You can remember the meaning of 谢天谢地 with this easy mnemonic:
In our culture, if you really want to thank the heavens for good fortune, you have to thank (谢) the sky (天), and thank (谢) the land (地) as well, because we believe deities are living in every corner.
Using mnemonics to remember the meaning of Chinese characters like can have an enormous impact on how long to learn Mandarin it takes.