Grahan

Eclipses occur when the Sun, Moon, and Earth align near the lunar nodes (Rahu-Ketu axis). A Solar Eclipse (Surya Grahan) happens at New Moon (Amavasya) when the Moon passes between the Sun and Earth, casting its shadow on Earth. A Lunar Eclipse (Chandra Grahan) occurs at Full Moon (Purnima) when the Earth's shadow falls on the Moon. The Moon's orbital plane is inclined ~5.14° to the ecliptic, so eclipses only occur when the New/Full Moon is near the ascending (Rahu) or descending (Ketu) node -- within about 18.5° for solar and 12.5° for lunar eclipses. The Saros cycle of ~18 years 11 days governs the periodicity.