The speed of light according to the Special Theory of Relativity has the same value C with respect to any inertial frame of reference in 4-dimensional pseudo-Euclidean space. An attempt to build an alternative physical model in 3-dimensional Euclidean space brings up a question: in what frame of reference does light move with the speed C? Series of experiments and mathematical reasoning suggest that: 1) the speed of a photon emitted by an atom equals to C if measured with respect to the atom if the atom is at rest or moving with a constant speed with respect to a 3-dimensional inertial frame of reference; 2) the speed of light acquires the value of a constant C near Earth, Sun and other large masses; the energy of the photon depends on the gravitational potential, 3) light has inertial properties.