An Etymological Dictionary of Astronomy and Astrophysics
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If event A must occur for event B to occur, then it is said that A is → necessary for B. If event A may cause B but there could be some other cause as well, then it is said that A is sufficient to cause B. See also → if and only if (iff).

→ necessary; → and; → sufficient; → condition.

1) The act of denying; → denial.
2) The absence or → opposite of something that is actual, positive, or affirmative.
3) A → negative statement, idea, doctrine; a contradiction, refutation, or rebuttal.
4) Logic: If p is a → proposition, then the statement "not p," denoted ¬ p, is the negation or opposite of p. If p is "It is sunny," then ¬ p is "It is not sunny." If p is → true, then ¬ p is → false, and vice versa.

Verbal noun of → negate.

A correlation between two variables such that as one variable's values tend to increase, the other variable's values tend to decrease.

→ negative; → correlation.

A type of polarization in which the direction of polarization becomes reversed.

→ negative; → polarization.

In computer programing, a function that is defined inside the definition of another function.

→ nested; → function.

A method in the evaluation of polynomials which involves fewer basic operations and allows simpler computation, especially for polynomials of high degree. More specifically, the polynomial P(x) = a₀ + a₁x + a₂x² + a₃x³ + ... + a_nxⁿ can be written in the nested form as: P(x) = a₀ + x(a₁ + x(a₂ + ... + x(a_{n - 1} + a_nx) ...)). For example, the polynomial P(x) = x³ - 5x² + 10x - 3 has the following nested form: P(x) = ((x - 5)x + 10)x - 3. Same as the → Ruffini-Horner method.

→ nested; → multiplication.

In optics, the process of combining two lenses having equal and opposite powers to produce a result having no power.

Verbal noun of → neutralize.

The transition between neutrino types (→ neutrino flavor) which is a probabilistic consequence of → quantum mechanics. A neutrino, when produced, is in a quantum state which has three different masses. Therefore, an electron neutrino emitted during a reaction can be detected as a muon or tau neutrino. In other words, the flavor eigenstates are different from the propagation eigenstates. This phenomenon was discovered in → solar neutrinos as well as in → atmospheric neutrinos. Neutrino oscillation violates the conservation of the → lepton number; it is possible only if neutrinos have a mass. First predicted by Bruno Pontecorvo in 1957, neutrino oscillation has since been observed by several experiments. It resolved the long-standing → solar neutrino problem. The smaller the mass difference between the flavors, the longer the oscillation period, so that oscillations would not occur if all of the flavors were equal in mass or were massless. Moreover, the oscillation period increases with neutrino energy.

→ neutrino; → oscillation.

A type of radioactive decay of atoms containing excess neutrons, in which a neutron is ejected from the nucleus.

→ neutron; → emission.

The reaction that transforms a → proton into a → neutron when a proton and an → electron are forced together to make a neutron: p + e^-→ n + ν^__e. In astronomy, this process occurs during the → core collapse of → massive stars which leads to the formation of → neutron stars.

→ neutron; → -ize; → -tion.

In → geometric optics, an expression relating the → focal lengths of an → optical system (f and f') and the object x and image x' distances measured from the respective focal points. Thus, ff' = xx'. Same as Newton's formula.

→ Newton; → equation.

A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.

→ Newton; → first; → law; → motion.

The universal law which states that the force of attraction between any two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them: F = G (m₁.m₂)/r², where G is the → gravitational constant.

→ Newton; → law; → gravitation.

The three fundamental laws which are the basis of → Newtonian mechanics. They were stated in Newton's Principia (1687). → Newton's first law, → Newton's second law , → Newton's third law.

→ Newton; → law; → motion.

For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.

→ Newton; → second; → law; → motion.

In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction.

→ Newton; → third; → law; → motion.

A particular solution of the → general relativity when the → gravitational mass is small. The → space-time is then approximated to the → Minkowski's and this leads to → Newtonian mechanics.

→ Newtonian; → approximation.

Same as the → gravitational constant.

→ Newtonian; → constant; → gravitation.

For a ring inclined to the planet's equator, the points at which the ring crosses the equator (nodes) slowly move around the planet (regress) in a direction opposite to that of the ring's orbital motion. (Ellis et al., 2007, Planetary Ring Systems, Springer).

→ nodal; → regression.

An act or instance of nominating, especially to office. The state of being nominated (Dictionary.com).

Verbal noun of → nominate.