Why the ‘Eastern Sea’ Sits on the Moon’s Western Edge

On July 7, astrophotographer Guy Bardon captured a rare, favorable view of one of the Moon’s most striking features: Mare Orientale, Latin for “Eastern Sea.” It’s the youngest of the Moon’s large impact basins, and normally it’s nearly impossible to see from Earth. This time, a bit of lunar libration, a subtle wobble in the Moon’s apparent orientation as it orbits, tilted the basin just enough into view, appearing foreshortened along the Moon’s western edge.

A billion-year-old bullseye

Mare Orientale formed more than 3 billion years ago, when an asteroid roughly 1,000 kilometers across slammed into the lunar surface. The impact left behind a set of concentric circular ridges, ripples frozen into the crust itself, similar in principle to the rings that spread outward when a stone hits water, just on a scale of hundreds of kilometers. Those rings are much easier to make out in direct images taken from lunar orbit than they are from Earth, where the basin sits at such an extreme angle that it appears squeezed into a sliver near the edge of the visible disk.

So why is the “Eastern Sea” in the west?

Here’s the fun bit of trivia buried in the name itself. Mare Orientale was named before 1961, back when the convention for labeling east and west on lunar maps was the opposite of what it is today. Astronomers later flipped the convention, but the name stuck. So a feature called the “Eastern Sea” now sits, somewhat confusingly, along the Moon’s western edge. It’s a small reminder that even our maps of the sky carry the fingerprints of the history of the people who drew them.

Source: NASA Astronomy Picture of the Day, July 10, 2026, “Western Moon, Eastern Sea.”

Image Credit & Copyright: Guy Bardon 

Source: Why the ‘Eastern Sea’ Sits on the Moon’s Western Edge – Scents of Science