TY - JOUR T1 - Proč je obvod Slunce menší než 2Pi r? A1 - Křížek, Michal JA - Obzory matematiky, fyziky a informatiky Y1 - 2017 VL - 46 IS - 3 SP - 7 EP - 18 SN - 1335-4981 N2 - In this paper we investigate differences between the Euclidean geometry and the spacetime geometry. We derive formulas for the proper radius and proper volume of a homogeneous mass ball. We shall see that the homogeneous ball, whose mass and radius is the same as that of the Sun, has its circumference about 3 km shorter than 2Pi r, where r is its proper radius. Similarly, the Earth has its proper volume about 457 km3 larger than the massless ball with the same circumference. The difference between the classical Euclidean geometry and the geometry of a curved spacetime will be most visible for balls corresponding to compact astrophysical objects such as, e.g., neutron stars. ER -