While David Warner had leapt and roared and punched the air upon reaching his century the previous day - and why not because it was a top-class innings? - Cook simply clenched his fist, raised his bat to the dressing room and the England supporters, exchanged a hug with his captain and took a brief look up to the sky.
The emotion? It looked like relief. Maybe it was tinged with exhaustion - it was bloody hot in Melbourne on Wednesday - and a natural English reserve that appeared exaggerated by Warner's lack of it the previous day. Or maybe it was simply an acceptance that, for all the significance of the personal milestone, the job was not done: England need more from him on Thursday.
It was interesting to see the reaction of the other players. Of course the England dressing room were delighted. But many of the Australia team applauded, too. Some went up to congratulate him personally. This has been a bad tempered and at times petty, petulant series. But Australia recognised here a good man who had been through hard times and, much as they want to win, they could respect that. You suspect that, on some level, the more experienced of them were even a little pleased for him.
Nobody should doubt the significance of this innings from a personal perspective. Cook has looked horrible at times on this tour. While he has experienced troughs of form before, this one had started to look like a nosedive. His reaction times and pace in the field appears to have slowed. The extra training sessions he has undertaken had started to look punishing and self-defeating.
People talked of 10 innings since he had scored a century, but it was 10 since he reached even 40. And while the whispers had been about his difficulties against the spin of Nathan Lyon, so great had been his struggle against the pace and accuracy of Mitchell Starc and Josh Hazlewood, that his issues against spin had barely seemed important.
It wasn't a particularly gritty innings, either. It bore some of the hallmark - the crisp timing and straight drives - of Cook at his best. And, most importantly, it came with his side under pressure and the tour in danger of reaching a tipping point. Sure, the Ashes have gone. That hurts. But a whitewash? That's agony and ignominy wrapped up in a humiliation. Despite an awful first session of the match, England find themselves with their noses in front at the end of day two. They could win this.
Whether this innings represented a return to form or the encore at the end of a fine career remains to be seen. Cook has only just turned 33, after all - that's 18-months younger than Shaun Marsh - and might be expected, in normal circumstances, to have his best years ahead of him.
But there's so much baggage; so many scars and memories. And where once he was a single-minded young sportsman hungry to prove himself, now he is a father for whom the game holds few surprises. He may well be sated. He may well struggle to regain that focus that once came so naturally.
At least if he goes now, he will do so - up to a point - on his own terms. This was an innings that guarantees he will not be dropped ahead of the New Zealand tour. And while his average and reputation may have dipped in recent times, it is worth recalling some of his achievements: the man of the series award when England won in Australia in 2010-11; the man of the series award when England won in India in 2012. No regular opener - and no England batsman - has ever scored more Test runs. His reputation should be assured.
It seems churlish to pick holes in such an innings. But it would be disingenuous not to recognise that the Australia attack, sans Starc, is significantly impoverished. Pat Cummins was far from his best, too, as he struggled with illness that reduced his pace and ability to contribute the volume of overs he might have done in normal circumstances.
And while there was a howl of disapproval after James Anderson suggested there was some doubt as to the strength of depth of Australia's seam resources, the evidence of this game supports his view. Jackson Bird is a decent bowler and might well get into the England squad. But he doesn't offer anything like the same level of threat as Starc.
Most of all, this sluggish surface has largely negated the greatest strength of the Australia attack: its pace. Cook was focussed on playing the ball around knee to hip height. There was nothing there to hurry him. If it has been produced in England, the Australia side would claim it has been "doctored." It is not so unlike the Edgbaston track where Cook last made a century. Nobody does attrition better than Cook.
That is not to diminish his efforts. This is a surface on which only three men in the Australian side were able to reach 30. It is a surface that punishes forcing shots and impatience with bat or ball. While others have struggled to come to terms with that fact - they were three batsmen dismissed via the inside edge in the Australian innings - Cook has understood it perfectly.
There will be frustration, though. Frustration that Cook and Stuart Broad produced their best form only after the Ashes had gone and frustration that it has taken so long to exploit the potential weaknesses in this Australia team.
But sometimes we crush the joy out of the moment with analysis and context. Sometimes it should be fine to enjoy the moment and forget about today until tomorrow. Cook, and England, deserved their day in the sun.