Word of the Week: Primary

Hey, Sky Fans! Happy Wednesday! We’re almost half-way there. The weekend will be here before we know it!

If you read along when we counted down the eight best moons of 2016, one for each night of Hanukkah, thanks. I mentioned a couple of orbital mechanics-related things in the posts about Neso and Charon that I thought I’d go back to. So, maybe now is a good time for this week’s Word of the Week!

In a system of things orbiting other things, for instance Uranus and Miranda, one of the objects is quiet a bit more massive than the other, and their combined center of mass, their barycenter, is located deep within the more massive object, close to its center. The smaller object still pulls on the bigger one gravitationally, but since the more massive one is so much bigger, that tug is very small by comparison.

Rather than calling the bigger thing just that, “the bigger thing in an orbital system,” we use the word primary to describe that more massive object. It’s a general term that lets us make our way through conversations a little quicker. The thing that orbits the primary is called a satellite, though we typically don’t refer to planets as satellites of their stars (I did in my picture). It’s not incorrect; it’s just… not correct.


The sun is our solar system’s primary. In the Uranus-Miranda system, Uranus is the primary. Mars is the primary, and Phobos and Deimos are satellites. It goes on. An interesting twist on  this are Pluto and Charon. They’re so close in mass that their barycenter is outside Pluto, so it’s a little odd to call it the primary. Also, the barycenter of the sun and Jupiter is just a bit above the sun’s surface.

There you have it, this week’s word of the week, Primary! Clear skies, everyone!



3 thoughts on “Word of the Week: Primary

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