And if there were no Moon? the Fun of Counterfactuals in Science
For example, consider what we can learn about Earth by asking what our planet would be like if the Moon had never formed. To understand the consequences of changing the cosmos, I began by considering Earth as it has actually been transformed by the presence of the Moon over the past 4.5 billion years. As Sir George Darwin deduced back in 1897, the Moon is spiraling away from Earth under the influence of the ocean tides that the Moon creates here. The fact that the Moon is moving away from us has been proven over the decades since 1969 when Apollo 11 astronauts left a reflector array on the Moon. By sending laser beams from Earth to the Moon and timing their round trips, astronomers have determined that the Moon is spiraling away from Earth at a rate of about 3.82 cm (1.5 in) per year. This means, of course, that the Earth and Moon were closer together in the past.
The recession of the Moon from Earth implies that the Moon is gaining energy, just as a ball can rise in the air only if it is given energy by being thrown upward. That energy given to the Moon comes from Earth, namely from our planet’s rotation – we are spinning more slowly today than we were when Earth first formed. Earth loses energy because the Moon creates tides on our rapidly spinning planet. These tides are not in a straight line between the centers of the two worlds. Rather, the high tide closest to the Moon is pulled ahead of the Moon by Earth’s rapid rotation. In turn, the water in that high tide gravitationally pulls the Moon in the direction of the Moon’s orbit, giving the Moon energy (like you would give a ball hanging on a string energy if you held the string and spun around). As a result the Moon (and the ball on the string) spiral outward. Earth, in turn, slows down because of friction between the water in the high tides and the body of Earth.