There is no mistake here.
Turn off is active. When transitively used ("subject turns object off"), the subject is turning off the object. When intransitively used ("subject turns off"), the subject turns itself off.
Be turned off is passive, indicating that the subject is not doing anything to turn itself off, and implying that some other force is acting on it to turn it off.
In this case, the active form was presumably used because the telescopes will (actively) use up their power (causing themselves to turn off) rather than someone flipping a switch somewhere to turn them off. So since it will be the telescopes' power drainage or mechanical failure that turns them off, the telescopes are described as turning themselves off.
It's the phrase 'cheek by jowl', which the OED lists under the entry for 'cheek':
5.a. cheek by jowl; †cheek by cheek (In 6–7 cheek(e to jowl, by chole, jole, joll, gig(g by geoul, jowl, 7–8 jig(g by jowl, 9 cheek by chowl, for chowl, and jowl, Sc. cheek-for-chow, dial. jig-by-jow.) Side by side; in the closest intimacy.
It's also listed under 'jowl | jole' (n1):
1.a. A jawbone, a ‘chaft’; a jaw; esp. the under jaw; pl. Jaws.
- Here perhaps belongs the phrase cheek by jowl, in earlier usage cheek by cheek: see cheek n. 5a. In this the j form is known from 1577, which is somewhat earlier than it is known in sense 1 above. The 17th c. variants cheek by chole, chowl, agree in form better with jowl n.2 or . But it is probable that, by the time the phrase came into use, all three ns. were already felt as one. The following examples supplement those under cheek n.
and that refers to 'jowl | jole' (n2):
The external throat or neck when fat or prominent; the pendulous flesh extending from the chops to the throat of a fat person, forming a ‘double’ chin; the dewlap of cattle; the crop or the wattle of a bird, etc.;
So the OED seems to be uncertain as to whether the 'jowl' in 'cheek by jowl' refers to the jaw, the neck, or the roll of fat which makes a 'double chin' - or even perhaps 'head'.
But 'walking cheek by jowl' definitely means walking very close together.
This is from The School of Manners, I'm guessing?
Walking with thy Superior in the house or Garden, give him the upper or righthand, and walk not just even with him cheek be joll, but a little behind him, yet not so distant as that it shall be troublesome to him to speak to thee, or hard for thee to hear.
So Garretson is saying, when you're walking with someone more important than you, don't walk side-by-side with them as if you were their best buddy, instead walk a little bit behind them (but not so far behind that it's a pain in the arse for them to talk to you).
Best Answer
The fact that the Chicago style manual has no clear definition reflects, I think, the fact that this is a fundamentally confusing term, due to a confluence of two sources of confusion:
Now, you and I could agree a clear answer to either of these questions (the second is less open to debate), but regardless, many other people will think the other way. It doesn't really matter whether we're right or wrong, if the aim is to create a clear communication, which after all is the primary function of language, whether spoken or written. Context will always help - if you're talking about something that happened in 1803 (eg the start of the Napoleonic Wars in Europe), and you say "diplomatic relations at the turn of the 19th century were at a low point" then people would guess what you mean.
So, I think that, if pushed to provide an answer, the Chicago Style manual would probably side with Wikipedia - that "The turn of the 19th century" means, say, 1795 to 1805. But it will still be a confusing choice of words to many.