A line connecting its widest points is called the major axis. A perpendicular
bisector of this axis is called the minor axis. The proportion of
the length of the minor to the major axis is an expression of the fatness
or thinness of the elipse and thus an indication of the viewer's angle
of view relative to the plan of the circle which the elipse represents.
The drawing to the left represents a stack of floating circles whose
planes are horizontal (level, parallel to the ground). The middle circle
at eye level is perfectly foreshortened (since the viewer's line of sight
to it is in the plane of that circle) thus it is represented by an elipse
so thin it is simply a straight line. The circles above and below
that are very thin elipses due to the acuteness of the viewer's angle of
sight. As the circles rise and fall above and below the horizon,
the elipses which represent them fatten. This is because the viewer is
looking "more into" them.
A cylinder may
be thought of a two stacked circles. If the cylinder is sitting upright
and below eyelevel then the top circle will be represented by an elipse
thinner than that rerpesenting the bottom circle. Note that the cylinder
walls are perpendicular to the major axes of both elipses and are tangent
to the elipses where the major axis contacts the elipse. Because of the
nature of the eliptical curve, there is a smooth transition from the elipse
to the wall of the cylinder regardles of the elipse's thinness.
Do this:
Draw six cans of various sizes sitting
upright scattered distances from you on your table.
