सिद्धान्तशिरोमणि: गणिताध्याय (भास्कराचार्य - ग्रहगणित, मध्यमाधिकार व स्पष्टाधिकार सटीक)

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The East-West line is roughly the join of the
extremities of the morning shadow as well as that in
the after-noon of a gnomon placed at the centre of a
circle drawn on a plane with any arbitrary radius,
when those shadows equal the radius of the circle. But
this line is to be deflected keeping its western point ie.
the extremity of the morning shadow fixed through a
distance K (sin δ₁ ~ sin δ₂) / cos ϕ at the eastern end perpendi-
cular to it, where the above distance is measured in units,
which measure K.
Comm. The east and west points are where the
celestial equator cuts the horizon. The east point is
thus the point where the Sun-rises when he is exactly at
the vernal equinox. The question is how to draw the
east-west line on a plane. For this we are asked to
draw a circle with any radius on that plane. The plane
is described here as अम्भःसुसमीकृतक्षिति that kind of sur-
face as is determined by the surface of water there. Such
a kind of surface forms approximately a horizontal plane
not of course exactly because such a surface is really
spherical, the earth being a sphere. But because the
radius of the earth is sufficiently large, we can take such
a surface to be a horizontal plane for all practical purposes.
Having drawn a circle place the gnomon vertical at the
centre. In the morning note the extremity of the shadow
when it equals the radius. In the afternoon also mark
the point when the shadow equals the radius. Join those
two points. It represents roughly the east-west line,
roughly because the Sun's declination changes in between
the two moments however small the change might be.
Ignoring the change in the declination, this line will
be east-west because of the following reason. The length
of the shadow is 12 tan z where 12 units are the measure
of the gnomon. But since the shadow on both the occasions
is equal to the radius of the circle z, the zenith-distance