Solar eclipse

In a solar eclipse the Moon passes between the Sun and Earth at new moon, blocking at least part of the Sun from some locations and casting a moving shadow on Earth's surface. The Moon's orbit is inclined by about five degrees to the plane of Earth's orbit around the Sun. As a result, most new moons pass above or below the alignment needed for an eclipse. Eclipses can occur when a new moon crosses the orbital plane near one of its two nodes; the periods when this alignment is possible are called eclipse seasons, which occur twice each year.

The Moon's shadow has a dark central region, the umbra, and a lighter outer region, the penumbra. Where the shadow falls determines what observers see. The Moon is much smaller than Earth, so the track of its central shadow across the surface is narrow compared with the wider area from which a partial eclipse can be seen.

A total solar eclipse occurs when the Moon completely covers the bright solar disk for observers inside the umbra. The narrow path across Earth where this happens is called the path of totality; observers in the surrounding penumbra see a partial eclipse. During totality, the Sun's faint outer atmosphere, the corona, becomes visible.

In a partial eclipse, the Sun, Moon and Earth are not aligned closely enough for the Moon to cover the entire disk as seen from a given location. An annular eclipse occurs when the Moon passes centrally across the Sun but appears smaller than it, leaving a bright ring of sunlight visible around the Moon. This happens when the Moon is relatively far from Earth in its elliptical orbit. In a hybrid eclipse, the observed form can change between total and annular along the eclipse path as the Moon's shadow crosses Earth's curved surface.

An eclipse's appearance depends on an observer's position. People inside the umbra can experience totality; those in the penumbra see only part of the Sun covered, and places outside the Moon's shadow see no solar eclipse at that time. Because the umbral path is small, totality is visible from a limited corridor even when a partial eclipse is visible across a much larger region.

Looking directly at the uneclipsed, partially eclipsed or annular Sun can injure the eyes. The American Astronomical Society advises using special-purpose solar filters that meet the ISO 12312-2 standard; ordinary sunglasses are not safe for this purpose. During a total eclipse, unaided viewing is safe only for the brief interval when the Moon completely covers the Sun's bright face, and only for observers within the path of totality. At every other phase, viewers must use appropriate filters or an indirect viewing method.