What is "upagraha"

In Hindu astrology, the term Upagraha ( Sanskrit: उपग्रह) refers to the so-called "shadow planets" (chayagrahas) that are actually mathematical points same as the two Lunar nodes - Rahu and Ketu are. These are portions belonging to the Sun and other planets respectively and produce painful consequences. The classic writers like Parashara and Varahamihira do not include their influences as do later writers like Vankatesa Sarma, author of Sarvartha Chintamani.