Geometry in projection: when a true circle is viewed at an oblique angle (not 90° to the line of sight), how will it appear in the projected view?

Introduction / Context:
Understanding how shapes transform under projection is critical in descriptive geometry, CAD visualization, and technical drawing. Circles rarely remain circles unless the viewing plane is perpendicular to the circle's plane.

Given Data / Assumptions:

• A circular feature (e.g., hole, boss) lies on a plane not perpendicular to the viewing plane.
• Standard orthographic or auxiliary projection is used.
• The observer's line of sight is not normal to the circle's plane.

Concept / Approach:
A circle projected onto a plane at an oblique angle maps to an ellipse. This result follows from affine projection geometry: the locus of points equidistant from a center on a tilted plane becomes an ellipse on the projection plane.

Step-by-Step Solution:

Verification / Alternative check:
Construct an auxiliary view normal to the circular plane; the feature will appear circular there, confirming the ellipse in the oblique principal view.

Why Other Options Are Wrong:

• Circular: Only when viewed normal to the circle's plane.
• Lengthened/Angular: Vague descriptors; the precise geometric result is an ellipse.

Common Pitfalls:
Dimensioning the elliptical apparent diameter as if it were true diameter leads to manufacturing errors; always show a true view for critical fits.

Final Answer:
Elliptical