How to set up a sundial
A horizontal sundial reads the sun's position where it is standing — 'sun time', not the time on your watch. Setting one up is three things: making it level, pointing it the right way, and knowing the two corrections that explain everything else.
How a horizontal sundial works
The shadow-casting edge is called the gnomon. To read correctly it must point at the celestial pole: in the northern hemisphere that means it points true north and rises at an angle equal to your latitude — roughly 51° in London, 55° in Edinburgh. The hour lines on the plate are cut for that angle, so a sundial cast for one latitude is most accurate at that latitude, and a few degrees out is a matter of minutes rather than hours.
Getting it level
A dial that leans keeps time but with a constant error — the shadow falls on the wrong side of the noon line all day. Check with a spirit level in two directions, left to right and front to back. A stone plinth or pedestal gives a flat, stable bed; on soil, set it on a compacted base or a paving slab rather than on loose earth that will settle.
Pointing it north
The noon line on the plate points to true north — not along the fence, and not necessarily along the bed. In the UK a compass is usually close enough for a garden, since magnetic north sits only a degree or two from true north here. If you want it exact, find the shortest shadow of the day: at solar noon the sun is due south and the shadow points due north.
Why it reads an hour fast in summer
From late March to late October the UK is on British Summer Time, an hour ahead of the sun. The dial is not wrong — add an hour in summer and you have clock time again. That is also why a sundial is the one garden ornament that is more interesting in winter, when it and the clock agree.
The equation of time
Even without summer time, the sun runs up to about fifteen minutes fast or slow depending on the month, because the Earth's orbit is elliptical and its axis is tilted. This is the equation of time, and it affects every sundial ever made. A well-set dial is a calendar as much as a clock — and reading it a few minutes out of step with your phone is the point, not a fault.
East, west, and why location matters
Sun time is local time. Each degree of longitude is four minutes, so a dial in Cornwall and one in Norfolk read differently when the sun crosses each. Keep that in mind when comparing a reading with another part of the country.
Brass outdoors, and what happens to it
Brass darkens outdoors to a brown-gold patina, and that is normal — some people polish it back once a year, and others leave it to age. Both are correct. If you do polish it, use a liquid metal polish and a soft cloth, and keep it away from the engraved numbers with abrasives that would soften them.
Written by Garden Ornaments & Accessories. Last updated .