According to Theory of Relativity, regular linear motion must not change speed of light, isn't? Moreover, explanation of Sagnac effect is simple and easy to un

According to Theory of Relativity, regular linear motion must not change speed of light, isn't? Moreover, explanation of Sagnac effect is simple and easy to understand when we accept Aether theory (2 paragraphs of texts), but explanation of same effect with Theory of Relativity needs 10 pages of texts with formulas and manipulations with time. I am sorry, but I cannot accept that anymore.