How to scan "ac veluti magno in populo ..."?

Question

In Virgil's Aeneid there is the following line:

ac veluti magno in populo cum saepe coorta est

and I am trying to figure out how to scan it. The first thing is that I thought the "a" in ac was short, so how can it begin a verse? Secondly, if I elide magno-in to a short vowel (as I would expect because I thought the "i" in in is short), then we have a problem because the first "o" in populo is short by nature, so it cannot begin a foot. However if we elide magno-in to a long vowel, then it matches pŏpŭlō fine, but we have a problem with "-ī măg". I guess this problem could be resolved by treating the first vowel in māgno as long.

In Lewis & Short there is no mark over the "A" in magnus, so I kind of always assumed it was a short A, but does having no mark mean it can be either?

Also, if I follow this last method of scanning, then I do not understand how I can elide magno in to a long vowel because my scansion guide says that when two vowels are elided, then "the length of the elision depends on the requirements of the second vowel". However, since I thought ĭn is short, then would it not require the elision to be short?

EDIT:

A moderator closed my question because in his sole opinion another answer consisting of a long dissertation on long by position "answered" my questions. If this other completely different question-answer pair "answered" my question it is not even remotely clear to me how. As I understand it, long by position is when the vowel is before a pair of consonants which is not the case here. For example, in Irby's "Basic Guide to Latin Meter and Scansion" (p. 583) it reads: "A vowel scans as long if (1) it is long by nature [NOT the case here in either word], (2) it is a dipthong [NOT the case here in either word], and (3) it is long by position--these vowels are followed by double consontants or a consonantal I [NOT the case here in either word].

I am kind of looking for an answer here to my particular question, not the advice to go read a 500-page textbook on Latin poetry.

Answer 1

In scansion, there are two related concepts that have to be distinguished. Vowels can be either long or short; syllables can be either heavy or light. (The term "long by position" should really be heavy by position, since it doesn't change the length of the vowel.)

First, let's mark the vowel lengths.

ac velutī magnō in populō cum sǣpe coorta est

Normally a macron isn't written over ae, since diphthongs are always long in Latin, but it doesn't hurt to illustrate that specifically.

Now, let's cut out the vowels that will get elided.

ac velutī magn' in populō cum sǣpe coort' est

And now comes the trickiest step: converting from vowel length to syllable weight. First, any syllable containing a long vowel is heavy:

ac velutī magn' in populō cum sǣpe coort' est

But also, any syllable with a coda consonant—that is, a consonant after the vowel—is heavy.

How do we know if there's a consonant after the vowel or not? Well, when multiple consonants come between vowels in Latin, the rule* is that the last consonant attaches to the start of the following syllable, and all other consonants attach to the end of the preceding syllable.

And in poetry, this even applies between words, just like elision does! So as a rule, any time there are two consonants after a vowel, that syllable has a coda consonant, and is heavy. This is known as being heavy (or "long") by position.

ac-ve-lu-tī-mag-n'in-po-pu-lō-cum-sǣ-pe-co-or-t'est

Now we can break this down into feet, as usual.

ac-ve-lu / tī-mag / n'in-po-pu / lō-cum / sǣ-pe-co / or-t'est

* There is one exception: the "muta cum liquida" rule, which says a stop consonant and a liquid consonant can go together at the start of the following syllable, instead of one of them being pushed to the end of the preceding syllable. But that doesn't matter here.