New moons aren’t much to look at. You can’t even see them most months of the year. That’s true for tomorrow’s new moon which will invisibly accompany the sun in its journey across the sky.
New moons occur about once a month when the moon passes between the sun and Earth. We can’t see them for two reasons: first, no sunshine touches the Earth-facing side when the moon lies in the same direction as the sun. It’s completely dark. From our perspective, the out-of-view lunar farside gets all the sunlight. Second, since the moon is nearly in line with the sun, it’s utterly lost in the glare of daylight.
We normally have to wait two days after new moon – when the moon’s orbital motion carries it to the left (east) of the sun – to see it as a thin crescent at dusk.
Most of the time the moon passes north or south of the sun at new phase because its orbit is tilted 5 degrees with respect to Earth’s. But 2.4 times a year on average, new moon coincides with the time the moon’s seesawing path slices through the plane of Earth’s orbit. For a brief time during that crossing, all three bodies are aligned and happy earthlings witness a solar eclipse.
If the alignment is imprecise, the moon blocks only a part of the sun, giving us a partial solar eclipse. If dead-on, we see a rarer total solar eclipse.
On October 23 this year, the lineup at new moon will be a good if imperfect one with a maximum of 81% of the sun covered. The partial eclipse will be visible across much of North America; from the eastern half of the U.S. and Canada the event will occur near sunset, adding a touch of drama to the scene.
I wrote earlier that we can’t see a new moon. That’s only partly true. We mostly pay attention to the sun’s changing shape during solar eclipses, but the dark, curving bite working its way slowly across the sun’s disk is none other than the new moon seen in silhouette.