Measurements play a vital role in modern science, but everyday life is unthinkable without them. For example, without measurements it is impossible to know what is near us and what is far away. If we make a list of all the measurements we take during the day, we will be surprised at how long it will be. Throughout its history, humanity has developed various measurement methods. With their help, we were able to determine the size of our planet, the extent of interstellar space, and even measure time. This book will discuss the mathematical methods on which astronomical, geodetic, calendar and metrological measurements are based.