1) Math.: For a → functiony =
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.

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.

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.

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

turning point

گشتگاه

gaštgâh (#)

Fr.: tournant

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.

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.