When plants align

[Smaug]

It's important to note a few things in addition to what he is saying. One, is that a conjunction doesn't mean the planets are all in a straight line, but rather that they're approximately in the same region of sky.

I'd like to add here that this somewhat depends on how you look at your problem. If you speak purely of a mathematical model presented by Newtonian mechanics than you can talk about exact periodicity, exact match of coordinates and all things analytically exact. Though even in a purely mathematical sence such an approach isn't reasonable as long as there's no way to actually acheive this "ideal exactness" due to extremely complex nature of the system being studied (chaotic behaviour). Also there always are different kinds of errors such as measuring errors, computational errors and other that often remove "ideal exactness" from actual studies even in spite of the fact that there are certain ways to somewhat neutralize said errors.

And if you consider actual Solar system than fundamentally there's no way to speak of something other than approximations.