Steve Lelievre's Equant Dial Program
Draws horizontal equant sundials featuring a graphical converter and movable ring (example). Uses millimetres for all lengths. Uses decimal degrees for all angles.
- No month labels or hour numbers are drawn by the program - you must add them afterwards using CAD or drawing software such as Inkscape.
- The dial is drawn with both calendar scales running anticlockwise. Perhaps the easiest way to figure out which month is which is to locate February - the only month with 29 daily tick marks. Remember
that for dials featuring Daylight Saving Time (DST), there are periods of 7 days in the spring and autumn that can fall in either DST or in Standard Time (ST). It is the handling of these periods that
leads to extra partial 'months' appearing in the calendar scales. For example, with North American DST rules, March is split into two 'months' : one runs from March 1 to March 14 for ST, the other runs
from March 8 to March 31 for DST. Likewise, we have a 'month' for November 1 to November 7 as DST, followed by November 1 to November 30 as ST. To aid understanding, it may help to look at an example.
- For the Hours To Sunset dial only, the hours scale runs anticlockwise (from 0).
- In Landscape orientation, A4 paper is 297mm wide × 210mm high and Letter paper is 279mm wide × is 216mm high, so for a drawing that will fit on either, use a width of 279mm or less and a height of 210mm or less.
- The gnomon footprint is drawn with the minimum length (substyle) needed to ensure its shadow always reaches the dial scale's innermost arc. Of course, a longer gnomon may be used
if desired (e.g. aethestic reasons, easier assembly).
- You can deal with time zone offsets outside the allowed range by adding or subtracting multiples of 15° and the adjusting hour label positions accordingly.
- If you want a PDF version of the diagram, either use your operating system's Print To PDF feature, or save it as an SVG file and then use an online conversion service such as www.fileformat.info.
- Labelling aids are extra marks on the dial face that can help with label placement. They are not relevant to dial use, and are intended to be deleted from the drawing once labelling is finished.
- The program cannot handle the 'midnight overlap' that affects dials in polar locations, so these latitudes are not allowed. In practice, due to the effect of atmospheric refraction on sunset times, and also
computational limits of the program, the maximum latitude supported is ±65.5° - slightly outside the Polar Circles.
- Although the program allows you specify them, dials for latitudes within about ±30° of the Equator will be unusable in practice. This is due to excessive bunching of lines making up the graphical
- Using a non-zero value for 'Gnomon offset' causes the toe of the gnomon to move away from the centre of the dial, which may reduce bunching of converter lines at the cost of more bunching elsewhere.
Positive values move the gnomon up the diagram.
- RGB colour codes used are Black:0,0,0; Red:255,0,0; Green:0,255,0; Blue:0,0,255; Pale Red:255,170,170; Pale Green:170,255,170; Pale Blue:170,170,255; White:255,255,255. Even white lines are present in the diagram; you just don't see them against the white background.
- For an explanation of the workings of these dials see Lelievre, Steve. 2017. "A Horizontal Tide Dial" in The Compendium, Journal of the North American Sundial Society,
v.24, no.4. ISSN 1074-3197.. It's not quite the same kind of dial, but it covers the general concept.
- This dial design was inspired by the Sawyer Equant.
Version 20200719. You are welcome to contact Steve Lelievre for general inquiries, suggestions for additional settings, and to report problems.
acknowledge original authorship. Steve Lelievre asserts his moral right to be known as the original author of this work. © Steve Lelievre, 2018.
Note, this program is incomplete. To Do: southern hemisphere code is not yet fully tested (beta status).
For an excellent source of general information on sundials, visit the North American Sundial Society website.