On Geometry the diameter is the a straight line that passes through the center and joins two opposite points, either on a circle, a closed curve, or a spherical surface. Every diameter will divide the circle in question into two semicircles.
The relationship established between the length of the circumference and its diameter is a constant called pi and is written as follows: π. Anyway, it might also be that we meet phi, which is pronounced fi. The most popular abbreviation for diameter turns out to be day or D. Meanwhile, its value is approaching 355/113, which is the same as 3.14159.
In the field of engineering and also from other technical areas, the symbol or the variable for the diameter is similar in both size and design to or, although it is important that this symbol is not confused with that of the empty set, which is similar, although in the case of the latter it is written in capital letters while the diameter is in lower case.
For his part, the angular diameter is the apparent dimension of the equatorial diameter of a celestial body expressed as an angle and assuming the observer at the vertex. In the case of the sun, the moon and the planets, the measurement of the angular diameter will be known through optical or micrometric procedures, while, in the case of the brightest stars, the angular diameter will be known from interferometric procedures, which they assume interference.
And in Hydraulics, the branch of the Physical which deals with the study of the mechanical properties of fluids, the hydraulic diameter It turns out to be a fairly common concept in the case of non-circular channel and tube fluids. From this concept it is possible to know more about the behavior of the flow in the same way as if it were a pipe with a circular section.