The amazing thing about the sundial in the church of Santa Maria degli Angeli et dei Martiri in Rome, which we described yesterday, is that you can actually see it in operation.
If you go to the church around midday (the exact timing varies through the year), a spot of sunlight shines through a small hole in the wall on the south side of the church, shown above. The light shines onto the floor, showing the elevation of the sun on the day. That information enables astronomers to work out the date and the timing of equinoxes and solstices. One obvious suggestion: go there on a sunny day.