Phil Plait, Stahp!

Quick! How many moons does the Earth have?

You might be forgiven for saying “One”, but it turns out the question isn’t all that easy to answer . . . because it’s it [sic] depends on what you mean by “moon”.

No, Phil, no.

That’s how Plait starts out his most recent article at Slate.com.

Just no. Forget the click-baity headline (“How Many Moons Does Earth Have? One More Now, Kinda.” ) And forget the explanation of resonant orbits and quasi-satellites. Earth has one moon, and one moon only, Vasili, the likes of Cruithne and OL339, a 150-meter diameter rock discovered in July in a resonant orbit that mimic’s Earth in nearly every way be damned.

OL339, the newest baby, is no moon.

A moon, or natural satellite, is defined as an object that orbits another object – its primary. Both Cruithne and OL339 have a primary – and it ain’t the Earth. It’s the Sun. So Cruithne and OL339, despite their resonant orbits, are as much moons of Earth as Mars is.

You may not a special irritation in my voice while I discuss this matter. It clearly bugs me. Because you can’t dither over what the definition of a moon is. A moon orbits a planet – and asteroids (or near-Earth asteroids, as Cruithne and OL339 are more properly called) orbit the Sun.

This kind of word parsing irritates me. It’s a parlor trick smug folk play on gullible fools who don’t know enough about astronomy to call horseshit on the shenanigans. Listen, for example, to Stephen Fry on the same question:

If I’d have been on that show, Fry would have gotten a punch up the bracket and further abuse. Because I agree with Rich Hall’s assessment: “Oh, you’re just making this up.” Cruithne does NOT orbit the Earth once every 770 years. No. It orbits the Sun about once every year.

I highly object to parsing astronomical facts to make something appear as it is not – something Plait should be fully aware of, once being president of the James Randi Educational Foundation. It ought to be clear what we mean by moon as it is clear what Uri Geller means by bending spoons.

New Scientist, to whom Plait offers a tip o’th’ hat, m’lady, for the news, gives us the news in a better, more straightforward, less clickbaity fashion:

Add one to the entourage. A newly discovered asteroid called 2014 OL339 is the latest quasi-satellite of Earth – a space rock that orbits the sun but is close enough to Earth to look like a companion.

The asteroid has been hanging out near Earth for about 775 years and it will move on about 165 years from now, say Carlos and Raul de la Fuente Marcos of Complutense University of Madrid in Spain, who have just described it.

Quasi-satellites orbit in resonance with Earth, allowing the planet's gravity to shift the rock's position much like an adult pushing a child on a swing, says Martin Connors, an astronomer at Athabasca University in Canada. The asteroid orbits the sun every 365 days, as Earth does, but Earth's gravity guides it into an eccentric wobble, which causes the rock to appear to circle backward around the planet.