A star field fills the frame. Earth sits in the middle of it as a circle seen from the side, with N at the top, S at the bottom and EQUATOR written across the waist. Gold rays cross the frame in one direction, every one of them parallel, and the caption on them says so. They run toward the north celestial pole. A cyan dot sits on the rim at your latitude, with a gold line from Earth's centre out to it and 51.5° written on the angle at the centre. A green line lies flat across the dot, tagged HORIZON. From the dot a cyan arrow runs up the frame toward that same pole, captioned to say so. Between the horizon and that arrow a cyan arc is drawn, and it reads 51.5° too. The same figure turns up twice, which is the whole lesson.