This is my exact wheelhouse: planets and dead languages.

I think you're correct, but I think some of it is genitive and some is nominative.

I believe the missing one is Terra and Saturnus is the one before it, and these are nominative.

This is also the traditional English pronunciation of Latin, not what we consider to be the classic pronunciation.

But I can barely understand the singing, so whatevs.

Heh, I dreamt of running the oracles last night.

This post was intended to be posted on February 15th. I've been continuing to try to research and try to cleanly put together the pieces as well as create an exploratory Fireteam to go back and search the Vault of Glass. However, I feel like I can sort of "see the writing on the wall" with the release of the Kate Remington's Respawn Podcast interview with Marty that is scheduled to come out today and reveal some background information behind "Music of the Spheres".

Marty recently reminded folks that the song snippet released with the Destiny Pre-release Alpha Lupi ARG, Eighth, has lyrics that were never determined or explained. A few short days later on January 8th I was researching and listening to “Eighth”, “Awakening”, and the recently fully released/leaked song “The Hope”. I was trying to decipher the lyrics (recently thought to be Latin or re-thought to be Latin again from 4 years ago). I've listened to these songs forwards, backwards, sideways, and upside down/inverted, many times. We've all heard these lyrics probably hundreds of times in the game and on the soundtrack and never realized the words we were hearing.
In the wee hours of the morning of the 8th I was researching Celestial Spheres, amongst many other things, I ended up falling down a rabbit hole on Google Image Search. When searching for something else I ran across a thumbnail of Thomas Wright's Celestial Map of the Universe from 1742. I was viewing 'related images' to it, and thats when it hit me, "like a dish washer dropped from three stories or something."

When seeing this particular image and thinking about the sound of the lyrics in my head: I then, as quickly as I could, loaded up the track Eighth and played it to confirm - I could hear!

(Note: I don't have the language (Old Latin???), spelling, or 'final sphere' nailed down or fully correct. The "operatic" style, syllabic nature, and "intonation" of the words being sung makes it especially difficult to decipher precisely)

Lunae
Mercurii
Lunae
Mercurii
Solis
Martis
Iovis
Solis
Martis
Iovis
Saturni
Dies/Deus/Deo/Dio/Terra/Terrum/Venus?
Saturni
Dies/Deus/Deo/Dio/Terra/Terrum/Venus?

The image above is the result of a later interpretation (Italian? Fra Mauro - Mappa Mundi) of the Ptolemaic System and Geocentric Model (below)

(Geocentric celestial spheres; Peter Apian's Cosmographia (Antwerp, 1539))

Marty has confirmed over on reddit the corrected order
And then posed another follow up question:

You folks should have helped the Alpha Lupi folks.

All words correct and yes it’s the order of planets/pieces in MotS - which is the classic order.

The final word is Terra.

Has anyone explicitly said where the titles come from?

Luna
Mercu
Venu
Luna
Mercu
Venu
Sola
Maru
Jova
Sola
Maru
Jova
Saturna
Terra
Saturna
Terra

I guess that I can hear Venu(s) wedged in there after Mercu(ry). I had originally thought that it sounded more like a continuation of the final syllable from Mercury, like an artistic flare or something. I was also going with the idea that Venus had been amended from the order for a specific reason, but that does not appear to be the case anymore.
On the other hand, I could hear Terra in there, just not confidently enough to nail it down 100% as the word that was being sung.