An Etymological Dictionary of Astronomy and Astrophysics

One of the pair of yellow lines in emission spectra of neutral sodium (Na I). D1 has a wavelength of 5895.94 Å and D2 is 5889.97 Å. This sodium doublet is one of the strongest absorption features in the spectra of late-type stars.

See also: Labelled D in a sequence of alphabetical letters first used by Joseph von Fraunhofer to designate spectral features in the solar spectrum, → Fraunhofer line.

One of the pair of yellow lines in emission spectra of neutral sodium (Na I). D1 has a wavelength of 5895.94 Å and D2 is 5889.97 Å. This sodium doublet is one of the strongest absorption features in the spectra of late-type stars.

See also: Labelled D in a sequence of alphabetical letters first used by Joseph von Fraunhofer to designate spectral features in the solar spectrum, → Fraunhofer line.

The inner → Saturn’s rings, with a width of 7,500 km, lying before the → C ring, at 66,900 km from the center of Saturn.

See also: → ring.

The inner → Saturn’s rings, with a width of 7,500 km, lying before the → C ring, at 66,900 km from the center of Saturn.

See also: → ring.

An → ionization front of → H II regions
whose expansion speed is comparable to the → sound speed in the gas (~ 10 km/sec for hydrogen at 10⁴ K). A D-type ionization front results from → R-type ionization front when its propagation speed decreases as the volume of gas ahead of the ionization front grows. If front velocity is equal to a lower limit (C₁² / 2C₂, where C₁ and C₂ are the sound speed ahead and behind the front respectively), the front is called D critical.

See also: D referring to a dense gas; → type; → ionization; → front.

An → ionization front of → H II regions
whose expansion speed is comparable to the → sound speed in the gas (~ 10 km/sec for hydrogen at 10⁴ K). A D-type ionization front results from → R-type ionization front when its propagation speed decreases as the volume of gas ahead of the ionization front grows. If front velocity is equal to a lower limit (C₁² / 2C₂, where C₁ and C₂ are the sound speed ahead and behind the front respectively), the front is called D critical.

See also: D referring to a dense gas; → type; → ionization; → front.

→ d’Alembert’s principle.

Etymology (EN): → d’Alembert’s principle; → Lagrangian.

→ d’Alembert’s principle.

Etymology (EN): → d’Alembert’s principle; → Lagrangian.

A hydrodynamical paradox arising from the neglect of → viscosity in the → steady flow of a fluid around a submerged solid body. According to this paradox, the submerged body would offer no resistance to the flow of an → inviscid fluid and the pressure on the surface of the body would be symmetrically distributed about the body. This paradox may be traced to the neglect of the viscous forces, which are indirectly responsible for fluid resistance by modifying the velocity field close to a solid body (Meteorology Glossary, American Meteorological Society).

See also: → d’Alembert’s principle; → paradox.

A hydrodynamical paradox arising from the neglect of → viscosity in the → steady flow of a fluid around a submerged solid body. According to this paradox, the submerged body would offer no resistance to the flow of an → inviscid fluid and the pressure on the surface of the body would be symmetrically distributed about the body. This paradox may be traced to the neglect of the viscous forces, which are indirectly responsible for fluid resistance by modifying the velocity field close to a solid body (Meteorology Glossary, American Meteorological Society).

See also: → d’Alembert’s principle; → paradox.

The statement that a moving body can be brought to a → static equilibrium by applying an imaginary inertia force of the same magnitude as that of the accelerating force but in the opposite direction. More specifically, when a body of mass m is moving with a uniform acceleration a under the action of an external force F, we can write: F = m . a, according to Newton’s second law. This equation can also be written as: F - ma = 0. Therefore, by applying the force -ma, the body will be considered in equilibrium as the sum of all forces acting on it is zero. Such equilibrium is called → dynamic equilibrium. Owing to this principle, dynamical problems can be treated as if they were statical.

See also: Named after the French mathematician and philosopher Jean le Rond d’Alembert (1717-1783), who introduced the principle in his Traité de dynamique (1743).

The statement that a moving body can be brought to a → static equilibrium by applying an imaginary inertia force of the same magnitude as that of the accelerating force but in the opposite direction. More specifically, when a body of mass m is moving with a uniform acceleration a under the action of an external force F, we can write: F = m . a, according to Newton’s second law. This equation can also be written as: F - ma = 0. Therefore, by applying the force -ma, the body will be considered in equilibrium as the sum of all forces acting on it is zero. Such equilibrium is called → dynamic equilibrium. Owing to this principle, dynamical problems can be treated as if they were statical.

See also: Named after the French mathematician and philosopher Jean le Rond d’Alembert (1717-1783), who introduced the principle in his Traité de dynamique (1743).

A second order, → partial differential operator in space-time, defined as: &#9643² = ∂²/∂x² + ∂²/∂y² + ∂²/∂z² - (1/c²)∂²/∂t², or &#9643² = ∇² - (1/c²)(∂²/∂t²), where ∇² is the → Laplacian and c is the → speed of light. This operator is the square of the → four-dimensional operator 
&#9643, which is Lorentz invariant.

See also: → d’Alembert’s principle; → operator.

A second order, → partial differential operator in space-time, defined as: &#9643² = ∂²/∂x² + ∂²/∂y² + ∂²/∂z² - (1/c²)∂²/∂t², or &#9643² = ∇² - (1/c²)(∂²/∂t²), where ∇² is the → Laplacian and c is the → speed of light. This operator is the square of the → four-dimensional operator 
&#9643, which is Lorentz invariant.

See also: → d’Alembert’s principle; → operator.

The → neutral → helium  → spectral line at 5876 Å.

See also: D3, because of confusion with the sodium → D lines.
When Joseph N. Lockyer first observed this line in the solar spectrum at the eclipse of 1868, helium was not yet isolated on Earth. Initially,
this line was thought to be the third member of the D1 and D2 line family
of sodium which lie in the same yellow part of the spectrum; → line.

The → neutral → helium  → spectral line at 5876 Å.

See also: D3, because of confusion with the sodium → D lines.
When Joseph N. Lockyer first observed this line in the solar spectrum at the eclipse of 1868, helium was not yet isolated on Earth. Initially,
this line was thought to be the third member of the D1 and D2 line family
of sodium which lie in the same yellow part of the spectrum; → line.