Scientific Rahu & Ketu

Gupta the Indian astronomer lived around 300 a.d. He understood that eclipses were caused by the shadows of the Earth or Moon. The story of Rahu and Ketu is just a simplification for the masses.

He also understood that an eclipse occurs when the Sun and Moon are close to the point in their respective orbits where the path of the Sun crosses the path of the Moon, or the node point.

The Sun moves against the background of stars around the entire sky in the course of a year. This path is called the ecliptic.

The Moon moves around the entire sky in the course of about 27 days, and its path is such that it crosses the ecliptic in two places. The place where the path of the Moon crosses the ecliptic moving northward is called the ascending node and the place where the path of the Moon crosses the ecliptic moving south is called the descending node.
Thus the Moon crosses the ecliptic at the ascending node, roughly 14days later crosses the ecliptic again at the descending node, and roughly 14 days later it is back to the ascending node crossing.

Things are somewhat complicated by the fact that the nodes move very slowly from one month to the next making it more difficult to predict eclipses.

However Hindu astronomers discovered that if they kept track of the location in the sky of the nodes, then they only need to check the proximity of the Sun or Moon to the nodes in order to be able to predict an eclipse. Since the nodes are theoretical points they are referred to as "shadow planets"

The ascending node is called Rahu and the descending node is called Ketu. Added to the seven visible planets, Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn, Hindu astrologers count a total of nine planets.