Why were the song dynasties divided in to two periods - norther and southern song?

The Song dynasty was divided into two periods, the Northern Song and the Southern Song, due to the invasion of northern China by the Jurchen-led Jin dynasty in 1127.

- The Northern Song period (960–1127) refers to the time when the dynasty ruled from its capital, Bianjing (present-day Kaifeng, Henan province).

- The Southern Song period (1127–1279) refers to the time when the dynasty relocated its capital to Lin'an (present-day Hangzhou, Zhejiang province) after losing control of northern China to the Jin dynasty.