Nemesis

Collisions of planets are extremely rare events - fortunately. But the orbits of planets follow chaotic rules and are sometimes subject to dangerous oscillations, especially if there is some kind of gravitational perturbance from other objects.

In this scene, the observer watches another planet approaching the one the observer is standing upon. The two planets are already much closer than the Earth and the Moon. It may be a matter of a few days at best when the two planets will crash into each other. Both planets will be completely destroyed in a gigantic fireball. The fireball of molten rocks will eventually cool down and form a new, larger planet.

There is still a chance that the two planets will pass each other untouched. But in this case, the gravitational influence of the two bodies will swing them into different orbits. They may collide later or may be heading for a totally different fate, very likely an extremely excentric orbit.