When the gods and asuras churned the ocean of milk and the nectar of immortality finally rose, Vishnu, disguised as the enchantress Mohini, began serving it to the gods alone. One asura, Svarbhanu, saw the trick, took the form of a deva and slipped into the line, seating himself between the Sun and the Moon. He had just swallowed the nectar when the two recognised him and cried out. Vishnu’s discus cut off his head before the amrita could pass his throat. The head, having tasted immortality, could not die; it became Rahu. The body became Ketu. Ever since, the story goes, Rahu pursues the Sun and Moon for betraying him, and when he catches one and swallows it, there is an eclipse, which ends because the swallowed light passes straight out through his severed neck.
The myth is old. A hymn in the Rig Veda already describes Svarbhanu piercing the sun with darkness, and the sage Atri restoring it. What is remarkable is what Indian astronomy later did with the figure. Rahu and Ketu became the two chaya grahas, shadow planets, and their positions were calculated precisely: they are the lunar nodes, the two points where the moon’s tilted orbit crosses the sun’s apparent path. Eclipses can only happen when the sun and moon are near one of these points, so the myth’s two dark pursuers correspond to real geometric positions that astronomers could compute and use to predict eclipses years ahead.
The astronomers were also clear about the physics. Aryabhata explained eclipses as the moon passing into the earth’s shadow, or the moon’s shadow falling on the earth, in the fifth century. Varahamihira, in the sixth, stated plainly in his Brihat Samhita that eclipses are caused by those shadows and not by a demon devouring the sun, while keeping the name Rahu for the node that produced them. It is an early and clear case of a tradition keeping a sacred story and a correct physical explanation side by side, each with its own job.
Many eclipse customs survive, from fasting and bathing afterwards to special precautions for pregnant women. Those precautions have no known physical basis, and the one genuine risk of an eclipse, damage to the eyes from looking at the sun, is prevented by proper eye protection, not by ritual. What the myth still carries well is its imagery: the brief darkening of the brightest things in the sky by something that was cut in two and cannot be whole. Rahu is the hunger that tasted immortality and cannot hold it, and the light always comes back out.
Part 26 of 30 — Series V: What Time Is Made Of
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