An Etymological Dictionary of Astronomy and Astrophysics

1. Optics: Any imaginary line passing through the center of curvature of a → spherical mirror or → lens and the corresponding geometrical center.

Principal axis is perpendicular to the mirror or lens at the pole. Rays of light parallel to the principal axis of a → concave mirror

will appear to converge on a point in front of the mirror somewhere between the mirror’s pole and its center of curvature. Same as → optical axis.

2. Mechanics: One of three mutually perpendicular axes of an object about which the → moment of inertia is maximum or minimum.

More specifically, an object has an infinite number of moments of inertia. If an object is not symmetrical about all planes through its → center of mass, then there will be one → axis of rotation about which the moment of inertia is largest, and there will be one axis of rotation about which the moment of inertia is smallest. These two axes will always be perpendicular to each other and are the principal axes of the object. The third principal axis of an object is the axis perpendicular to these two axes.

In general the → angular momentum (L) of a body spinning about a point O is not in the same direction as the axis of rotation, or that of the → angular velocity angular velocity (ω); that is L is not parallel to ω. For certain bodies, however, there can be certain axes for which L and ω are parallel. In that case L = Iω, where I is the moment of inertia about the axis in question.

In a symmetric rigid body, the axes of symmetry coincide with the principal axes of the moment of inertia

See also: → principal; → axis.