How many moons does Earth have?

How many moons does Earth have? It’s a question with a very simple answer, and a more complex one.

The simple answer is that Earth has only one moon, which we call “the moon”. It is the largest and brightest object in the night sky, and the only solar system body besides Earth that humans have visited in our space exploration efforts.

The more complex answer is that the number of moons has varied over time.

When Earth first formed, around 4.5 billion years ago, it had no moons, but that soon changed. Researchers believe that the proto-Earth was struck by an object the size of Mars, nicknamed Theia, blasting much of its crust into orbit. This debris eventually formed into the moon we know today.

Although the moon is our only permanent natural satellite, astronomers have discovered many other near-Earth objects that could be considered honorary ‘mini’ moons.

These fall into a few groups. First there are temporary satellites; objects that have been captured by Earth’s gravity, putting them into orbit before they eventually escape again. We know of only two – a small asteroid called 2006 RH120, which orbited Earth for nine months in 2006 and 2007, and 2020 CD3, another small asteroid spotted just before it flew away from Earth in March 2020, having spent almost three years in orbit.

Then there are objects that orbit around the sun in Earth’s vicinity. Two of these, 2010 TK7 and 2020 XL5 are known as Trojans, and occupy gravitationally stable points in space known as Lagrange points, which are created by the interaction between Earth and the sun’s gravity and follow our planet’s orbital path. The Lagrange points also seem to collect large amounts of dust particles, which some astronomers have dubbed Kordylewski clouds or “ghost moons”.

Some objects known as quasi-satellites don’t follow Earth’s orbit, but take 365 days to orbit the sun just like our planet, making them appear to be in orbit despite being outside Earth’s gravitational influence.

Other close objects approaching our planet before heading in the opposite direction around the sun until meeting Earth again on the other side. These trace out the shape of a horseshoe, so are known as horseshoe orbits.

Finally, Earth is also orbited by many artificial satellites that occasionally get mistaken for potential new moons. In 2015, astronomers excitedly announced the observation of one such object, only to later realise it was actually the Gaia space telescope.