An Etymological Dictionary of Astronomy and Astrophysics
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Same as → aberration of starlight .

→ stellar; → aberratio.

The ratio of the peak intensity of the central core of the image of a point source to the corresponding intensity contained in an image obtained in the absence of aberration. An optically perfect system has a Strehl ratio of unity. The ratio may be computed by integrating the area under a → modulation transfer function (MTF) curve and dividing it by the integral over the diffraction-limited MTF.

In honor of Karl Strehl (1864-1940), the German physicist and mathematician who introduced the concept; → ratio.

The process whereby the amount of → water vapor in the air exceeds that needed to → saturate. In other words, the condition of air in which the → humidity is above the level required for saturation at a given temperature (i.e. the → relative humidity is greater than 100%). When the temperature drops below freezing, this can lead to a situation where more water vapor is present in the air than the air can hold. At every temperature, there is a maximum amount of water vapor that can be supported in the air. The higher the temperature, the more water vapor can be accommodated. But if the air that is already at 100% relative humidity is cooled then it becomes supersaturated, and this situation is unstable. As a result, the excess water vapor crystallizes out, either into water droplets or directly into ice.

→ super-; → saturation.

A geometrical method in land surveying for the determination of the relative position of points. In contrast to → triangulation, trilateration involves measuring the lengths of the three sides of touching or overlapping triangles and not their angles.

→ tri-; → lateral; → side; → -tion.

An → orbital migration of low-mass → planets in which no gap is created in the → protoplanetary disk. According to planetary models, beyond a critical core mass for the forming planet, a gap in the protoplanetary disk is created. The critical mass depends on the mass and → metallicity of the disk and therefore it does not have a singular value, but has been shown to be between about 10-30 Earth masses. Compare with → Type II migration.

→ type; → migration.

The → orbital migration of forming → planets that → accrete enough disk material to exceed the critical mass. This type migrates in a gap created in the → protoplanetary disk. Compare with → Type I migration.

→ type; → migration.

1) The act of vibrating. The state of being vibrated.
2) A continuing → periodic  → oscillation relative to a fixed reference point or → equilibrium state.

Verbal noun from → vibrate.

Of or pertaining to → vibration. → vibrational mode.

→ vibration; → -al.

The energy due to the vibration of the molecules making up atoms (→ molecular vibration). A molecule in space can have energies in various forms: → rotational energy, vibrational energy, or electronic energy. These energies of molecules are → quantized and a particular molecule can exist in different rotational and vibrational → energy levels. The molecules can move from one level to another level only by a jump involving a finite amount of energy. → Quantum mechanics predicts that any molecule can never have zero vibrational energy, that is atoms can never be completely at rest relative to each other. The harmonically oscillating molecules can undergo vibrational changes determined by simple selection rules obtained from → Schrödinger equation.

→ vibrational; → energy.

The frequency at which the atoms in a molecule vibrate. The frequencies of → molecular vibrations in diatomic molecules are in the order of 10^-12 to 10^-14 Hz. In such molecules, the only → vibrational mode available is along the bond. More complicated molecules have many types of vibration and stretching modes.

→ vibrational; → frequency.

Any of the ways in which a → molecule vibrates. Each vibrational mode has a different → frequency frequency. The number of vibrational modes of a molecule is determined by the number of atoms in it. The number of vibrational modes for a non-linear molecule is 3N - 6, where N is the number of atoms making up the molecule. For a linear molecule it is 3N - 5.

→ vibrational; → mode.

A transition between two → quantized  → levels of a → molecule that have different vibrational energies.

→ vibrational; → transition.

A slight change in the → energy level of a → molecule due to → vibrational transition and/or → rotational transition.

→ vibrational; → rotational; → transition.

of a gas included in the composition of a → gas mixture, the ratio of mass of this gas to the mass of the whole mixture. Same as → weight fraction and → weight-fraction concentration.

→ weight; → concentration.

Same as → weight concentration.

→ weight; → fraction; → concentration.