Cruithne: Earth’s Second Moon?
We're so used to the Earth having only one moon that we don't even bother to give it a specific name. But what if it's time to give up that habit? Let’s talk about Cruithne – a mysterious object that some consider the “Earth’s second moon”. Read on and find out why this idea came about and how true it is. You can also download the Sky Tonight app to see Cruithne moving in the sky.
Contents
- What is Cruithne?
- Cruithne’s special relationship with the Earth
- More facts about Cruithne
- Cruithne: to sum up
What is Cruithne?
Asteroid 3753 Cruithne is a near-Earth object with a diameter of 2.07 km (not potentially hazardous, just in case you were worried). It was discovered in 1984, and in 1997, scientists revealed its unusual orbit which led to it being referred to as the “Earth's second moon”.
Cruithne’s special relationship with the Earth
Cruithne moves near the Earth in a unique way that sets it apart from most asteroids:
- Cruithne's period of revolution around the Sun is approximately 364 days, which is almost equal to that of the Earth (an astronomer would say, they are in 1:1 orbital resonance). This gives the illusion that Cruithne is following the Earth in its orbit.
- Cruithne is in a normal elliptic orbit around the Sun, but from the perspective of the Earth, it follows a rather bizarre trajectory that resembles a kidney bean.
- Over a span of 770 years, Cruithne traces a series of bean-shaped loops around the Earth’s orbit, forming a distinctive horseshoe pattern with our planet nestled in the gap.
Is Cruithne Earth’s second moon?
While Cruithne's orbit and proximity to the Earth might suggest a moon-like association, it does not meet the criteria to be classified as the Earth's moon. First and foremost, Cruithne does not orbit the Earth; at times, it’s on the other side of the Sun, well outside of the Earth's gravitational influence. At some points in its trajectory, Cruithne comes into the neighborhood of Venus and Mercury and outside the orbit of Mars. Since Cruithne and Earth orbit the Sun at similar distances, they are considered co-orbital objects.
Is Cruithne Earth’s quasi-moon?
A quasi-moon is like a companion to a planet that doesn't orbit it directly but follows a similar path and maintains a stable position relative to that planet. Sounds a lot like Cruithne, doesn’t it? Well, don't jump to conclusions. As of March 2024, the Earth has six quasi-satellites, and Cruithne is not one of them. The reason is that quasi-satellites typically seem to make an oblong loop around a planet. Cruithne’s path around the Earth, as we mentioned above, resembles a horseshoe. However, it can eventually morph into a quasi-satellite orbit in a few centuries. Studies have shown that these shifts (from horseshoe to quasi-satellite orbits and vice versa) are quite common among Earth’s co-orbital asteroids. For example, asteroid 469219 Kamo‘oalewa, which is currently one of our quasi-satellites, switches repeatedly between these configurations.
More facts about Cruithne
How big is Cruithne?
Cruithne is about 2.07 kilometers in diameter, which makes it larger than 97% of near-Earth asteroids. For a quick size reference, it is as tall as 5 Empire State Buildings, 6 Eiffel Towers, or 22 Statues of Liberty placed on top of each other. Since we're discussing Cruithne as a 'second moon,' it's worth noting that it is 1,678 times smaller than our Moon.
Is Cruithne visible from the Earth?
Cruithne is not visible from the Earth with the naked eye due to its small size and great distance from our planet. Its magnitude is 17.6, which is relatively dim. To put this into perspective, it is dimmer than Pluto, so observing this asteroid requires advanced telescopes and equipment to track its movements across the sky. By the way, the free app Sky Tonight can help with finding Cruithne: just type its unusual name into the search bar, and the app will show its current location.
What does the name Cruithne mean?
The name “Cruithne” is derived from the Irish language and refers to the first tribes to populate the British Isles. It was also the name of a legendary king of the Picts, the early Celtic inhabitants of Scotland.
Cruithne: to sum up
Cruithne is a near-Earth asteroid that has piqued interest with its unusual orbit and close relationship with our planet. Some have even hastened to dub it the Earth's second moon. However, this statement is not true, because Cruithne does not revolve around the Earth: they both orbit the Sun at the same distance, which makes them co-orbital objects. Over time, Cruithne can become the Earth’s quasi-moon.
