The first reason is that the two dynasties were established in different places.
The establishment of the Western Han Dynasty was the rise of the world at the end of Qin Dynasty. Liu Bang, the ancestor of the Han Dynasty, was divided into Hanzhong kings by Xiang Yu after attacking Xianyang, and later began to fight for hegemony between Chu and Han.
After defeating Xiang Yu, Chang 'an was the capital, and because the capital was in the west, it was called the Western Han Dynasty. In the Eastern Han Dynasty, Liu Xiu, Emperor Guangwu of Han Dynasty, broke out in the late New Dynasty. The imperial clan of the Han Dynasty, represented by Liu Xiu, took advantage of the situation to rebuild and restore the Han Dynasty.
The Song Dynasty was called the Northern Song Dynasty, and so was the Southern Song Dynasty. The establishment of the Northern Song Dynasty was that Zhao Kuangyin became a yellow-robed man after the mutiny in Chen Qiao. He ascended the throne in Kaifeng and proclaimed himself emperor, established the Northern Song Dynasty, and then gradually unified the whole country.
The Southern Song Dynasty was an exiled regime in the Song Dynasty, which was established after the invasion of the Jin Army to the south and perished in the Northern Song Dynasty. Because the capital is in the south, it became the Southern Song Dynasty.
Location is just one of them, followed by the change of economic and political center of gravity.
Before the establishment of the Han Dynasty, it was the Qin Dynasty. After the Qin Dynasty unified the six countries, Xianyang, the former capital, was the national capital. In order to prevent the restoration of the old six countries, the rich and noble of the six countries were concentrated in Xianyang, and the western regions became the political and economic center of the country.
By the end of the Western Han Dynasty, Luoyang, the eastern capital, had become the political and economic center of the country, and a large part of Liu Xiu's success in starting his troops came from the support of Nanyang strongmen.
Of course, the geographical difference of the capital is not the main factor. For example, the Eastern Jin Dynasty was also established in the south, but it was also called the Eastern Jin Dynasty, and the reasons for the change of economic and political center of gravity also accounted for a large part.
So was the establishment of the Northern Song Dynasty. When Song Taizu and Zhao Kuangyin established the Northern Song Dynasty, the North was the political and economic center of the country.
In the Southern Song Dynasty, the Jin people invaded south, and a large number of Han people in the north crossed south.
Therefore, China's political and economic center of gravity has shifted to the south. For example, the Southern Song Dynasty court and the Southern Ming Dynasty court.
Therefore, the names of Southern Song Dynasty, Northern Song Dynasty, Eastern Han Dynasty and Western Han Dynasty are divided from geographical location, economic and political center of gravity and other factors.