There is one Han Dynasty. It is divided into two parts because Wang Mang usurped the throne in 8AD. The family name of Han Dynasty is Liu. The farmers rising coming up later overthrew Wang Ming. Liu Xiu, from the noble family of Han Dynasty got the throne and became the first empror of Eastern Han. Western Han's capital is Chang'an, present Xian. The capital of Western Han Dynasty is Luoyang. Luoyang is east of Xian. That's why there are western and eastern Han. People called them by adding western and eastern before Han to distinguish those two periods.