Phra Rahu is a mythical god that swallows the sun or the moon causing eclipses. He is depicted in art as a dragon with no body riding a chariot drawn by eight black horses. Rahu is one of the navagrahas (nine planets) in Vedic astrology. The rahu kala is considered inauspicious. According to legend, during the Samudra manthan, the asura Rahu drank some of the divine nectar.
But before the nectar could pass his throat, Mohini (the female avatar of Vishnu) cut off his head. The head, however, remained immortal.
It is believed that this immortal head occasionally swallows the sun or the moon, causing eclipses.
Then, the sun or moon passes through the opening at the neck, ending the eclipse.
Astronomically, Rahu and Ketu denotes the two points of intersection of the paths of the Sun and the Moon as they move on the celestial sphere.
Therefore, Rahu and Ketu are respectively called the north and the south lunar nodes.
The fact that Eclipses occur when Sun and Moon are at one of these points gives rise to the myth of the swallowing of the Sun.
In Buddhism Rahu is one of the krodhadevatas (terror-inspiring gods).
In Thailand Rahu will readily devour all of your bad luck or even neutralise the bad luck and spit out good luck and fortune to you if you pray or offer to it.
The best day to offer Rahu is every wednesday after sunset and offering.