Quick answer: a sundial only reads correctly when three things line up — the gnomon points at true north rather than magnetic north, the dial plate sits dead level, and the gnomon’s angle matches your latitude. Get those three right and the largest error left is the equation of time, which pulls a sundial between 14 and 16 minutes off twice a year.
Why magnetic north is the wrong north
Most sundials set up with a phone end up aimed at magnetic north. Magnetic north is not the north a sundial cares about.
The gap between the two — magnetic declination — runs about 10 to 16 degrees across the continental United States, and it drifts a fraction of a degree every year. Ten degrees of error on a 10-inch horizontal dial is roughly 40 minutes of clock time, enough to make a perfectly good sundial look broken.
To find true north without a compass, use the shadow method. Plant a 1/4-inch straight rod in level ground before 11 a.m., mark the tip of its shadow, then mark the tip again in the afternoon at the same distance from the rod. The line between the two marks runs east to west, and true north sits perpendicular to it. Street grids are not a reliable shortcut, because very few are aligned to true north.
What angle should the gnomon sit at?
The gnomon is the raised edge that casts the shadow, and its angle above the dial plate has to equal your latitude. At 40 degrees north the gnomon rises at 40 degrees; at 30 degrees north, 30 degrees.
A dial built for the wrong latitude is not simply slow or fast. Its hour lines fan out at the wrong spacing, so the error grows and shrinks through the day instead of holding constant. Check the specification before you buy: a dial cut for one band reads honestly only inside that band.
What else throws a sundial off?
On a horizontal dial the shadow sweeps 15 degrees every hour, so a single degree of tilt is worth about four minutes of error. That is why a dial resting on a wobbly table can never be trusted, even after the alignment is perfect.
Longitude adds a second correction. Standard time is set at the centre meridian of your time zone, and every degree you sit east or west of it moves solar noon by four minutes. If you live three degrees west of your zone’s meridian, add 12 minutes.
The last correction is the equation of time — the gap between clock noon and solar noon caused by Earth’s elliptical orbit and axial tilt. It peaks near 16 minutes in early November and bottoms out near 14 minutes in mid-February, and it passes through zero four times a year. A sundial that reads perfectly in mid-April will read several minutes fast by November.
Where does a sundial belong in a garden?
Placement is mostly about midday sun and a surface you can actually level. A south-facing patio or a low garden wall that catches sun between 10 a.m. and 2 p.m. will show the shadow moving in a way you can read at a glance. Corners that fall into shade by noon, or spots under a tree that leafs out in May, work for only part of the year.
A cast iron or steel dial has one practical advantage: the metal holds its plane. Once you level a 6 lb dial on a stone step, it stays level through a winter of freeze-thaw cycling between 20°F and 100°F without warping. That is what makes a rusted tabletop sundial a reasonable choice for a patio table — the weight keeps the dial honest, and the finish takes the weather without a cover.
If the spot you have in mind is still bare, giving a patio corner a sense of place is usually a matter of one object that belongs there rather than three that do not. It is the same rule that applies to a tabletop.
Set it, check it against a clock at solar noon, and leave it alone for a season. The four-minute-per-degree arithmetic does not change, and the dial only gets more honest as the alignment settles.