Moon and Twins

The Moon passes especially close to the star Pollux tonight — the brighter “twin” of Gemini. The star stands just to the right of the Moon at nightfall. The other twin, Castor, is farther along the same line.

The Moon always passes closer to Pollux than to Castor. That’s because of the relationships of the three bodies to the ecliptic — the Sun’s path across the sky.

The Moon’s orbit is tilted a bit with respect to the ecliptic. Over the course of a month, it meanders to either side of the ecliptic. And over a period of almost two decades, its maximum distance from the ecliptic varies as well. At most, the Moon can appear 6.6 degrees from the ecliptic — the width of three fingers held at arm’s length. So the Moon can pass close to, or even cover up, any star or planet within that distance from the ecliptic.

Pollux and Castor are outside that zone. Pollux is 6.7 degrees from the ecliptic, while Castor is about 10 degrees away. So the Moon can sometimes appear to almost touch Pollux, while always keeping a little bigger gap with Castor.

Over millennia, the ecliptic shifts with respect to the background of stars. So for thousands of years, the Moon could sometimes pass in front of Pollux, hiding it from view. That last happened about 2100 years ago. Pollux will move back into the Moon’s range in about 10,000 years — setting up some especially close encounters.

Script by Damond Benningfield