Have you ever tried to see a small object alternately with one eye or the other? It seems to be moving, when in fact it is stable. This phenomenon is called parallax. Now think that you have a head the size of the Earth, so the one eye is at the northern hemisphere and the other eye is at the southern hemisphere and both of them are located at the same meridian. So if you observe the Moon alternately with the one eye or the other, you’ll realize that the position of the Moon is changing. This is the key to measure the distance to the Moon geometrically and we use the digital tool of Stellarium for that.