What’s creating this giant hole in space? At first glance the featured image looks like it could be a black hole, ringed by a field of stray moons. It isn’t. It’s a shadow, Earth’s shadow, and the picture behind it is one of the more patient pieces of astrophotography you’ll see this year.

People have noticed that Earth casts a dark, curved shadow on the Moon during a partial lunar eclipse since at least the time of Aristotle, who took the curve as evidence that Earth itself is round. But there’s a catch: no single eclipse ever shows the whole shadow. The Moon only ever grazes part of it, so the rest of the circle has always had to be inferred rather than seen.

Building a Full Circle Out of 22 Years of Partial Ones

Using modern digital technology, though, images from many separate lunar eclipses can be combined to reconstruct Earth’s complete shadow, and that’s exactly what today’s featured image compilation does. It’s the work of a persistent astrophotographer who spent 22 years photographing partial lunar eclipses and lining up the results so the darkened part of the Moon always points toward the same center. Stack enough of those partial bites together and the outline finally closes into a full, unbroken circle, something Earth never actually shows you in a single night.

The Moon isn’t eclipsed every month, despite completing an orbit roughly every 29.5 days, the reason there isn’t a “moon-th” eclipse on schedule is that the Moon’s orbit is slightly tilted relative to Earth’s orbit around the Sun. Most full Moons pass just above or below Earth’s shadow rather than through it, which is exactly why a project like this needed over two decades to gather enough material.

A Faint Blue Band Hiding in the Data

Look closely at some of the individual eclipse images making up the compilation and there’s a faint blue band visible along part of the shadow’s edge. That color comes from Earth’s atmosphere, which filters out more red sunlight than blue as light grazes through it on its way to the Moon, the same basic physics that turns the deep interior of the shadow blood-red during a total eclipse, just showing up at the opposite end of the color spectrum near the edge.

One More Chance to See It, Soon

Later this week, a new lunar eclipse will occur and will be best visible in parts of North and South America, Europe, and Africa, on the night of August 27–28, 2026, a deep partial eclipse will send 96% of the Moon’s disk into Earth’s umbra, the deepest partial eclipse since 2024 and the most striking lunar eclipse until a total eclipse at the end of 2028.

Image Credit & Copyright: Tim Martin
Source: https://apod.nasa.gov/apod/astropix.html