Let 
be the orthic
triangle of a triangle 
. Then each side of each triangle meets the three sides
of the other triangle, and the points of intersection lie on a line 
called the orthic axis of 
.
The orthic axis is central line 
, has trilinear equation
 |
It is perpendicular to the Euler line.
It passes through Kimberling centers 
for 
, 232, 468, 523 (isogonal
conjugate of the focus of the Kiepert hyperbola),
647, 650, 676, 1637, 1886, 1990, 2485, 2489, 2490, 2491, 2492, 2493, 2501, 2506,
2977, 3003, 3011, 3012, and 3018. The anticomplement
of the orthic axis is the de Longchamps line.
The orthic axis is the perspectrix of the medial triangle and tangential triangle, as well as (by definition) the orthic triangle and reference triangle.
It is the radical line of the coaxal system consisting of (circumcircle, nine-point circle, orthocentroidal circle, orthoptic circle of the Steiner inellipse, polar circle, tangential circle). This includes the particular cases of the circumcircle and the nine-point circle (Casey 1888, p. 176; Kimberling 1998, p. 150), as well as of any two of the circumcircle, nine-point circle, and polar circle (Tummers 1960-61).
The angle between the orthic axis and Gergonne line is equal to that between the Euler line and the Soddy line (F. Jackson, pers. comm., Nov. 2, 2005).
