MOMENTS after the lengthy discussions concluded and the decision was made, Neil Lennon walked down the tunnel and out into the bowl of Parkhead.

It was 1am. The stadium was empty and silent. The only illumination was from the moonlight. He had never seen the place looking so serene. The date was June 8, 2010, and Lennon had just agreed the deal to become the permanent manager of Celtic.

Two days later Lennon painted that picture at his first media conference, unprompted. He would effortlessly cough up colour and insight like that for the next four years. Another line from that day: it was the first time he talked about getting Parkhead rocking again and bringing back "the thunder". All of it was a dream for the headline writers and for his club's marketing department.

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After the limp surrender of the Tony Mowbray regime Lennon restored Celtic. By "thunder" he meant noise, passion and spectacle and he delivered that, at least in some memorable spikes in Celtic's campaigns.

He said something else on that first day which is oddly relevant to his departure. He had walked out into the stadium in the dead of night "to get a breath of fresh air because we had been in there for a fair length of time". What is he doing now, having tendered his resignation, if not exactly the same thing? Lennon has been with the club for a fair length of time, since 2000 as player, coach and manager, with only a brief interruption when he went south and felt a bit lost without Celtic in his life. He's leaving to get his breath of fresh air, a permanent departure from his club and a new chapter in a career which will be fascinating to follow from afar.

The time comes when any Celtic or Rangers manager recognises that he has trundled to the end of the line. Only the successful ones have the privilege of an exit strategy and it is to Lennon's credit that - like Martin O'Neill and Gordon Strachan, and unlike Mowbray - he leaves on his own terms.

It was far from guaranteed that it would end well when Celtic placed the team in his hands four years ago. Boardroom reservations were apparent from the fact he was initially given only a 12-month contract. Aged only 38, Lennon understood why he was often described as a "rookie boss" but I felt he bristled about that a little and was glad to win a Scottish Cup after his first season, and a league title at the end of his second, so that mild disrespect was removed.

Celtic blew a winning position in his first league campaign by losing in Inverness Caledonian Thistle with four games left, a result which can still send a shiver up his spine. Ten games into the following league season the wheels threatened to come off the cart. Celtic were 3-0 down at half-time against Kilmarnock (that was still the score past the 70th minute) and Lennon began to get that feeling of detached helplessness managers experience when things are spiralling out of their control.

Celtic salvaged a draw and it was an ordeal he came to identify as the turning point in his management.

Three consecutive league titles were collected after that, each easier than the one before. The winning margin over Rangers in 2011/12 was 20 points and would have been a very comfortable 10 even if the Ibrox club had not been docked the same number for going into administration. Without Rangers his winning margins were 16 points in 2013 and 29 this season.

The removal of the eternal rival reduced the pleasure of finishing above others. The dulling of the contest led attendances to fall at Parkhead and diminished Lennon's sense of achievement.

In the cup competitions his record was poor, just two wins in nine campaigns. Some of the defeats provoked fierce criticism from supporters who pointed to the resources and asked how Celtic lost to Ross County, Kilmarnock, Hearts, St Mirren, Morton and Aberdeen. There was defeat in a final to Rangers, too, but also Scottish Cup triumphs in 2011 and 2013. A crippled Rangers was seen as a free pass to collect a treble but only in 2012/13, with a league and Scottish Cup double, did Celtic lift more than one trophy.

That was the season when the thunder was at its loudest. On November 7, 2012, the decibel levels were deafening. Celtic's 2-1 defeat of Barcelona in the Champions League group stage will forever remain the iconic 90 minutes of Lennon's reign. The team grew in Europe under him, posting steadily improving results in each of his first three seasons and culminating in qualification for the last-16 of Uefa's premier competition.

From there, the evidence of decline became obvious. Over his four seasons Celtic went out of Europe in August, then December, then March, then December again. What turned out to be his final European tie brought the bitter sting of losing 6-1 to Barcelona.

The peak came in 2012/13, the season of the double and the last 16, the team of Victor Wanyama, Gary Hooper and Kelvin Wilson. He accepted the departure of Wanyama and Hooper as an inevitable consequence of the Celtic business model, just as he was sounded resigned to Fraser Forster and Virgil van Dijk also leaving sooner rather than later.

A Celtic manager is under annual pressure to deliver Champions League group football with foundations built on sand. On balance his judgment in the transfer market was above average. James Forrest, one of his favourites, was a rarity when it came to home-grown talent.

It has become a cliche to write about the "real" Lennon, and how he is a far more thoughtful, composed and intelligent man than many believe him to be. But it is true. For a while he was quick to anger on the touchline, losing his rag at refereeing decisions so often that he admitted major shareholder Dermot Desmond had given him a "talking down" at times. He is calmer now. Experience, maturity and the absence of Rangers made life easier.

Mercifully he leaves at peace, not hounded out by hatred. He had to deal with far too much of that. He was among the victims of a campaign of letter bombs and bullets sent through the post in 2011. It was in that year, too, that a thug leapt out of the stand at Tynecastle and attacked him. There were times when Lennon looked exhausted by the stress of his existence back then - he had a bodyguard on matchdays - but his resilience was consistently impressive.

This week he decided he had ridden the rollercoaster for long enough. There was no intrigue over his departure. Like his mentors, O'Neill and Strachan, four or five years was enough. The thunder he whipped up reached its crescendo in 2012. Now that it has died down, for Celtic and for him, the time had come to move on.