epicyclic theory نگرهی ِ اپی-چرخهای negare-ye apicarxe-yi
*Fr.: théorie épicyclique*
The theory that describes the Galactic dynamics, that is the orbits of stars and gas
clouds in the → *Galactic disk*, as well as the spiral
→ *density wave*. Formulated by Bertil Lindblad (1895-1965),
the epicyclic theory assumes that orbits are circular with small deviations. Star orbits
are described by the superposition of two motions:
*i*) a rotation of the star (epicenter) around the Galactic center at
the circular angular velocity, Ω, and *ii*)
a retrograde elliptical motion at
→ *epicyclic frequency*, κ.
The epicyclic motion in the Galactic plane occurs in a retrograde sense to conserve
→ *angular momentum*. In general Ω and κ
are different and, therefore, orbits do not close. However, seen by an
observer who rotates with the epicenter, orbits are closed ellipses. → *epicyclic*; → *theory*. |