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How does the Moon’s distance from Earth determine whether a solar eclipse is total or annular?

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near and far sides of Earth's Moon — Moon (Left) Near side of Earth's Moon, photographed by the Galileo spacecraft on its way to Jupiter. (Right) Far side of the Moon with some of the near side visible (upper right), photographed by the Apollo 16 spacecraft.

The Moon’s distance from Earth is a primary factor in determining whether a solar eclipse is total or annular because it dictates the Moon's apparent size in the sky relative to the Sun.12

Because the Moon travels in an elliptical orbit, its distance from Earth varies. This change in distance alters the Moon's angular diameter as seen from Earth:23

Total Eclipse: When the Moon is near perigee—its closest point to Earth—it appears large enough to completely cover the Sun’s disk. Observers within the path of the umbra experience a total eclipse.24
Annular Eclipse: When the Moon is near apogee—its farthest point from Earth—it appears smaller than the Sun. In this case, the Moon cannot fully cover the solar disk, leaving a visible ring of light around its silhouette. This is known as an annular eclipse.24

Essentially, the Moon must be close enough to Earth for its disk to appear equal to or larger than the Sun's disk to produce a total eclipse.23