The noon workshop

Draw the hours with sunlight.

A sundial follows the Sun’s apparent motion rather than the conventions of a clock. Its geometry connects your latitude to a carefully aligned shadow edge. Begin with a horizontal dial, a true north-south line, and a clear understanding that solar noon and twelve o’clock are usually different moments.

Open the lay out an hour line tool ↗

Study 05 / dial / An original diagram

Three useful starting points

Observe · understand · practice

The shadow edge does the reading

On a conventional horizontal dial, the working edge of the gnomon points parallel to Earth’s rotation axis. In the northern hemisphere it rises toward true north at an angle to the horizontal equal to the local latitude. The southern-hemisphere arrangement is mirrored toward true south. A thick decorative pointer can create confusing shadows, so identify which straight edge is intended to mark time. Keep the dial plate level and use true directions rather than an uncorrected magnetic compass reading. A correctly cut dial set at the wrong latitude will show systematic errors. Start with a temporary card model so you can check the alignment before committing to a permanent material.

Hour lines are not evenly spaced

The Sun’s hour angle changes by fifteen degrees per solar hour, but the projection onto a horizontal plate does not divide the plate into equal angles. For a northern horizontal dial, the hour-line angle from noon is found from the tangent of the solar hour angle multiplied by the sine of latitude. The calculator shows the absolute offset and whether the line belongs before or after noon. It uses hours from six to eighteen and latitudes away from the equator and pole. Some drawn hour lines may receive no useful sunlight in a particular season or location. Buildings, trees, and the gnomon itself can also limit which hours are readable.

Solar time and clock time

A dial indicates local apparent solar time. Civil clocks additionally reflect a time-zone reference longitude, any seasonal clock adjustment, and the changing difference between apparent and mean solar time known as the equation of time. Longitude contributes roughly four minutes per degree relative to the time-zone meridian. The equation of time changes through the year and must be taken from an appropriate dated table or calculation. Label a first dial as solar time instead of adding a single permanent correction. Observe its noon shadow on several clear days, record the date and clock reading, and compare the pattern. A photograph without date and alignment notes cannot establish the dial’s accuracy.

The workbench

Lay out an hour line

This northern-hemisphere horizontal-dial helper calculates an angle measured away from the noon line. Enter latitude and a solar hour, including decimals for half hours. The displayed direction distinguishes morning from afternoon. The dial must still be level and aligned with true north; this tool does not determine civil clock corrections.

Take it further

A small next session

Make a level card plate and draw the noon line first. Use the planner to mark nine, ten, eleven, thirteen, fourteen, and fifteen solar hours for your latitude. Add a triangular gnomon with its working edge at the correct angle. Place the model where it is safe from wind and compare morning and afternoon readings. Record alignment changes separately from time corrections, so you can tell a construction problem from the expected difference between sunlight and a civil clock.