One of the first things I did when I moved to Belfast was walk across the footbridge. I always like the way things look from the water. The flat expanse of ocean between me and the shore provides a clear view of the city.

It was low tide when I went out on the bridge, and I immediately noticed the huge change in the water level between low and high tides. I wasn’t used to seeing 12-foot swings in the tide. Yards of mussels were exposed, clinging to the stone pilings that support the bridge.

The tides are a fascinating phenomenon and seeing their evidence got me thinking about what is responsible for them — the moon.

Tides are critical to many types of sea life and shore birds. They create entire ecosystems, like salt marshes, that depend on the consistent ebb and flow. Even more than that, they are a constant reminder of our wider solar system and the happy, albeit violent, coincidences that led to the world we live in.

As most learned in ninth grade earth science, the tides are controlled by the pull of gravity by the moon on the Earth’s oceans. The interaction between the sun and the moon and the rotation of the earth causes the cyclical rising and receding of the water. When the moon teams up with the sun — when they are lined up on the same side of the earth — it boosts the effect.

This is something we take for granted when looking up at the sky or observing the tides. The moon has always been there. It seems to have been meant to be there, since so much of life depends on it. However, the fact that Earth has a moon was not a foregone conclusion.

Around 4.5 billion years ago the Earth was a hot young proto-planet spinning around the newly born sun. At that time a rogue planet roughly the size of Mars collided with it, blasting huge chunks of the planet into space. Over time that material, left over from the massive collision, coalesced into the moon.

The impact had to have happened at a very precise angle for this to have worked. If it had hit the Earth directly it would have destroyed both of the planets, if it hit in a glancing blow the Earth wouldn’t have ejected enough material to form the Moon. It had to happen just so.

If it hadn’t happened as it did, the Earth would be a very different place. The moon acted during the earlier, more active years of the solar system kind of like a shield protecting our planet from comets and asteroids. As the foreign objects would circle in, threatening to impact Earth and wipe out early life, the moon would pull them in with its gravity and take the hit for us. Without the moon, early life would have experienced far more impacts and resulting extinction events.

The tides were critical in this period. The water would rise and fall over mineral-rich rock, which might have contained the ingredients for the first specks of life. Since then the Earth has matured, and we tend to take for granted how lucky we are to have that face in the sky.

So next time you are out digging steamers at low tide or fishing as it ebbs, give a thought the moon.