An Etymological Dictionary of Astronomy and Astrophysics
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The state of being similar; likeness; resemblance.

→ similar; → -ity.

1) A transformation that preserves angles and changes all distances in the same ratio.
2) A transformation of the form B = X^-1AX relating two → square matrices A and B.

→ similarity; → transformation.

The property of events occurring → simultaneously.

Noun from → simultaneous; → -ity.

1) Math.: A point at which a given mathematical object is not defined.
2) Physics: A point in → space-time in which gravitational force causes space-time to have infinite curvature; matter is crushed to infinite density and zero volume. This happens at the center of a → black hole where space and time cease to exist as we know them, and the known laws of physics break down. See also → Schwarzschild singularity; → initial singularity.

→ singular + → -ity.

The general term for all forms of short-lived phenomena on the Sun, including → solar flares, → sunspots, → prominences, etc., indicating that the Sun is an active star.

→ solar; → activity.

Same as the → solar cycle.

→ solar activity; → cycle.

The total → radiant energy, in all wavelengths, emitted by the Sun in all directions. It is 3.828 × 10²⁶ W or 3.828 × 10³³ erg sec^-1 (International Astronomical Union, Resolution B3, 14 August 2015, Honolulu, USA). This is the luminosity unit conventionally used to give the luminosities of stars. See also: → solar constant. When the Earth first formed, 4.56 billion years ago, the Sun radiated 30% less energy than it does today, thus giving rise to the so-called → faint early Sun paradox. Ever since then, its power has increased by 7% every billion years (I. Ribas, 2009, arXiv:0911.4872).

→ solar; → luminosity.

The proportion of the solar matter made up of → chemical elements heavier than → helium. It is denoted by Z, which represents the sum of all elements heavier than → helium, in mass fraction. The most recent determination of the solar Z gives a value of 0.0134 (Asplund et al. 2009, ARAA 47, 481), corresponding to the present-day photospheric composition.

→ solar; → metallicity.

The rate of change of the Sun's position with respect to the local standard of rest toward the → solar apex.

→ solar; → velocity.

The state, property, or quality of being solid. Solidness.

→ solid; → -ity.

The average → sound power passing through a unit area perpendicular to the direction that the sound is traveling. It is usually expressed in watts per square meter.

→ sound; → level.

The expression of sound intensity in decibel units. The sound intensity level (L_I), in decibels, is computed as: L_I = 10 log (I/I₀), where I is the measured sound intensity and I₀ is the reference intensity (1 x 10 ^-12 watt per square meter).

→ sound; → intensity; → level.

The number of → overtones present in a sound and their respective intensity. Like → loudness, it is a subjective quantity and cannot be measured with instruments.

→ sound; → quality.

The velocity of a star relative to the Sun.

→ space; → velocity.

The theory formulated by A. Einstein in 1905, which is based on the following two → postulates:
1) → Principle of relativity: The laws of physical phenomena are the same when studied in terms of two reference systems moving at a constant velocity relative to each other.
2) → Principle of constancy: The → velocity of light in free space is the same for all observers and is independent of the relative velocity of the source of light and the observer.
The term "special theory of relativity" refers to the restriction in the first postulate to reference systems moving at a constant velocity relative to each other (→ inertial reference frame). See also → general relativity.

→ special; → relativity.

Same as → relative density.

→ specific; → density.

The ratio of the density of a substance at the temperature under consideration to the density of water at the temperature of its maximum density (4 °C).

→ specific; → gravity.

The dimensionless ratio of the mass of water vapor to the total mass in a particular volume. → humidity

→ specific; → humidity.

A measure of the amount of radiation received per unit solid angle per unit time per unit area normally from an element of surface.

→ specific; → intensity.

The state or character of being → specific.

→ specific; → -ity.