Long before this story begins, aliens swept through the Solar System and wept as they slammed the Moon into the Pacific Ocean and extremely slowed the Earth's rotation, making a day as long as a year. They said that they had to do this, though their motives remain unclear. In any case, they are gone now, and a Flower Forest (capitalized in the original) covers Europe and Africa on the side of the Earth that now faces the Sun.
The Moon lies partly embedded on the night side of the planet: in effect, an enormous mountain that reaches hundreds of miles above the atmosphere. It is the realm of a powerful scientist who can grant wishes, but he warns our protagonists that he is not all-powerful. There is a man who asked for high intelligence, and got it, but at the cost of staying plugged into a machine; he has no opportunity to use the intellect that he gained. And there are others in equally creative predicaments. Clearly, one must be careful what one asks for when bargaining with this man.
The protagonists ask if the historic rotation of the Earth in 24-hour days can be reestablished. Indeed it can, but after rotating for only twelve hours, the machine fails and the Earth stops spinning. The Flower Forest now faces away from the Sun, while the ex-lunar laboratory faces it, to the scientist's glee.
This was published as a novella or novel, perhaps in the 70s or 80s, in English. I suspect that Robert Silverberg may be the author despite the obvious errors in physics.