Translation of a passage discussing zodiac systems


My friend and I are reading 宝石泥棒 by Masaki Yamada. In the beginning of chapter
five, there are a few paragraphs that discuss constellations and zodiac
systems, but we can't figure out how to translate some of them. Here is the
text (paragraphs are numbered):

>   1.
> 今夜の〝宿〟は、斗宿{いて}だった。黄道{こうどう}星座の〝いて座〟が、今夜の宿星となっているのだ。斗、牛、女、虚、危、室、壁……など赤道帯を二十八に区分し、旅人に見たてられた月が、毎夜、それらの〝宿〟を移り歩いていく——いわゆる黄道二十八宿は、中国で生まれたものと考えられがちだが、似たようなものはインドやアラビアにも存在したという。
>   2.
> 黄道二十八宿の本来の意味は、見失われて久しくなるが、夜空を二十八に区分し、星座の移り変わりを暦{こよみ}の基底に据{す}えるというその方法だけは、この時代の人間に引き継がれている。
>   3.
> 本来の意味が失われるのも当然といえばいえる。参星{オリオン}、昴{すばる}などの宿星に変わりはないものの、星座そのものは大きく変化しているのだ。総じて、夜空は明かるさを増したようである。
>   4. 美しいと形容するだけでは、言葉が充分ではない。たぎった星が、空を白く燃えあがらせている印象なのだ。
>

Our translation for paragraph 1 is:

> "Tonight's 'mansion' was the Dipper. That is, the standard zodiac's
> Sagittarius constellation was the sign for this night. The equatorial belt
> is divided into 28 parts, The Dipper, The Ox, the Girl, The Emptiness, The
> Rooftop, The Wall etc., and the moon, which has been likened to a traveler,
> goes through those "mansions". The so-called 28 mansions of the ecliptic are
> thought to have been born in China, but similar systems exist in India and
> Arabia."

Paragraph 2:

> "The proper meaning of the 28 mansion system has long since slipped into
> obscurity, but the division of the night's sky into 28 parts, and the method
> of basing calendars on the passage of constellations, those alone have been
> passed down to the people of the current age."

Now, the part that we are having issue with is paragraph 3. We are not sure
how to break down this sentence. Is `当然といえばいえる` a set phrase? If so, what does
it mean? Or is it just `当然といえば` and the second `いえる` belongs to the noun
phrase that precedes 当然?

The sentence that follows seems to say

> "Although Orion and Subaru have not changed, the constellation systems
> themselves have changed greatly. Generally, it seems the night's sky has
> increased in brightness".

We are not sure if this translation is right. I'm not sure how the fact that
the constellation systems have changed relates to an increase in the night's
brightness. Moreover, we don't know how to connect that to the sentence with
the 当然. Is it the constellation systems as in "The Chinese system", "The
western system" etc. that have changed, or the actual constellations in the
sky, with the exception of Orion and Subaru, that have changed?

The final paragraph's translation seems to be:

> "Words are insufficient even for describing it as beautiful. The impression
> is that overflowing stars are causing the sky to burn in white fire."

Although this seems straight-forward (in a poetic sort of way), we can't quite
make out a coherent meaning for the entire passage.

So please help us with paragraph 3, and if possible, the overall meaning of
paragraphs 2-4, as we seem to have lost the forest for the trees.




This is related to the pattern Ａ（だ）（と）いえばＡ´ and other ～といえば expressions.  

> このコーヒーは **おいしいといえばおいしい** けど、高くない？  
>  The coffee _**is**_ good, but don't you think it's a little expensive?

> 明日 **といえば** 、テストがあるって先生が言ってなかったっけ？  
>  **Speaking of** tomorrow, didn't the teacher say that we have a test?

> 飲み物 **といえば** ビールでしょ！  
>  **When it comes to** drinks nothing beats beer! yeah?

Your example feels a bit different, but I still think the idea is related:

> 本来の意味が失われるのも当然（だ）といえば、（そう / 当然だと）いえる  
>  If you were to say that it was only natural that the original meaning be
> lost, you could (say that).

In other words, "It wouldn't be at all wrong to say that it's perfectly
natural that the original meaning be lost".

