The setting is a human colony on the Moon, some time in the future after a nuclear holocaust on earth. This colony contains the only human survivors of the human race.The imagery is mainly of the desolate splendour of this setting, the mountains and sky. The moral is never to forget one's roots - in this case amplified on a gigantic scale; the survivors of the human race must never forget where they came from, and aim to return. This is why the trip outside is a pilgrimage, when Mervin's dad takes him to view the earth which appears beautiful but still deadly in the aftermath of the nuclear holocaust. This is the first time that Mervin sees his original home and he is filled with sadness over the loss, but at the same time he instinctively realises that one day, when at last it's safe to go back, his descendants will do so. The tone is sad and elegiac, but retains a glimmer of hope.
"If I Forget Thee, O Earth" is a short story by Arthur C. Clarke that was first published in 1951. This was only six years after the United States had used atomic bombs against civilians in the Japanese cities of Hiroshima and Nagasaki. This gave rise to a nuclear arms race between the United States and Soviet Union and a policy known as "Mutually Assured Destruction," in which both countries have enough nuclear weapons to completely annihilate their opponents—and perhaps humanity as a whole.
In Clarke's story, the earth has been made uninhabitable by nuclear war and the characters live in a lunar colony. The main image of the story is that of the beautiful but poisoned Earth seen from the moon. The theme is the power of technology to destroy the world if used unwisely and perhaps to provide some hope of redemption in the hands of a few wise people. The moral is that with nuclear weapons, climate change, and heedless environmental degradation, we now have the potential to make our planet uninhabitable and therefore we need to be very careful not to do so.