Navigating to the Moon
There is a poetic beauty to the Apollo flights which lies in the fact that the crews navigated between worlds by sighting on the very same stars their ancestors would have employed to guide boats and ships across the oceans of Earth. The maritime connection even extended to the instrument used for the task, because the Apollo spacecraft had its own sophisticated version of the sextant, an optical device used for centuries by sailors to measure angles between Earth’s horizon and the Sun and stars. Yet sighting on celestial objects was only one of a range of techniques that NASA brought to bear on the problem of guidance and navigation, skills that had to be mastered to ensure that 400,000 kilometres of space between Earth and the Moon were crossed in both directions accurately and safely. These skills required consummate finesse in the measurement of extremely subtle parameters, and high mathematical competence to interpret the results correctly, as excessive errors were utterly and lethally unforgiving. This region of space, encompassed by the Moon’s orbit around Earth, is termed cislunar space, pronounced with a soft ‘c’. Finding a way across it is therefore called cislunar navigation.