Notes on setting a dial

A dial set by magnetic compass carries the local magnetic declination as a permanent error, often several degrees and in places more than fifteen. Steel in a plinth or a wall makes it worse. Set by the sun instead.

The noon shadow. Look up today's figure in your plate's clock-correction table and work out the clock time of local noon. Hourline's workbench shows the longitude allowance too. At that moment the shadow of a plumb line points exactly along the meridian. Mark two points on it and you have true north–south.

Equal shadows. If you don't trust your clock, stand a rod upright on a level board. Mark the tip of its shadow in mid-morning and draw a circle through the mark, centred on the rod. In the afternoon, mark where the shadow tip touches the circle again. The line from the rod to the midpoint of the two marks points north (south, in the southern hemisphere).

A horizontal dial must be level in both directions. A tilt along the noon line moves morning and afternoon readings the same way; a tilt across it moves them in opposite directions. Use a good spirit level on the plate itself, not the plinth.

A vertical dial's noon line (or substyle, for a declining dial) must be set with a plumb line, not by eye against brickwork. Courses are rarely level.

The sloping edge, the style, must be parallel to the earth's axis. On a horizontal dial that means it rises at your latitude; Hourline prints the angle on the gnomon template. The gnomon must stand square to the plate. Check with a set square on both sides.

A gnomon has thickness, and both of its edges cast shadows: the west edge in the morning, the east edge in the afternoon. Enter the thickness on the workbench and the morning lines are moved to start from one edge and the afternoon lines from the other, leaving a gap the width of the gnomon along the noon line.

The tip of the gnomon, the nodus, casts the point that runs along the date curves. If you want the curves to be right, the nodus must sit at the stated height exactly above the marked foot.

Three things separate sun time from clock time. Longitude: clocks keep the time of a zone meridian, and every degree east or west of it is four minutes. Hourline can build this into the lines, so the plate is drawn for your exact spot. Summer time: an hour, which you can build into the numerals. The equation of time: the sun itself runs up to about sixteen minutes fast in early November and fourteen slow in mid-February, because of the earth's tilted, elliptical orbit. No straight line can absorb that, which is why each plate comes with a table, and why the noon analemma is drawn as a figure-eight.

Accuracy: Hourline's almanac is good to about half a minute of time and a few hundredths of a degree in the hour-line angles, far finer than any cut line or shadow edge.

Very few walls face due south. A wall that turns away from south is said to decline, and its hour lines are no longer symmetrical. The substyle tilts away from the vertical and the noon line stays plumb. Measure the declination with the declinometer and take more than one reading.

A wall facing nearly due east or west gets a dial with parallel hour lines and a style that runs parallel to the wall. It shows only morning or only afternoon hours. A wall facing away from the sun shows only the early and late summer hours. Hourline draws all of these, and tells you when a wall can't carry a useful dial.

Look up before you fix anything: eaves, gutters and neighbouring trees take hours off a wall dial.

Slate takes fine lines from a V-bit or a laser and lasts for centuries; paint-fill the lines. Brass and bronze engrave well with a router or rotary engraver; fibre lasers mark rather than cut them. Stainless steel and Corten suit plasma and waterjet for plate and gnomon, with lines etched or engraved. Hardwoods such as oak and iroko make handsome dials but move; seal the end grain. Cut a prototype in card or acrylic first. A free paper proof costs nothing and catches most mistakes.

All Hourline lettering is single-stroke, like a pen plotter's, so an engraving bit or laser traces it once, cleanly. If you prefer filled lettering, you can offset the strokes in your CAM software.

Below the equator the gnomon points to the south celestial pole, the noon shadow points south, and the hours run anticlockwise on a horizontal dial. Enter a negative latitude and Hourline turns the plate accordingly. The noon line still reads “up” on the drawing. A wall dial wants a wall facing roughly north.

Hourline does not draw reclining or inclining dials, on surfaces neither horizontal nor vertical. It also leaves out analemmatic (human-gnomon) dials, bifilar dials and portable altitude dials. Plates on a sloping roof or a tilted monument therefore need a specialist. These may come later. Hourline will not pretend to draw them before they are right.