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

The point on → horizon in direction of → geographic south pole.

→ south; → point.

A point where the → flow  → velocity is → zero. For example a point around an obstacle where a → flow tube splits into two portions.

→ stagnation; → point.

1) Math.: For a → function y = f(x), a point at which the → tangent to the graph is horizontal. In other words, a point where the → slope is zero: dy/dx = 0.
2) Of a planet, the position at which the rate of change of its apparent → right ascension is momentarily zero.

→ stationary; → point.

The point of the surface of a celestial body (including the Earth) at which the Sun is directly overhead at a particular time.

→ sub-; → solar; → point.

The point on the Earth, or other body, at which a particular star is directly overhead at a given time.

→ substellar; → point.

The point on the surface of a celestial body where the star is perceived to be directly overhead (in zenith). The sublunar point and subsolar point are the equivalent points for the Moon and Sun, respectively.

→ sub-; → terrestrial.

The accuracy with which a telescope can be pointed to a particular coordinate in the sky.

→ telescope; → pointing; accuracy.

The definite pressure and temperature at which all three phases of a substance coexist in → phase equilibrium. The triple point of water has a pressure of 4.58 mm of Hg and a temperature of 273.16 °K.

→ triple; → point.

Noqté, → triple; segâné, from sé→ three + -gâné suffix forming plural entities, from Mid.Pers. -gânag, -gâna.

The closest point in the path of a sound wave to the center of a star, as studied in → asteroseismology. Starting from the surface, the sound wave first moves into the star almost straight toward the center. Its path then deflects, because of the increasing → sound speed, so that it misses the center of the star. After the turning point, the wave moves out again until it reaches the surface, where it is reflected. If exactly an integer number of wavelengths fits between two reflections at the surface, the sound wave corresponds to a → standing wave with a specific pattern of → node lines on the surface.

→ turn; → point.

Same as → main-sequence turnoff.

→ turnoff; → point.

The intersection point between the → celestial equator and the → ecliptic from where the Sun apparently enters into the northern celestial hemisphere.

→ vernal; → point.

A configuration of magnetic field lines around a → magnetic null point resembling a cross. See also → O-point.

X, the letter of alphabet whose shape evokes the configuration; → point.

The point at which the → strain caused by a → stress on a material begins to increase without further increase in the stress. This point marks the end of → elastic deformation and the beginning of → plastic deformation. Same as → elastic limit.

→ yield; → point.

1) General: A starting point for making a measurement.
2) A basic parameter on which a → photometric system relies and must be determined each night. The zero points of → magnitudes and color indices (→ color index) depend on atmospheric transparency that varies from night to night.

→ zero; → point.

The lowest possible energy that a → quantum mechanical system may possess. It is the energy of the → ground state of the system. The term "zero point" refers to the observed fact that → vacuum fluctuations persist at → absolute zero temperature. Same as → vacuum energy.

→ zero; → point; → energy.