orbit

Half the Moon is always sunlit; phases are how much of that lit half we see as the Moon orbits Earth. The synodic month lasts 29.5 days, moving through new, crescent, quarter, gibbous, and full phases. Phases are NOT Earth's shadow—that's an eclipse. The Moon always shows us the same near side due to tidal locking, a gravitational dance between Earth and Moon.

Most stars in the galaxy are in binary systems. If our solar system had two suns, Earth's orbit might be stable — or chaotic. The climate would be stranger, the sky alien, and the evolution of life might have gone very differently.