I have a question about the provenance of ancient documents.  For
example how do we know that Julius Caesar was assassinated in 55 BC
(or whenever it was)?  To continue using ancient Rome as an example,
Tacitus wrote a lot of Roman history.  But he wrote on scrolls which
have long since turned to dust.  I read on a website that Tacitus'
work was preserved and copied in a monastery in Fulda, Germany.  But
the historical record of this monastery only goes back as far as
Charlemagne.  To the best of my knowledge you can't trace a copy of
Tacitus back farther than that.  In fact, back in the 1700s a lot of
people thought Tacitus' work was a forgery.  So how do historians
trace the provenance of ancient documents to be sure the history we
read really happened?

Hi.

This is an excellent question. Proving that something happened 2,000
years ago is obviously challenging for a number of reasons: lack of
formal records, differences in the calendar, and, as you mention,
historical accounts that are not necessarily credible. So how is that
historians can make any claims at all? How do historians know that
Julius Caesar was assassinated on exactly March 15th in 44 BC? Indeed,
how do historians know that Julius Caesar was assassinated at all?
Heck, how do we even know that Julius Caesar really existed?

Fortunately, in the case of Roman history, there are many sources of
evidence. As you've mentioned, one area of proof is the written
historical record. In and of themselves, Tacitus' writing don't
necessarily prove anything. However, if the same claims made by
Tacitus appear in the works of other historians, then they gain
additional credibility. For example, similar details about Julius
Caesar's assassination also appear in the works of another Roman
historian, Seutonius. For an English translation of Seutonius'
account, visit this "Ancient History Sourcebook" page at Fordham
University:
http://www.fordham.edu/halsall/ancient/suetonius-julius.html

Suppose that multiple written accounts are still not convincing, where
do historians turn next? Suppose you say, "Well, okay, maybe Julius
Caesar really existed and was assassinated, but why should I believe
that he was assassinated on March 15th? Maybe one of the ancient
historians got the date wrong, and the rest copied him." Sure, that's
possible, but how can we explain away the existence of an ancient
Roman coin that features Caesar's assassin Brutus and displays the
words "Eid Mar" (middle day of March)? Indeed such coin actually
exists. See this page from Wild Winds Numismatics:
http://www.wildwinds.com/coins/sear5/s1439.html

In the case of Roman history, coins provide an important part of the
historical record. Indeed, there is a wealth of scholarship relating
to Roman history as discovered through coins. See this essay, "Roman
Coins and Roman History" by Daniel J. Taylor on the web site of
Lawrence University:
http://www.lawrence.edu/dept/art/buerger/essays/romanc.html

There are also books such as Michael Grant's "Roman History From
Coins" and C.H.V. Sutherland's "Roman History and Coinage, 44 BC - Ad
69: Fifty Points of Relation from Julius Caesar to Vespasian." Read
descriptions about the books at Barnes and Noble:
http://search.barnesandnoble.com/booksearch/isbnInquiry.asp?userid=68OE3OV2CG&isbn=0521095492
http://search.barnesandnoble.com/booksearch/isbnInquiry.asp?userid=68OE3OV2CG&isbn=0198721242

Details about Roman history are also often confirmed in art and
architecture. The Romans were great sculptors, and details about
historical individuals and events are often revealed through
sculpture. The Romans made busts and statues of their leaders, and
many have survived through this day. See this page featuring busts of
Caesar from the Classics department at Beloit College:
http://www.beloit.edu/~classics/main/courses/history222/julius/ 

In a few cases, Roman art and architecture provides a direct
historical record. Accounts about Roman emporer Trajan might exist in
contemporary written histories, and you might learn about his invasion
of Dacia (modern day Romania) in one of those histories. However, an
equally important source of information would be the Column of Trajan
in Rome. The column depicts a sculpted relief of the events of the
Dacian invasion. See the Trajan's Column project at McMaster
University:
http://cheiron.humanities.mcmaster.ca/~trajan/index.html

In summary, we can never be exactly sure that ancient history happened
exactly the way the ancient historians said that it did. However,
details from other historians as well as from things such as coins,
art, and architecture provide clues that help to confirm contemporary
historical accounts.

search terms: Roman history, coins, art, architecture, sculpture

I hope this helps.

This is a good answer.  I'll be the first to admit my question was
very broad.  I'm going to ask a follow up question specifically on the
provenance of Tacitus.

I'm not much of a historian, but I learned while doing genealogy is
that the more information you have to back up your claims, the better.
So my guess is that Tacitus's work is not the only historic document
that historians relied on.

--Chris

juggler-ga gave an excellent response but their is one thing that he
left out:

Textual Criticism

Actually, he did refer to it...he just didn't call it by name.

One of the ways which the provenance of ancient texts is verified is
called "textual criticism".

Most web searches will bring up sites on Biblical textual criticism
but there are Tacitean texual critics as well.

Basically, they take all copies of a manuscript and compare what is
said to what is known from other manuscripts, the contradictions among
the similar texts and such things as writing style and copyist error.
It is a process of elimination more than anything else. Once they are
through whittling down what they have the end result is what has
veracity.

Of course, there is no way to be 100% certain that something is
correct but by this process they can come very close to knowing the
real truth.

Here are a couple of websites that will explain different aspects of 
this science:

http://www.tektonics.org/copyisterrors.html

http://www.christianseparatist.org/ast/hist/textcrit.htm

http://www.stilez.freeserve.co.uk/apol/solomon.html