Sundial of the month March

Double line-of-light sundial

Instead of a pole style shadow, a line of light may be used to read a sundial.
The horizontal sundial shown here forms a line of light using the slit between two east-west aligned stone slabs.
The slabs are inclined according to local latitude.

The thickness of the slabs reduces the duration of the visibility of the line of light, despite the bevelling used here.
In this example, the useful readout of the sundial would be limited to the period between the hours of 8 and 16, local apparent time.

A second sundial enables the user to read the other hours.
Here, the slit between two vertical slabs, aligned north-south, forms the line of light.
This sundial works during the early hours of 4 to 8, and during the late hours of 16 to 20 hours, local apparent time.

The figure shows the total hour line pattern.
The readout is in local apparent time; one sundial uses Roman numerals, the other Arabic.
A special date line works with an index fixed between the east-west slabs.