1. Draw two concentrical circles : one of radius r and the other one of
radius r*sin(Lat)
2. Now draw a sheaf of 24 equispaced lines from its center as if it
were an equatorial dial.
3. These lines intersect the circles at points I' and I'', II' and
II'', and so on up to XXIV' and XXIV''.
4. Now trace horizontal lines from the inner points and vertical lines
from the outer points. Let's call
I the point where the lines from I' and I'' intersect, II the point for
II' and II'', and so on.
5. If we connect these points we just have the analemmatic ellipse,
right? Well, but if we trace lines
from the center to these points we get a horizontal dial for that
latitude. Isn't that nice?
But I think Anselmo makes one small error. Yes, the points at step 4 are the
hour marks of an analemmatic dial if, by 'horizontal lines' he means
east-west lines, but to draw a Horizontal Sundial you want to draw
north-south lines from the inner points and east-west ones from the outer
points.
This is because an analemmatic dial's major (longer) axis is east-west,
whereas for a horizontal dial constructed this way the major axis is
north-south.
Chris,
You are right. I over-simplified the procedure I saw in Fantoni's
book. Indeed you have to trace
horizontal (ie., EW) and vertical (ie, NS) lines in all points. The
lower intersection marks the analemmatic
points and the upper the horizontal-dial points. I do not know if there
exists an additional simplification, perhaps
taking somewhere the colatitude, because I hadn't understood very well
what Britannica tries to say.
Best regards,
Anselmo.
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