What causes a solar eclipse?

A solar eclipse occurs when the moon passes between Earth and the sun and its shadow falls on part of Earth's surface. In the narrow path of totality, where the darkest part of the shadow lands, the moon completely covers the solar disc and the sun's corona becomes visible. Outside that path a partial eclipse is seen. The reason totality is possible at all is a coincidence of scale: the sun's diameter is roughly 400 times the moon's, and it sits roughly 400 times further away, so the two appear almost exactly the same size from Earth. No other planet in the solar system has this arrangement.

Eclipses do not happen monthly because the moon's orbit is tilted about five degrees relative to Earth's orbital plane, so at most new moons the shadow passes above or below Earth. Alignment occurs only near the nodes where the orbits intersect, producing solar eclipses roughly two to five times a year somewhere on Earth, with total eclipses at any given location separated by centuries on average. When the moon is near the far point of its slightly elliptical orbit it appears too small to cover the sun completely, producing an annular eclipse with a ring of sunlight around the moon.

Safety is not optional here. Looking at the partially eclipsed sun without proper filters can cause permanent retinal damage without any pain, since the retina has no pain receptors, and the damage may not be apparent for hours. Ordinary sunglasses are not adequate. Certified eclipse glasses meeting the ISO 12312-2 standard, or indirect viewing through a pinhole projector, are the safe methods. The only moment it is safe to look directly is during totality itself, and only within the path of totality, and only until the sun's edge reappears.