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

A collection of 109 impressive celestial objects compiled for amateur astronomers. These objects (→ star clusters, → nebulae, → supernova remnants, and → galaxies), selected from the → New General Catalog and the → Index Catalog, are not present in the → Messier catalog.

Named after Patrick Caldwell Moore (1923-2012), English amateur astronomer, who compiled the catalog in 1995; → catalog.

The unit of → magnetic flux. The flux through 1 square cm normal to a magnetic field of 1 → gauss. It is equal to 10^-8 → weber (Wb)s.

After James Clerk Maxwell (1831-1879), British outstanding physicist, who made fundamental contributions to electromagnetic theory and the kinetic theory of gases.

A type of → Wheatstone bridge used for measuring → inductance in terms of → resistance and → capacitance.

→ maxwell; → bridge.

A division in Saturn's ring in the outer part of the C ring. It is about 87500 km from Saturn's center and is 500 km wide. The gap was discovered in 1980 by Voyager 1.

Not discovered by J. C. Maxwell, but named in his honor; → maxwell; → gap.

A → thought experiment meant to raise questions about the possibility of violating the → second law of thermodynamics. A wall separates two compartments filled with gas. A little "demon" sits by a tiny trap door in the wall. He is able to sort hot (faster) molecules from cold molecules without expending energy, thus bringing about a general decrease in → entropy and violating the second law of thermodynamics. The → paradox is explained by the fact that such a demon would still need to use energy to observe and sort the molecules. Thus the total entropy of the system still increases.

Named after James Clerk Maxwell (→ maxwell), who first thought of this experiment; → demon.

A set of four fundamental equations that describe the electric and magnetic fields arising from varying electric charges and magnetic fields, electric currents, charge distributions, and how those fields change in time. In their vector differential form, these equations are:
i) ∇.E = ρ/ε₀ (→ Gauss's law for electricity),
ii) ∇.B = 0 (→ Gauss's law for magnetism),
iii) ∇ x E = -∂B/∂t (→ Faraday's law of induction),
iv) ∇ x B = μ₀J + μ₀ε₀∂E/∂t (→ Ampere's law), with c² = 1/(μ₀ε₀), where E is → electric intensity, B is → magnetic flux density, ρ is → charge density, ε₀ is → permittivity, μ₀ is → permeability, J is → current density, and c is → speed of light.

→ maxwell. It should be emphasized that the equations originally published by James Clerk Maxwell in 1873 (in A Treatise on Electricity and Magnetism) were 20 in number, had 20 variables, and were in scalar form. The German physicist Heinrich Rudolf Hertz (1857-1894) reduced them to 12 scalar equations (1884). It was the English mathematician/physicist Oliver Heaviside (1850-1925) who expressed Maxwell's equations in vector form using the notations of → gradient, → divergence, and → curl of a vector, thus simplifying them to the present 4 equations (1886). Before Einstein these equations were known as Maxwell-Heaviside-Hertz equations, Einstein (1940) popularized the name "Maxwell's Equations;" → equation.

Every part of a deformable electric circuit tends to move in such a direction as to enclose the maximum magnetic flux.

→ maxwell; → rule.

The distribution law for kinetic energies (or, equivalently, speeds) of molecules of an ideal gas in equilibrium at a given temperature.

→ maxwell; → Boltzmann's constant; → distribution.

The incompatibility between → Galilean relativity and Mawxell's theory of → electromagnetism. Maxwell demonstrated that electrical and magnetic fields propagate as waves in space. The propagation speed of these waves in a vacuum is given by the expression c = (ε₀.μ₀)^-0.5, where ε₀ is the electric → permittivity and μ₀ is the magnetic → permeability, both → physical constants. Maxwell noticed that this value corresponds exactly to the → speed of light in vacuum. This implies, however, that the speed of light must also be a universal constant, just as are the electrical and the magnetic field constants! The problem is that → Maxwell's equations do not relate this velocity to an absolute background and specify no → reference frame against which it is measured. If we accept that the principle of relativity not only applies to mechanics, then it must also be true that Maxwell's equations apply in any → inertial frame, with the same values for the universal constants. Therefore, the speed of light should be independent of the movement of its source. This, however, contradicts the vector addition of velocities, which is a verified principle within → Newtonian mechanics. Einstein was bold enough to conclude that the principle of Newtonian relativity and Maxwell's theory of electromagnetism are incompatible! In other words, the → Galilean transformation and the → Newtonian relativity principle based on this transformation were wrong. There exists, therefore, a new relativity principle, → Einsteinian relativity, for both mechanics and electrodynamics that is based on the → Lorentz transformation.

→ Newton; → Maxwell; → incompatibility.

Region in a → field of force in which the potential decreases abruptly, and in the surrounding region of which the potential is larger.

→ potential; → well.

1) In a good or satisfactory manner; thoroughly, carefully, or soundly.
2) A hole drilled or bored into the earth to obtain water, petroleum, natural gas, brine, or sulfur (Dictionary.com).

1) M.E., from O.E. wel(l) (cognates Du. wel, Ger. wohl).
2) M.E. well(e), O.E. wylle, wella, welle (cognates: O.Saxon wallan, O.Fris. walla, O.H.G. wallan, Ger. wallen "to bubble, boil").

1) Xoš "good, well, sweet, fair, lovely," probably related to hu- "good, well," → eu-. Xub, ultimately from Av. huuāpah- "doing good work," → operate.
2) Câh "a well," from Mid.Pers. câh "a well;" Av. cāt- "a well," from kan- "to dig," uskən- "to dig out;" O.Pers. kan- "to dig," akaniya- "it was dug;" Mod.Pers. kandan "to dig;" cf. Skt. khan- "to dig," khanati "he digs," kha- "cavity, hollow, cave, aperture."

A string of → symbols from the alphabet of the → formal language that conforms to the grammar of the formal language. → closed wff, → open wff.

Wff, pronounced whiff; → well; → form; → formula.

A set in which every → nonempty → subset has a minimum element.

→ well; → order; → set.

1) A well used in Antiquity from bottom of which the sky could be observed during the day with a better contrast. The aperture of the well reduced the light diffused by the sky.
2) A vertical tunnel in → Paris Observatory (built in 1667), from the top roof to the underground vaults, 55m deep. This well had the purpose of observing the stars near the → zenith and measure their → parallaxes resulting from the Earth motion around Sun. A long plumb line was attached to the top of the well. Astronomers thought they could measure the stellar shifts with respect to the plumb line. The problem was, however, the lack of stability of the images, because the well acted in fact as a chimney generating turbulence. So that the zenithal well was hardly used. See also: → zenith telescope.

→ zenithal; → well.