Solar and Lunar Eclipses: The Astronomy, Rahu-Ketu and Sutak Explained

A solar eclipse (Surya Grahan) happens on a new moon (Amavasya), when the Moon passes directly between the Sun and Earth and its shadow falls on part of the Earth. A lunar eclipse (Chandra Grahan) happens on a full moon (Purnima), when the Earth lies between the Sun and Moon and the Moon enters Earth's shadow.

It does not happen every month because the Moon's orbit is tilted by about five degrees to the Earth's orbit around the Sun. An eclipse can occur only when a new or full moon takes place near one of the two points where the orbits cross — the lunar nodes.

Those two crossing points are exactly what Indian astronomy names Rahu (ascending node) and Ketu (descending node). In the Puranic story from the churning of the ocean (Samudra Manthan), the demon Svarbhanu drank a share of the nectar of immortality, was beheaded by Vishnu, and the head (Rahu) and body (Ketu) live on and periodically swallow the Sun and Moon. The astronomical and mythological accounts meet at the node: an eclipse occurs when the Sun or Moon is near Rahu or Ketu.

A total solar eclipse covers the Sun completely, a partial one covers part of it, and an annular one leaves a bright ring because the Moon is at a distance where it looks slightly smaller than the Sun. Lunar eclipses are total, partial or penumbral (a faint dimming as the Moon passes through Earth's outer shadow). Worldwide there are between two and five solar eclipses a year, but any single place sees a total solar eclipse only very rarely.

In Hindu tradition Sutak is a period of ritual impurity before an eclipse. It begins about twelve hours before a solar eclipse and about nine hours before a lunar eclipse, and ends when the eclipse ends (moksha) and a bath is taken. Sutak is traditionally observed only if the eclipse is visible from your location; for a partial view of a distant eclipse that you cannot see, most almanacs say it does not apply.

During Sutak many families avoid cooking and eating, and temples close and reopen after purification. Children, older people and the ill are traditionally exempt from the fasting rules. Many households add tulsi leaves or kusha grass to stored food, chant mantras, and bathe and clean the house after the eclipse ends.

Some beliefs — for example that food becomes unsafe or that pregnant women should stay indoors — are widespread. There is no medical evidence that an eclipse harms a pregnancy or a baby, or that eating during one is dangerous. If a belief worries you or a family member, a doctor can address the concern; observing or skipping the customs is a personal choice.

Looking at the Sun directly during a partial or annular eclipse, even briefly, can permanently damage the retina — and there is no pain to warn you. Use certified eclipse glasses (ISO 12312-2) or a pinhole projector; sunglasses, exposed film and phone cameras without a proper solar filter are not safe. A lunar eclipse is safe to watch with the naked eye.