What would happen if Earth had no moon?
Earth is unique because it has an exceptionally large satellite: the Moon. The Moon has a special relationship with Earth. If we had no moon, life on Earth would be significantly different.
The Moon is actually partly made of the foundation of the Earth. When Earth was a molten lava infant around 30 million years ago, a giant 'impactor' hit Earth, taking some of its mantle (layer of silicate rock) with it when it ricocheted off into space. This impactor merged with the hot mantle to form the Moon.
Around one billion years ago the Moon stopped being geologically active, but the Moon still has a number of significant impacts on Earth.
Without the Moon, eclipses would not exist and our nights would be much darker, because the Moon provides light to the Earth by reflecting light from the sun. Scientists believe without the Moon, during the night it would be so dark you would not be able to see you hand in front of your face - without the aid of artificial light.
There would be no calendar system based on months. The word ‘month’ actually stems from ‘moon’. This is because most calendars are based on the phases of the moon.
The Earth’s tides would also be different. If the Moon wasn’t affecting tides, the sun’s gravitational influence would essentially ‘take over’ tidal changes. This means tides would be about a third the size they are now, because the sun doesn’t have the same ‘pull’ as the Moon.
The most significant change for life on Earth, relating to tidal changes, would be the length of a day. Billions of years ago when Earth was in its infancy, Earth spun on its axis at a significantly faster pace. The daily cycle was less than 10 hours long. It is due to the ebb and flow of tides that breaks the Earth’s spin.
Without the Moon, the Earth would spin fastest and days would be shorter. It is the Moon that helps slow down Earth’s rotation, this is very slight reduction in speed - around a few milliseconds per year, but remains a significant impact over billions of years.
And not only that. Without the Moon, the orientation of the Earth's axis would become unstable and would experience chaotic variations over time. Without the Moon there would be no consistent seasons, meaning weather would be completely chaotic. Mars, for example, has two tiny moons and its axis of rotation has changed 60º in the last 10 million years (in comparison, the Earth's axis oscillates only 1.5º every 41,000 years). The climatic consequences of a chaotic variation in planetary rotation axis makes it very difficult for life to evolve. This means without our moon, perhaps life on Earth would ever have existed.
