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How eclipses occur at the lunar nodes, their astronomical mechanics, Sutak rules, and how we compute them from first principles
In the myth of the Samudra Manthan (Ocean Churning), a demon named Svarbhanu disguised himself as a god and drank the nectar of immortality. The Sun and Moon recognised him; Vishnu severed his head with the Sudarshana Chakra. The head became Rahu — the ascending lunar node; the torso became Ketu — the descending lunar node. Ever since, Rahu and Ketu circle the sky, periodically swallowing the Sun and Moon — and an eclipse occurs. What the myth encodes is astronomical fact: eclipses happen precisely where Rahu and Ketu are.
The Moon's tilt: The Moon's orbital plane is tilted 5.15° relative to the ecliptic (the Sun's apparent path). Most of the time the Moon passes above or below the Sun's shadow.
The nodal points: The Moon crosses the ecliptic plane twice a month. These crossing points are the lunar nodes — ascending (Rahu) when moving north, descending (Ketu) when moving south. Here the Moon's ecliptic latitude is zero.
Solar eclipse condition: New Moon + near a node = solar eclipse. The Moon passes between Earth and Sun; the Moon's shadow falls on Earth's surface.
Lunar eclipse condition: Full Moon + near a node = lunar eclipse. Earth passes between Sun and Moon; the Moon enters Earth's shadow.
Not every New Moon and Full Moon is an eclipse — there are only about 2–3 eclipse seasons per year. Each season lasts about 34–38 days, when the Sun is within 15–18° of a node. A single season may produce 2–3 eclipses: one solar and one lunar, or two solar and one lunar. This is why eclipses tend to come in clusters.