Vastu is science related to Land and the effect of the celestial body. According to mythology. A Mani from Lord Shiva's Eye drops and created a dragon called MOONGA. The dragon creates havoc among Devi Devatas .So Lord Shiva decides to destroy it. Moonga is pressed into the land in head facing North East direction. Moonga pleads Lord shiva to have mercy. So Shivji showing mercy on Moonga blesses it to become the Lord of land and names it Vaastu.