Possibly a better explanation than the sibling comment: The plane of the moon's orbit wobbles relative to the Earth's equator, with a period of 18.6 years. The

Possibly a better explanation than the sibling comment: The plane of the moon's orbit wobbles relative to the Earth's equator, with a period of 18.6 years. The "standstill" says that the moon apparently follows the same track from each orbital pass (month) to the next - it refers to the change in the moon's declination (angle above the horizon) at the same point in its orbit on successive months. At the time of a standstill, this change is at its minimum value, because the difference between the moon's orbital plane and earth's equator is momentarily at one extreme of the wobble (the peak or trough of a sinusoid.) "First in >18 years" is a clickbait headline - the interval is always that, there's nothing special about this one. It's as regular as saying "first full moon in 29.5 days".