometry. Neither needs a battery.

How the compass half works

A magnetic needle, free to rotate, aligns itself with the Earth's magnetic field and points toward magnetic north. This part is straightforward and works anywhere on the surface of the planet, unaffected by weather, charge, or time of day.

How the sundial half works

The sundial uses a raised blade, called a gnomon, angled to match your latitude. When the gnomon is angled correctly and the compass is oriented to true north, the sun casts a shadow from the gnomon onto a graduated dial beneath it. As the sun moves across the sky through the day, the shadow moves across the dial, indicating the approximate hour.

Reading it, step by step

First, use the compass needle to orient the whole instrument so it points north. Second, raise the gnomon to the angle marked for your latitude — most portable sundials have an adjustable hinge for this. Third, read the hour where the gnomon's shadow falls on the dial.

Accuracy

A well-made sundial compass is accurate to within about ten minutes in good sunlight, once correctly oriented and latitude-adjusted. This is remarkable for an instrument with no moving parts beyond the needle and gnomon hinge.

Solar time versus clock time

A sundial reads solar time, which is based on the sun's actual position, not the standardised clock time your phone shows. During daylight saving months, add one hour to the sundial's reading to match clock time. There is also a small, predictable seasonal variation called the equation of time, generally a few minutes either way, which most users simply accept as part of the instrument's charm rather than correcting for.

Why it still works after four hundred years

Sundials predate mechanical clocks by centuries and remained the most reliable timekeeping method well into the modern era, because they rely on astronomy rather than mechanism. Nothing to wind, nothing to charge, nothing to break — as long as the sun is out, it works.