How does the Moon block the Sun?

Short answer

It's a coincidence of scale: the Moon is about 400 times smaller than the Sun, but it's also about 400 times closer to Earth, so both disks appear almost the same size. When it passes in front at new moon, its silhouette covers the Sun's light and a total eclipse occurs.

Published: October 6, 2026 · Updated: October 6, 2026 · Edited by Above Ten Thousand · Editorial policy

Types of solar eclipse ESAEuropean Space Agency (CC BY SA), vía commons

The coincidence of 400

That such a modest body can block our star is a geometric coincidence. The Moon is about 400 times smaller than the Sun, but it is also about 400 times closer to Earth. Since nearby objects look larger than distant ones, the two end up appearing almost the same size in the sky. When the Moon sits between Earth and the Sun during a total eclipse, it appears to completely cover the sunlight, leaving only the star's outer atmosphere visible.

Why it doesn't happen every month

The Moon passes through its new phase once a month, but it almost always does so above or below the line connecting Earth and the Sun, because its orbit is not in the same plane. Precise alignment only occurs during eclipse seasons: two periods a year, about 35 days each, when the Sun, Moon and Earth can line up. Solar eclipses happen at new moon and lunar eclipses at full moon. And since the area from which totality is visible is limited, for any given place these are infrequent events: the next total eclipse visible from the contiguous United States will be on August 23, 2044.

A show with an expiration date

The Moon is drifting away: its orbit grows by about 1.5 inches (3.8 cm) each year. Its apparent disk will therefore keep shrinking until it stops fully covering the Sun. According to NASA, at that rate it will take more than 600 million years for total eclipses to disappear completely. Alignments will still occur, but the near-perfect fit that today reveals the solar corona is a temporary feature.