The movement begins with a brief introduction for the winds that simply outlines the key, B flat:
Then the violins have the melody:
|Click to enlarge|
Beethoven's metronome marking here, quarter note at 60, is quicker than most performances, though some conductors, following the "historically-informed performance" principles, and playing on original instruments, do adhere pretty strictly to Beethoven's tempi. This opening melody is a classic period, meaning that it is an eight-measure phrase divided in two four-measure segments. The first ends on the dominant, a half-cadence, and the second with a perfect authentic cadence on the tonic. I wrote about this kind of musical architecture here. Now you may have noticed that there are actually NINE measures in the theme, not eight. The reason is that there is an extra measure inserted between the two segments in which the winds add to their brief introduction. This kind of procedure, internal expansion of a conventional phrase, is a fruitful one and we will see it again.
When we listen to the performance, you will notice that Barenboim takes this movement much slower than quarter note = 60. His tempo is somewhere around quarter note = 30 or 35. Remember Beethoven's tempo is adagio molto e cantabile. This continues for a few phrases, then a new theme in a new tempo is introduced: Andante moderato, quarter note = 63. Which is absurd, of course. The difference between adagio molto and andante moderato is considerably greater than the difference between 60 and 63 beats per minute! Barenboim's choice, to slow down the adagio to something that really sounds like adagio molto, makes sense. Here is the new theme, given to the second violins:
Sorry for the break: it's a long theme and takes up a couple of lines in the score. That final F# is the link to a repeat of the phrase. The key is now D major and this is another eight measure period. There is no clear half cadence after the first four measures, however as the whole phrase tends to alternate between I and V with a dominant pedal. This section modulates back to B flat and we hear a variation on the first theme, in the original tempo with a lot of delicate filigree in the first violins.
Then the Andante moderato returns, but this time the flutes and oboes have the theme and the key is G major instead of D. On the next return to the adagio, the key is E flat and the winds have a variation on the original theme. The horn gets some nice solos and the strings have a pizzicato accompaniment. In the next section, modulating back to B flat, the first violins offer another variation in filigree, this time with a change of meter to 12/8, which is identical to simply doing triplets (or sextuplets). This has been prefigured in the strings as their pizzicato was in triplets in the previous section. After more and more elaborate variations in the first violins, the winds return with a simpler statement of the theme.
A loud fanfare in the winds and brass introduces yet another variation, mostly in the first violins. I have talked about "delicate filigree" which is a feeble attempt to describe the wonderful expressive things that are going on here. It is very tempting to simply refuse to talk about this sort of movement, but I have tried to give a bit of an idea of what is going on in case it might open a door for you.
Here is Daniel Barenboim conducting the West-Eastern Divan Orchestra at the Proms last year in 2012: