How does the Moon's shadow cause a solar eclipse?

A solar eclipse occurs when the Moon passes between the Sun and Earth and its shadow reaches Earth. It is an alignment event, not a change in the Sun’s brightness. From a location within the shadow, the Moon blocks part or all of the Sun’s visible disk. Different places on Earth can experience different amounts of covering during the same event.

The Sun is an extended light source, so its shadow has more than one region. Within the umbra, the central total-shadow region, the Sun is completely hidden. Within the penumbra, only part of the Sun is hidden. The diagram shows a central alignment with a small dark region and a wider partial-shadow region on Earth.

The Moon must be near new moon for a solar eclipse, but a new moon does not guarantee one. The Moon’s orbit is tilted relative to Earth’s orbit around the Sun. Usually the Moon’s shadow passes above or below Earth rather than across it.

Distances also affect the result. When the Moon appears too small to cover the whole solar disk, a central eclipse can be annular, leaving a ring of the Sun visible. The illustration is a simplified total-eclipse geometry; its object sizes and distances are chosen to make the shadow regions clear.