If we didn’t have the Moon, the most obvious immediate effect would be that the tides would be much smaller, since our natural satellite’s gravity accounts for roughly two-thirds of the tidal ‘tug’ on Earth’s seas. This could have a big impact on life on our planet since there are some important species whose life cycles are synchronised with the tides. Even land-based creatures have behaviour patterns based on the changing brightness of the Moon in the night sky throughout the month. On a larger scale, though, the long-term consequences for Earth itself could be more dramatic. The Moon’s gravity helps to stabilise our planet’s rotation and keeps its axis tilted at a more or less constant 23 degrees, producing seasons that keep most parts of the world at moderate temperatures. Without the Moon, Earth might become like Mars, whose axis tips back and forth between 15 and 35 degrees over tens of thousands of years, carrying the planet from one climate extreme to another. As if that weren’t bad enough, some astronomers believe the Moon also plays an important role in ‘sweeping up’ rogue asteroids and comets that would otherwise hit our world, so without it, we might suffer impacts much more frequently.
Answered by Giles Sparrow.