When a rocket launch is scrubbed because the window closed, it sounds like an excuse, especially when the sky looks clear. In reality, a launch window is a precise calculation, not a preference: the rocket must leave at the moment when Earth's rotation, the target orbit, and the destination all line up, and missing that moment by minutes can mean missing the target by thousands of kilometres. Understanding windows explains most launch delays that have nothing to do with weather.

This guide explains the three reasons windows exist, in plain language and without equations.