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Why are sun, moon, rahu and ketu called planets or grahas in Jyotish?

In ancient times, when human beings observed the heavenly bodies in the sky, they called the fast moving bodies as planets or grahas and the steady bright objects in the backdrop as stars or nakshatras. So, they ended up calling all major bright and comparatively faster heavenly bodies as grahas. So, all such objects observable by naked eyes, like Sun, Moon, Venus, Mercury, Saturn, Jupiter, Mars were called planets or grahas. Rahu and Ketu are not physical bodies but are calculations of the eclipses. But, as they have major importance in a horoscope they too are considered planets or grahas. 

This concept of planets in astrology is very different from the astronomical concept of planets.