  This paper analyses changes in public opinion by tracking political
discussions in which people voluntarily engage online. Unlike polls or surveys,
our approach does not elicit opinions but approximates what the public thinks
by analysing the discussions in which they decide to take part. We measure the
emotional content of online discussions in three dimensions (valence, arousal
and dominance), paying special attention to deviation around average values,
which we use as a proxy for disagreement and polarisation. We show that this
measurement of public opinion helps predict presidential approval rates,
suggesting that there is a point of connection between online discussions
(often deemed not representative of the overall population) and offline polls.
We also show that this measurement provides a deeper understanding of the
individual mechanisms that drive aggregated shifts in public opinion. Our data
spans a period that includes two US presidential elections, the attacks of
September 11, and the start of military action in Afghanistan and Iraq.
