Solar and lunar eclipses
A solar eclipse occurs at new Moon when the Moon passes between the Sun and the Earth and casts its shadow on the Earth; inside the umbra the eclipse is total, in the penumbra partial, and when the Moon is farther from Earth the eclipse is annular. A lunar eclipse occurs at full Moon when the Moon enters the Earth’s shadow and is visible from the whole night side of the Earth. Eclipses do not happen every month because the Moon’s orbit is inclined ≈ 5° to the ecliptic and an alignment is possible only near the nodes where the orbit crosses the ecliptic. The Sun and Moon have almost the same angular diameter (≈ 0.5°), so total eclipses are rare, seen along a narrow track, and totality lasts at most about 7.5 minutes. Eclipses repeat with the Saros period of about 18 years 11 days. On 12 August 2026 a total solar eclipse crossed Greenland, Iceland and Spain. Watch a solar eclipse only through certified eclipse glasses (ISO 12312-2) or indirectly, by projecting the image through a pinhole onto a screen – ordinary dark sunglasses, X-ray film, binoculars and telescopes can blind you; only during totality itself (when the Sun is completely covered) may it be seen with the naked eye, and the glasses go back on as soon as the first edge of the Sun reappears.
A safe model of a solar eclipse: in a dark room use a torch (Sun), a small ball (Moon) and a large ball (Earth) to make shadows; move the Moon ball to show umbra and penumbra on the Earth. Never look at the real Sun.