Original: In his book , The Holy Science , Sri Yukteswar explained that the descending phase of Satya Yuga lasts 4800 years , the Treta Yuga 3600 years , Dwapara Yuga 2400 years , and the Kali Yuga 1200 years .
Simplified: Hindus believe that there are periods of time called yugas ( '' ages '' or '' eras '' ) : Satya Yuga , Treta Yuga , Dvapara Yuga , and Kali Yuga .