The Moon didn't "appear" in the sense of suddenly popping into existence. It formed over a long period of time, alongside the Earth, billions of years ago.

Here's what scientists believe:

* The Giant-Impact Hypothesis: The most widely accepted theory is that a Mars-sized object called Theia collided with the early Earth. This impact ejected a massive amount of debris into space, which eventually coalesced to form the Moon.

* Age of the Moon: Radiometric dating of lunar samples indicates the Moon is about 4.51 billion years old. This is just slightly younger than Earth, which is estimated to be about 4.54 billion years old.

So, while we can't pinpoint an exact moment when the Moon "appeared", we know it formed very early in the history of our solar system, and it's been a constant companion to Earth ever since.