As of Tuesday, Sept. 23, the moon phase is Waxing Crescent, and according to NASA's Daily Moon Observation, there's just 3% visible tonight. What are moon phases? NASA tells us that moon phases are part of a 29.5-day cycle of the moon’s orbit. This is how we get full moons, half moons, and moons that appear completely invisible. There are eight main moon phases, and they follow a repeating cycle:New Moon - The moon is between Earth and the sun, so the side we see is dark (in other words, it's invisible to the eye).