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

→ d'Alembert's principle.

→ d'Alembert's principle; → Lagrangian.

To burn suddenly and violently with great heat and intense light. → deflagration.

From L. deflagratus, p.p. of deflagrare "to burn down," from → de- + flag(rare) "to blaze, glow, burn" (L. fulgur "lightning;" PIE *bhleg- "to shine;" cf. Gk. phlegein "to burn, scorch," Skt. bhárgas- "radiance, lustre, splendour," O.E. blæc "black") + -atus "-ate"

Taškaftidan, from taš "fire," variant of âtaš→ fire + kaftidan "to explode," → explode.

A rapid → chemical reaction in which the → output of → heat is enough to enable the reaction to proceed and be accelerated without input of heat from another source. The effect of a true deflagration under confinement is an → explosion. See also: → detonation; → explosion.

Verbal noun of → deflagrate.

One of the five → Lagrangian points, denoted L1, which lies between the two bodies on the line passing through their center of mass. In a → close binary star system mass transfer occurs through this point.

→ inner; → Lagrangian points.

1a) A lagging or falling behind; retardation.
1b) Mechanics: The amount of retardation of some motion.
1c) Electricity: The retardation of one alternating quantity, as current, with respect to another related alternating quantity, as voltage.
2) To fail to maintain a desired pace or to keep up; fall or stay behind (Dictionary.com).

Possibly from Scandinavian; cf. Norwegian lagga "to go slowly."

Lek, from lek lek kardan "to walk slowly, to lag behind."

1) A body of seawater that is almost completely cut off from the ocean by a barrier beach.
2) The body of seawater that is enclosed by an atoll.

Lagoon, from Fr. lagune, from It. laguna "pond, lake," from L. lacuna "pond, hole," from lacus "pond;" → nebula.

Mordâb "lagoon," literally "dead water," from mord, mordé "dead" + âb "water."
The first element from mordan, mir- "to die," marg "death," mard "man;" Mid.Pers. murdan "to die;" O.Pers. marta- "dead," martiya- "man;" Av. mərəta- "died, dead," amərətāt- "immortality;" cf. Skt. mar- "to die," mriyáe "dies;" Gk. emorten "to die," ambrotos "immortal;" L. morior "to die" (Fr. mourir), mors, mortis "death" (Fr. mort), immortalis "immortal;" Lith. mirtis "mortal;" O.C.S. mrutvu "dead;" O.Ir. marb; Welsh marw "died;" O.E. morþ "murder;" PIE base *mor-/*mr- "to die."
The second element âb "water," from Mid.Pers. âb "water;" O. Pers. ap- "water;" Av. ap- "water;" cf. Skt. áp- "water;" Hitt. happa- "water;" PIE āp-, ab- "water, river;" cf. Gk. Apidanos, proper noun, a river in Thessalia; L. amnis "stream, river" (from *abnis); O.Ir. ab "river," O.Prus. ape "stream," Lith. upé "stream;" Latv. upe "brook."

A giant → H II region lying in the direction of → Sagittarius about 5,000 → light-years away. It represents a giant cloud of interstellar matter which is currently undergoing star formation, and has already formed a considerable cluster of young stars (NGC 6530).

→ lagoon; → nebula.

A set of second order → differential equations for a system of particles which relate the kinetic energy of the system to the → generalized coordinates, the generalized forces, and the time. If the motion of a → holonomic system is described by the generalized coordinates q₁, q₂, ..., q_n and the → generalized velocities  q^.₁, q^.₂, ..., q^._n, the equations of the motion are of the form: d/dt (∂T/∂q^._i) - ∂T/∂q^._i = Q_i (i = 1, 2, ..., n), where T is the kinetic energy of the system and Q_i the generalized force.

→ Lagrangian; → equation.

1) Of or relating to Joseph-Louis Lagrange (1736-1813), see below.
2) Same as → Lagrangian function. The Lagrangian of a → dynamical system describes its → dynamics and when subjected to an → action gives rise to → field equations and a → conservation law for the theory. Lagrangians are the keys for the mathematical formulation of field theories ( → field theory).
See also:
→ inner Lagrangian point, → Lagrangian density, → Lagrangian dynamics, → Lagrangian formalism, → Lagrangian function, → Lagrangian method, → Lagrangian multiplier, → Lagrangian particle, → Lagrangian point.

After the French/Italian mathematician Joseph-Louis Lagrange (1736-1813), who was the creator of the → calculus of variations (at the age of nineteen). He made also great advances in the treatment of → differential equations and applied his mathematical techniques to problems of → mechanics, especially those arising in astronomy.

A quantity, denoted L_d, describing a continuous system in the → Lagrangian formalism, and defined as the → Lagrangian per unit volume. It is related to the Lagrangian L by:
L = ∫∫∫L_d d³V.
Lagrangian density is often called Lagrangian when there is no ambiguity.

→ Lagrangian; → density.

A reformulation of → Newtonian mechanics in which dynamical properties of the system are described in terms of generalized variables. In this approach the → generalized coordinates and → generalized velocities are treated as independent variables. Indeed applying Newton's laws to complicated problems can become a difficult task, especially if a description of the motion is needed for systems that either move in a complicated manner, or other coordinates than → Cartesian coordinates are used, or even for systems that involve several objects. Lagrangian dynamics encompasses Newton dynamics, and moreover leads to the concept of the → Hamiltonian of the system and a process by means of which it can be calculated. The Hamiltonian is a cornerstone in the field of → quantum mechanics.

→ Lagrangian; → dynamics.

A reformulation of classical mechanics that describes the evolution of a physical system using → variational principle The formalism does not require the concept of force, which is replaced by the → Lagrangian function. The formalism makes the description of systems more simpler. Moreover, the passage from classical description to quantum description becomes natural. Same as → Lagrangian dynamics.

→ Lagrangian; → formalism.

A physical quantity (denoted L), defined as the difference between the → kinetic energy (T) and the → potential energy (V) of a system: L = T - V. It is a function of → generalized coordinates, → generalized velocities, and time. Same as → Lagrangian, → kinetic potential.

→ Lagrangian; → function.

Fluid mechanics: An approach in which a single fluid particle (→ Lagrangian particle) is followed during its motion. The physical properties of the particle, such as velocity, acceleration, and density are described at each point and at each instant. Compare with → Eulerian method.

→ Lagrangian; → method.

Math.: A constant that appears in the process for obtaining extrema of functions of several variables. Suppose that the function f(x,y) has to be maximized by choice of x and y subject to the constraint that g(x,y)≤ k. The solution can be found by constructing the → Lagrangian function  L(x,y,λ) = f(x,y) + λ[k - g(x,y)], where λ is the Lagrangian multiplier.

→ Lagrangian point; → multiplier.

Fluid mechanics: In the → Lagrangian method, a particle that moves as though it is an element of fluid. The particle concept is an approach to solving complicated fluid dynamics problems by tracking a large number of particles representing the fluid. The particle may be thought of as the location of the center of mass of the fluid element with one or more property values.

→ Lagrangian; → particle.

On of the five locations in space where the → centrifugal force and the → gravitational force of two bodies (m orbiting M) neutralize each other. A third, less massive body, located at any one of these points, will be held in equilibrium with respect to the other two. Three of the points, L1, L2, and L3, lie on a line joining the centers of M and m. L1 lies between M and m, near to m, L2 lies beyond m, and L3 on the other side of M beyond the orbit. The other two points, L4 and L5, which are the most stable, lie on either side of this line, in the orbit of m around M, each of them making an equilateral triangle with M and m. L4 lies in the m's orbit approximately 60° ahead of it, while L5 lies in the m's orbit approximately 60° behind m. See also → Trojan asteroid; → Roche lobe; → equipotential surface; → horseshoe orbit.

→ Lagrangian; → point.

The delay between → sunset and → moonset.

→ moonset; → lag.

1) General: Same as → phase difference.
2) Cepheids: The observed phase difference between luminosity and velocity in classical (radially pulsating) → Cepheids. On the basis of adiabatic pulsation theory, one would expect the maximum luminosity to occur when the radius of the star is minimal. This means that the maximum outward velocity would be one quarter period out of phase with the maximum velocity. However, in the observations the maximum luminosity and maximum outward velocity are nearly in phase. This effect is due to the → kappa mechanism which is responsible for driving the → pulsations. The pulsations in Cepheids are excited by the helium → partial ionization zone, He⁺↔ He⁺⁺, which is located below the He ↔ He⁺ and H ↔ H⁺ zones. These latter two regions are too shallow to contribute significantly to the driving of the fundamental modes of Cepheids; so their only effect is to introduce a phase shift.

→ phase; lag, possibly from a Scandinavian source; cf. Norw. lagga "go slowly."

Degarsâni, → difference; fâz→ phase.

A bright cloud-like feature that appears in the vicinity of a sunspot. Plages represent regions of higher temperature and density within the chromosphere. They are particularly visible when photographed through filters passing the spectral light of hydrogen or calcium.

From Fr., from It. piaggia, from L.L. plagia "shore;" noun use of the feminine of plagius "horizontal;" frpm Gk. plagios "slanting, sideways" from plag(os) "side" + -ios adj. suffix.

Plâž, loan from Fr., as above.