[Legate of Amon Lanc]

Originally Posted by Mänwe

"3018 August ...being hunted both by the Elves and Sauron's servants, he took refuge in Moria; but when he had at last discovered the way to the West-gate he could not get out." - (Appendix B, The Tale of Years)

The choice of words here for me suggest that before hand he had been under no pressure to actually hide. Yet being now hunted by both sides he seeks refuge.

Aragorn let us not forget entered Moria through the eastern gates, when Gollum in my view if he had travelled through Moria was one way. West to east. If Aragorn was tracking him he wouldn't make such a mistake as to travel in the opposite direction.

Note: Gollum, as the quote from the Tale of the Years implies, entered through the Eastern gate, not western, as you say. Because, it says he "took refuge in Moria..." and then "when he at last discovered the way to the West-gate..." i.e. he entered the other way than West, and that's East. (assuming he didn't use any other entrance, which I doubt) And, how would he even come to entering through the Western gate? And, how (why) would he get to Eregion in the first place? Aragorn entered through the eastern gate, so if he was tracking Gollum, it would make sense for him to go from east to west, in the track of Gollum.

However, I don't think Aragorn came to Moria in the pursuit of Gollum. In Unfinished Tales, Christopher Tolkien mentions:

In a letter written in 1964 my father said:

There are, of course, quite a lot of links between The Hobbit and The Lord of the Rings that are not clearly set out. They were mostly written or sketched out, but cut out to lighten the boat: such as Gandalf's exploratory journeys, his relations with Aragorn and Gondor; all the movements of Gollum, until he took refuge in Moria, and so on.

Another part from the Unfinished Tales, which I quote in fullness:

It seems clear that pursued both by Elves and Orcs Gollum crossed the Anduin, probably by swimming, and so eluded the hunt of Sauron; but being still hunted by Elves, and not yet daring to pass near Lórien (only the lure of the Ring itself made him dare to do this afterwards), he hid himself In Moria. 10 That was probably in the autumn of the year; after which all trace of him was lost.
What then happened to Gollum cannot of course be known for certain. He was peculiarly fitted to survive in such straits, though at cost of great misery; but he was in great peril of discovery by the servants of Sauron that lurked in Moria, especially since such bare necessity of food as he must have he could only get by thieving dangerously. No doubt he had intended to use Moria simply as a secret passage westward, his purpose being to find "Shire" himself as quickly as he could; but he became lost, and it was a very long time before he found his way about. It thus seems probable that he had not long made his way towards the West-gate when the Nine Walkers arrived. He knew nothing, of course, about the action of the doors. To him they would seem huge and immovable; and though they had no lock or bar and opened outwards to a thrust, he did not discover that. In any case he was now far away from any source of food, for the Orcs were mostly in the East-end of Moria, and was become weak and desperate, so that even if he had known all about the doors he still could not have thrust them open. It was thus a piece of singular good fortune for Gollum that the Nine Walkers arrived when they did.

From this it looks unlikely that Gollum would enter Moria once, then leave and enter again. Also, even to Gollum Moria certainly was not a nice place, and he didn't want to go there once, the more he wouldn't like to return in there - like Aragorn or Gandalf.

Back to Aragorn. I think Aragorn's words imply that he entered through the eastern gate and then again left through the same gate. He could, as well, have come through and open the Western gate by just pushing it (as Gandalf says), but a) he says "I passed once the Dimrill Gate", which does not seem like he passed through Moria, but just the gate (might be he even just walked three steps in, then decided that it's not much nice and left), b) he says "I also came out again", which not only does not say that he must've left through the other gate, but may as well sound like he came out through the same entrance. And c), mainly, Aragorn does not seem to know at all, how to navigate Moria. Gandalf himself explains gaps in his knowledge by it being long ago since he went there, and that he was going from east to west, not from west to east, like Fellowship now went. But, while Gandalf at least tries to lead the Fellowship, Aragorn doesn't at all. Which would seem like he never was too deep in there, and not taken the whole journey even from East to West.

Whatever the reason for his entering to Moria was, it was surely not nice, and I'd consider it something like a "ranger issue", not concerning Gandalf or the Wise or the Dwarves. That's just my feeling, not based on any evidence. However, it certainly wasn't anything super-important, because it's not mentioned anywhere.

Originally Posted by MatthewM

I know what you mean, as I am also re-reading The Lord of the Rings, you always catch something new, or delve deeper into certain details that you hadn't before. Although I don't know the answer to your question, I must say one thing- I actually like how the movie presents Gandalf as advising against the entry into Moria. I don't understand how Aragorn would know not to enter, while Gandalf was unsuspecting to the danger that was within. Perhaps Gandalf knew he was supposed to fall there. I don't know. But from a logical standpoint, it makes more sense to me that Gandalf knew the evils of Moria in the film, rather than in the book where he seemed clueless.

In the book, Gandalf does not know what's Durin's bane, as well as none else, except Dáin who saw it. Gandalf's words referring to balrog before the battle on the Bridge are just:

Originally Posted by The Bridge of Khazad-dûm

Then something came into the chamber – I felt it through the door, and the orcs themselves were afraid and fell silent. It laid hold of the iron ring, and then it perceived me and my spell.
What it was I cannot guess, but I have never felt such a challenge.

This is the first time Gandalf "encounters" the Balrog (through closed door). He does not even come to a speculation, it seems even now, he doesn't know much, and surely he doesn't come to the idea that this can be the Balrog. The moment he learns is only when the Balrog actually shows:

Originally Posted by The Bridge of Khazad-dûm

"Ai! ai!" wailed Legolas. "A Balrog! A Balrog is come!"
Gimli stared with wide eyes. "Durin's Bane!" he cried, and letting his axe fall he covered his face.
"A Balrog," muttered Gandalf. "Now I understand." He faltered and leaned heavily on his staff. "What an evil fortune! And I am already weary."

Had he known what the Durin's Bane is, I agree he'd probably try to avoid the journey through Moria at all costs. But in the book, he doesn't know. You have probably been deceived by Saruman's words when he is browsing through the book and speaking to (not present) Gandalf "You know what the Dwarves awoke..." Do not let the movies poison your mind!