I am utterly disappointed by the translation of 先天八卦 and 後天八卦 as Earlier Heaven Ba Gua and Later Heaven Ba Gua. The meaning of the original term in Chinese is totally lost. At first I thought it might be the poor work of a translation software. So I looked up Google Translator for 先天 and 後天. To my surprise, it translates correctly the former as “innate” and the latter as “acquired”.
先天 describes something that comes with birth while 後天 describes something that is acquired after birth. 先天八卦 was the original Ba Gua discovered by Fu Xi 伏羲 through his observation of nature. 先天八卦 describes his observation of the world in its natural form. This is how Wu Ji gives birth to Tai Ji 無極生太極, Tai Ji gives birth to the binary basics 太極生兩儀, the binary basics give birth to the four images 兩儀生四象 and finally the four images give birth to the eight trigrams 四象生八卦. Everything comes naturally and that is why the picture is named 先天八卦. The idea can only be described by a lengthy sentence. As a term, I would prefer transliteration rather than translation.
It is unfortunate that even some Chinese scholars got the wrong meaning of Xian Tian Ba Gua and Hou Tian Ba Gua leading to an ignorant invention of Zhong Tian Ba Gua 中天八卦. This is another example of adding feet to the picture of a snake 畵蛇添足. Of course they will not admit that it is their invention and try to link the idea with Shuo Gua Zhuan 説卦傳 or even Guei Zang Yi 歸藏易 and Lian Shan Yi 連山易 to gain credibility.