An Etymological Dictionary of Astronomy and Astrophysics
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1) The fact of being necessary or indispensable.
2) Something necessary or indispensable.

→ necessary; → -ity.

A filter having a flat response over the range of wavelengths of interest. Also called neutral filter or gray filter.

→ neutral; → density; → filter.

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.

The Newton's equations of motion, if they hold in any → reference frame, they are valid also in any other reference frame moving with uniform velocity relative to the first.

→ Newtonian; → principle; → relativity.

The laws of physics are unchanged under → Galilean transformation. This implies that no mechanical experiment can detect any intrinsic diff between two → inertial frames. Same as → Galilean relativity.

→ Newton; → relativity.

The instability of a physical or mathematical system that arises from the nonlinear nature of relevant variables and their interactions within the system.

→ nonlinear; → instability.

The property, condition, or state of being → nonlinear.

→ non-; → linearity.

General: The fact or condition of being normal.
Chemistry: Measure of the number of gram-equivalent weights of a compound per liter of solution.

From → normal + → -ity.

The density of an atomic nucleus (about 10¹⁴ g/cm³).

→ nuclear; → density.

Number of a particular type of object found in each unit volume.

→ number; → density.

A probability determined by the long-term frequency of an event; in contrast to subjective probability.

→ objective; → probability.

1) The state or quality of being → objective. Opposite of → subjectivity.
2) Ability to view things objectively.
3) External reality.

→ objective + → -ity.

1) General: The quality or state of being oblique.
2) The → angle that an object's → rotational axis makes with its → orbital axis. In other words, the angle between its → equatorial plane and → orbital plane. Same as → axial tilt.

State noun of → oblique.

The angle between the Earth's → equatorial plane and the → ecliptic. Its present value is 23°26'13.5'' (or 23.43708 degrees). The effects of → precession and → nutation cause it to change between extreme values of 22.1 and 24.5 degrees over a period of approximately 41,000 years. It is currently decreasing.

→ obliquity; → ecliptic.

The state or quality of being obscure.

→ obscure + → -ity.

1) General: The character of something that can be observed.
2) Astro.: The possibility that an object be observable at a particular time.
3) Control theory: A measure for how well internal states of a system can be inferred by knowledge of its external outputs.

From → observable + → -ity.

1) General: The state or quality of being opaque.
2) A measure of the absorption of photons on their way from the stellar center to the surface. Opacity depends upon the frequency of the radiation, the density, the chemical composition, and the thermodynamic state of the gas. For a given density, the hotter the gas the lower the opacity, since the gas absorbs less readily, as described by → Kramers' law. Conversely, the cooler the gas the higher the opacity. See also → stellar pulsation, → kappa mechanism, → valve mechanism, → partial ionization zone.

From Fr. opacité, from L. opacitatem (nom. opacitas) "shade, shadiness," from opacus "shaded, dark, opaque."

Kederi, from keder "opaque," from Ar. kader + -i suffix forming nouns from adjectives.

The capability of being put into use, operation, or practice.

→ operable; → -ity.

1) An appropriate or favorable time or → occasion.
2) A situation or condition favorable for attainment of a goal (Dictionary.com).

→ opportune + → -ity.

The property possessed by some substances and their solutions of rotating the plane of vibration of → polarized light. When a beam of → linearly polarized light is sent through an optically active substance, such as crystalline quartz and sugar solution, the direction of vibration of the emerging linearly polarized light is found to be different from the original direction. Those which rotate the → plane of polarization to the right, for an observer looking in the incoming beam, are called → dextrorotatory or right handed; those which rotate it to the left, → levorotatory or left handed. Optical activity may be due to an asymmetry of molecules of a substance (solutions of cane sugar) or it may be a property of a crystal as a whole (crystalline quartz).

→ optical; → activity.