Earth has a second quasi-moon — a small asteroid that has been orbiting us for 100 years
Asteroid Cruithne (about 5km wide) has been in a complex resonant orbit with Earth for approximately 100 years, co-orbiting the Sun in a way that traces a horseshoe pattern relative to Earth over a 770-year cycle. It is not gravitationally bound to Earth like the Moon, so it's called a 'quasi-moon' or 'co-orbital asteroid'. Several other small objects share similar relationships with Earth. At some point in the distant future, gravitational perturbations will eject Cruithne from this relationship — but for now, Earth has more than one companion in orbit.
We're taught Earth has one moon, full stop. Finding that 'moon' is actually a simplified answer — and that Earth has multiple gravitational companions in various types of co-orbital relationships — makes the solar system feel more complex than the textbook diagram suggests.
“Earth has a second quasi-moon — asteroid Cruithne, 5km wide, has been in a complex resonant orbit with us for ~100 years. Not gravitationally bound like our Moon, but a companion nonetheless. 🌍 #OddlyHuman”