What to Know About April’s ‘Micromoon’

The moon is not about to change its size. Almost since its formation more than four billion years ago, it has maintained a mean diameter of 2,159.2 miles. That’s less than the distance from Los Angeles to New York. As NASA puts it, if the Earth were the size of a nickel, the moon would be about the size of a coffee bean. But that doesn’t mean the moon can’t look bigger or smaller—and this Saturday, April 12, it will be comparatively tiny, amounting to what sky watchers call a micromoon. Here’s what you need to know.

On any given night, the moon can appear to change its size—sometimes dramatically. Thanks to the so-called moon illusion, it may appear especially large—sometimes huge—when it is low in the sky, hugging the horizon, and then appearing to shrink steadily as it climbs in the sky. But your eyes and, significantly, your brain, are lying. To prove it, simply stretch your arm out, close one eye, and hold your index finger up. Your fingernail and the moon will appear about the same size, regardless of how big or small the moon looks. What’s going on is a trick of the mind; at the horizon, you make an unconscious comparison between the size of the moon and the size of objects like houses and trees, with the moon appearing to dwarf them. High overhead the moon is the one that gets dwarfed by the emptiness of the sky. Your mind thus concludes that it has shrunk.

The micromoon is something else. On average, the moon is 238,855 miles from Earth, but that figure changes. The moon’s orbit around the Earth is elliptical, with a low point, or perigee, of about 224,000 miles, and a high point, or apogee, of 251,650 miles. When we see the moon at perigee, closer to Earth, it naturally looks bigger—creating the phenomenon known as a supermoon, when the lunar disk appears 7% larger and 15% brighter than it typically does. At apogee—which is where the moon will be this weekend—it will appear 14% smaller and 30% dimmer, creating the illusion of the micromoon.