In the figure below:
* The blue sphere is the earth, represented in twelve panels as it moves around the sun.
* The magenta circle is the location of Jaipur, India - on the day depicted in each panel, Jaipur will trace the little circle (not a great circle) represented by the magenta curve from midnight to midnight.
* The green line is the axis of rotation of the earth.
* The grey circle is the plane of the ecliptic.
* The light blue line is the normal to the ecliptic, and the intersection points of this line with the celestial sphere are the ecliptic poles.
* In the panels, I kept the sun in a fixed location, moved the earth around it, and then rotated the view such that the earth is again centered. But this will effectively rotate the fixed stars or the zodiac.
* The little blue circle is the tangent plane to Jaipur on the earth, at the point of time where the sun is at its zenith for that particular day.
* Thus, the black line will be the local "UP" direction in Jaipur at noon-time on that day.
* So then, the sun, at noon-time, will be tilted from the "STRAIGHT UP" direction by a certain amount, represented by the angle between the black and yellow lines.
* The slope of each rashivalaya will be the angle between the blue tangent plane and the yellow line.
* The rashivalaya will also point towards the ecliptic pole, which is the intersection of the light blue line with the earth sphere.
* For both Cancer and Capricorn, the direction to which the rashivalaya points will also be true north. But not so for the other signs of the zodiac.
* So if the sun is at its highest point, and is also coincident with the first point of one of the zodiacal signs, then the rashivalaya for that sign will not cast a shadow.
Will elaborate more when I get a chance.