Earth’s sole satellite has loomed large in human history since ancient times. Its orbit around Earth inspired our calendar month. Its gravitational pull coaxes Earth’s oceans (and large lakes) into daily cycles of high and low tide. The moon might even affect how you toss and turn at night (one study showed that people sleep worse during a full moon). In fact, scientists suspect that our satellite’s stabilizing effect on Earth’s wobble and climates helped life evolve here. Without the moon, there might not have been moon gazers.
Our satellite is literally a chip off the old block, formed around 4.5 billion years ago when a roving body the size of Mars collided with the infant Earth and knocked a cloud of debris into orbit. That debris coalesced into the moon. But while an atmosphere protected Earth from all but the largest meteor impacts, the moon’s airless surface came under constant assault over billions of years. The powdery lunar dust is pocked by craters and dented by dark basins people once thought were seas (they’re dry, although the moon may contain ice in it deepest crevices). Our satellite is also the only heavenly body visited by human beings, whose footsteps are easy enough to follow when you touch down in your spaceship. They remain unchanged on the charcoal-gray surface.