An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

Of, pertaining to, characterized by, or forming an axis.

→ axis; → -al.

The angle at which a planet's axis of rotation is tilted, with respect to that of the planet's orbit.

→ axial; → inclination.

1) A change in the orientation of the → rotation axis of a non-spherical, spinning body caused by → gravity. A rotating top will precess in a direction determined by the → torque exerted by its → weight. The precession → angular velocity is inversely proportional to the spin angular velocity, so that the precession is faster and more pronounced as the top slows down.
2) In particular, the gradual shift in the orientation of Earth's rotation axis which traces out a conical path once every ~ 26,000 years. The cause of the Earth's precession is the → equatorial bulge of the Earth, brought about by the → centrifugal force of the → rotation, which has changed the Earth from a perfect sphere to a slightly flattened one, thicker across the equator. The attraction of the Moon and Sun on the bulge is the reason which makes the Earth precess.

→ axial; → precession.

A symmetry with respect to a line. A geometric configuration is said to have axial symmetry if it remains unchanged when rotated about a given line.

→ axial; → symmetry.

The angle between an object's → rotation axis and its → orbital axis. In other words, the angle between its → equatorial plane and → orbital plane. Same as → axial inclination and → obliquity, but not to be confounded with → orbital inclination.

→ axial; → tilt.

A birefrigent crystal, such as mica, that is characterized by having two optical axes along which light is propagated with equal velocities.

Biaxial, from → bi- "two" + axial, from → axe; → crystal.

Math.: having three axes of the same length. Also equiaxed.
Physics: A crystal exhibiting similar dimensions in all directions.

→ equi-; → axis.

Characteristic of optical analyses that are limited to infinitesimally small apertures. Also called first-order or Gaussian optics.

→ para-; → axial.

From par(â)-, → para-, + âseyi→ axial.

A ray that lies close to and almost parallel to the optical axis and behaves according to paraxial equations.

→ paraxial, → ray.

The angle between the Sun's → rotation axis and perpendicular to the → ecliptic plane. In other words, the inclination of the Sun's → equator with respect to the → ecliptic plane. It is 7.25 degrees.

→ solar; → axial; → tilt.

Crystal with double refraction possessing only one → optic axis.

→ uni-; axial adj. from → axis; → crystal.