I read an article that says it happens every 19.6 years, but this is the first time since 1226 that it has happened during earth’s night so we can see it. I don’t understand how that happens that it is only 1 out of the last 42 times it has occurred.
In: Physics
Each planet’s orbit is independent of the others. Meanwhile none of the orbits have any sway over the rotation of the Earth. So you’ve got 4 extremely different variables (Earth’s orbit, Earth’s rotation, Jupiter’s orbit, and Saturn’s orbit) all running independently, just waiting on all of them to line up perfectly. 1 out of 42 isn’t that bad of a win when you think about just how unlikely it would be that al of those variables line up.
You may as well pick 4 random dudes from across the globe, and call it a win when all 4 of them happen to crave mashed potatoes on the same day.
They are usually not appearing this close together. Because their orbits are not perfectly planar circles, they are not usually appearing this close when they pass each other.
Earth takes a year to go around the Sun, Jupiter and Saturn take several years. So during the few days they’re really close in the sky, the positions of everything relative to the sun don’t change very much – but only the region of the solar system “behind” the Earth compared to the sun is visible at night from the Earth. If that conjunction, or anything, happens and the Earth isn’t between it and the Sun, then it’s hard to see from Earth because the bright Sun is illuminating all the dust in our atmosphere and making the sky too bright to see dim things.
Because they’re not always this close. 19.6 is the periodicity of the moment they’re the closest considering their orbits. Since orbit shapes aren’t perfect, they can be more or less close *in our sky* (they’re still ducking far from one another). And last time they were this close (in the 17th century), the sun was in the way so we couldn’t see.
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