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I was watching Crash Course Astronomy and came across a sentence with the word "setup" that I cannot get.(The sentence is at about the 4th to 5th minute of the video.)

At one perihelion pass, one side of Mercury faces the Sun. Then, 88 days later or so, it approaches perihelion again. But it's spun 1.5 times, and this means the exact opposite side of Mercury faces the Sun at this closest approach. 88 days later, Mercury has spun 1.5 times again and the whole thing repeats. It turns out it is a perfectly legitimate stable configuration, just like the one to one spin/orbit setup.

I have looked up all the meanings of "setup" in various dictionaries, but still I cannot understand what it means there. The first definitions to come up were "arrangement" and "organization", but at that moment it was still unclear to me whether that definition fit.

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    Did you not find any dictionaries telling you that setup means arrangement, configuration, situation? The "one-to-one spin/orbit" configuration referred to would be when a planet spins on its own axis once every time it makes a single orbit round its star (so it always presents the same side to the star, while the other side remains in constant darkness). That "stable configuration" also applies to the Moon as it orbits Earth, which is why we can't directly observe the far side of the Moon from Earth.
    – FumbleFingers
    Feb 3, 2018 at 17:40
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    The first dictionary site I checked had as the first definition: "1. organization; arrangement", which is exactly the definition that applies here.
    – The Photon
    Feb 3, 2018 at 17:41
  • Yes, the first definitions come up were "arrangement" and "organization", but at that moment it was still unclear to me. I should have thought over the whole piece more carefully.
    – Dmytro O'Hope
    Feb 3, 2018 at 20:00
  • For the next question, you may want to explain which of the definitions you found that seemed to you like it might be the closest one. Sometimes it's hard for someone fluent in a language to understand why it's not easy to pick out the right definition. The more we understand why the dictionary didn't help, the better we can explain what the right definition is and maybe give you some advice on how to pick out the correct definition next time.
    – ColleenV
    Feb 4, 2018 at 13:11

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Setup: 1. The way in which something is constituted, arranged, or planned.

It is about the way that a certain planet system is arranged around a star. The "one spin/orbit setup" means that the planet has settled in such a way (arrangement) of orbiting that when it is in a certain position in an orbit (say, perihelion), it has made one full rotation around its axis since the last time that it passed this very same spot.

Apparently, this paragraph is saying that earlier, astronomers thought that only one spin/orbit arrangement is stable; if a planet doesn't make a full rotation, or makes 1.x rotations as it turns one complete orbit around its star, it's not stable and it will gradually move into a different orbit. E. g. if it was making 0.8 rotations in a higher orbit, it would move to a lower orbit which would take 0.8 the time, and the relation will become 1 to 1. Now, they found out that a different arrangement can also be stable.

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