Same sentence, different question:
「ではみなさんは、そういうふうに川だといわれたり、乳の流れたあとだといわれたりしていたこのぼんやりと白いものがほんとうはなにかご承知ですか。」
Well everyone, this vague white thing which has been called a river and has been called a trail of flowing milk, do you know what it really is ?
Literally I translate:
xがなにかご承知ですか as
"is there some knowledge of x" ->
"Does anyone know about x".
Is this correct? Is it a common way to ask a question? What other ways could you ask "does anyone know about x?"?
It is a perfectly normal way to elicit a response. my self I would take the meaning in as "Are you aware of x". We use this question at work all the time when bugs and system error are found. having said that. I would probably not use it while speaking with friend. But would use it if I were an old man telling a story to kids or a narrator of a documentary.
easy alternatives could be
なにか分かりますか？
なんなんでしょう？
なにかご存知ですか？ (a little more on the polite side)
Xがほんとうはなにかご承知ですか literally translates to "Do you know what X really is?" Although ご承知ですか may be relatively less uncommon than ご存じですか (honorific "to know ～"), Xがなにか is a very common way to ask questions like this.
Xがなにか here forms an embedded question which can be literally translated as "what X is". Similarly, Xがだれか means "who X is", Xがいつだったか means "when X was", and so on. Details are explained in this question: Usage of か after a clause?
- 分かりますか
- 知っていますか
- ご存知{ぞんじ}ですか・ご承知{しょうち}ですか
The last two are rather formal and are often used interchangeably.
Like...
ご存知の通り and ご承知の通り
There is one case I can think of though where they are not really fully interchangeable...
「承知いたしました！」 - "Understood!"
This makes sense if you think about the Kanji though, as 承知 is like saying「承{うけたまわ}って知る」, which carries the idea of receiving knowledge, whereas 存知 just indicates the presence of knowledge.
Here's a comparison with 了解 which has a similar meaning...
