meaning - 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.