A 'fake moon' has been following Earth around the sun since 100 BC, and astronomers just noticed

• An asteroid has been trailing Earth around the sun since 100 BC, astronomers estimate.
• The space rock at first appears to be another moon, but it's actually orbiting the sun and not Earth.

An asteroid has been following Earth around the sun for the last two millennia, and astronomers just noticed it.

Scientists first discovered the space rock, called 2023 FW13, in March using the Pan-STARRS observatory in Hawaii.

At first glance, the asteroid seemed to be orbiting our planet, which would make it another moon.

But this tailgating asteroid is actually orbiting the sun, the space news website Sky & Telescope reported. It just happens to be traveling at roughly the same path and pace as our planet.

"Earth plays essentially no role in its motion," Alan Harris, a scientist specializing in near-Earth objects at the Space Science Institute, told Sky & Telescope. "[It's] in no way associated with Earth other than by chance."

That makes 2023 FW13 a "quasi-moon," or a "fake moon."

What makes this fake moon unique

Usually quasi-moons trail Earth for just a few decades, but this one is different. From data they've collected about its orbit, astronomers calculated that the asteroid has been in the vicinity of our planet since about 100 BC, the year Julius Caesar was born.

Other telescopes have confirmed the asteroid's existence, according to Space.com, and the Minor Planet Center at the International Astronomical Union officially listed it as a known object in April. That's when French astronomer and journalist Adrien Coffinet first reported the discovery.

The asteroid is roughly 20 meters long, which is about the size of a semi-truck, and it comes within about 9 million miles of Earth at the closest point of its path, according to Space.com. For reference, the moon — the one we all know and love — is about 238,855 miles away. It's unlikely to hit Earth, though, Harris said.

"The good news is, such an orbit doesn't result in an impacting trajectory 'out of the blue,'" he told Sky & Telescope.

This isn't Earth's first extra moon. In 2016, the Pan-STARRS observatory discovered a sneaky rock satellite that may be a fragment of the moon — the big one that we can all see in the sky. For now, that's still the only moon that matters to our lives here on Earth.