In the Bible, in the Book of Psalms, there is a line "If I forget thee, O, Jerusalem." This is in Psalm 137. It refers to a time when the Israelites were exiled from their homes (to Babylon) and were wishing they could be back home. It is a way of acknowledging that Jerusalem was their only true home.
In the Clarke story, people have essentially been exiled from Earth. The title alludes to the Biblical passage and it means that the people up on the Moon are acknowleding that Earth is their real home. They will yearn to return to Earth for as long as it takes for the planet to stop being radioactive...
i think it means that if some1 were 2 4get were they came from then the earth will perish