a coordinate dependent speed of light. Hendrik Lorentz (1913) obtained coordinates similar to (, , ) while studying Einstein's equivalence principle and the uniform gravitational field. A detailed description was given by Friedrich Kottler (1914), who formulated the corresponding orthonormal tetrad, transformation formulas and metric (, ). Also Karl Bollert (1922) obtained the metric () in his study of uniform acceleration and uniform gravitational fields. In a paper concerned with Born rigidity, Georges Lemaître (1924) obtained coordinates and metric (, ). Albert Einstein and Nathan Rosen (1935) described (, ) as the "well known" expressions for a homogeneous gravitational field. After Christian Møller (1943) obtained (, ) in as study related to homogeneous gravitational fields, he (1952) as well as Misner & Thorne & Wheeler (1973) used Fermi–Walker transport to obtain the same equations. While these investigations were concerned with flat spacetime, Wolfgang Rindler (1960) analyzed hyperbolic motion in curved spacetime, and showed (1966) the analogy between the hyperbolic coordinates (, ) in flat spacetime with Kruskal coordinates in Schwarzschild space. This influenced subsequent writers in their formulation of Unruh radiation measured by an observer in hyperbolic motion, which is similar to the description of Hawking radiation of black holes. Horizon Born (1909) showed that the inner points of a Born rigid body in hyperbolic motion can only be in the region . Sommerfeld (1910) defined that the coordinates allowed for the transformation between inertial and hyperbolic coordinates must satisfy . Kottler (1914) defined this region as , and pointed out the existence of a "border plane" () , beyond which no signal can reach the observer in hyperbolic motion. This was called the "horizon of the observer" () by Bollert (1922). Rindler (1966) demonstrated the relation between such a horizon and the horizon in Kruskal coordinates. Radar coordinates Using Bollert's formalism, Stjepan Mohorovičić (1922) made a different choice for some parameter and obtained metric () with a printing error, which was corrected by Bollert (1922b) with another printing error, until a version without printing error was given by Mohorovičić (1923). In addition, Mohorovičić erroneously argued that metric (, now called Kottler–Møller metric) is incorrect, which was rebutted by Bollert (1922). Metric () was rediscovered by Harry Lass (1963), who also gave the corresponding coordinates () which are sometimes called "Lass coordinates". Metric (), as well as (, ), was also derived by Fritz Rohrlich (1963). Eventually, the Lass coordinates