I'm not seeing a comment pointing out why 28 days isn't a moon thing, so I'll take a shot. If you watch new moon to new moon or full moon to full moon, it's a 29.5 day cycle. It's true, the moon's orbit is only 28 days. However, that's 360° of travel. We don't track the moon against the stars for its cycle though, it's tracked against the sun. A full moon sits opposite the sun, a new moon in line with the sun, etc. So, in that 28 day orbital period, the earth has also orbited about 1/13 of an orbit around the sun, changing the position of the sun against the stars . That means the moon has to travel an extra 28° of orbit to reach the new moon position again - about an extra day an a half.