Posted to the homepage on August 11, 2013.
The Moon—like the sun, stars and Earth—is easy for a human being to take for granted. But the Earth's moon is truly exceptional, and should be appreciated for shaping the exceptional world we live on. Earth is the only planet with a single moon, and relative to the Earth, Luna is the largest moon in the solar system. The Moon thus exerts a strong, solitary influence on the planet it was torn from. Evidence suggests that the Moon was blown into orbit by a massive asteroid impact about 4.5 billion years ago, shortly before the emergence of life on Earth. Even if this cataclysm was incidental to abiogenesis, life as we know it would not exist without the Moon. On Starts With a Bang, Ethan Siegel says that a moonless Earth would have drastically shorter days, and our tilt towards the sun would vary over time. As Ethan explains, "without our Moon, there would be nothing preventing catastrophic shifts in our rotational axis" and at times—like Mercury—we would have no seasons. So the Moon not only stabilizes our world, but adds another layer of periodicity, of yin and yang, to the forces that shape our lives.