Perhaps they wanted the extra time to organise a party, or perhaps they were (foolishly) waiting for better weather, but this week, four months after the fact, Britain celebrates the 60th anniversary of Queen Elizabeth II’s accession to the throne.

Having passed George III’s mark last year, the Queen is now the second-longest serving British monarch; she will overtake Queen Victoria in September 2015. (And another seven months or so from then to overtake the claimed reign of James Edward, the Old Pretender.)

Since she seems in fine health, and her mother lived to be 101, no one would bet against her still being there in three or four years. At the age of 86 she is already, by quite a margin, the longest-lived British monarch (Victoria is next at 81), and in January she became the longest-lived British head of state: Richard Cromwell, briefly Lord Protector in 1658-59, beat every previous monarch for longevity, although he lived most of his life in obscurity.

While we don’t usually invest people with a lot of credit just for not dying, on all accounts the Queen has been an exceptionally able, hard-working and conscientious monarch. To keep that up for 60 years is a serious achievement. When contrasted with some of the other members of her family, the Queen’s dedication to her task has been outstanding.

Part of the reason monarchs these days are able to be popular and respected is that they have so little real responsibility; they are well insulated from most of the sources of public controversy. Elizabeth’s grandfather, George V, established the modern pattern: that the monarch acts only on the advice of the prime minister, and that their only independent act is to appoint a new prime minister when necessary.

In exceptional cases that might involve the dismissal of a government that is acting improperly, but even that is only possible if the monarch can find other advisers who are prepared to take responsibility for it.

Plenty of scope remains for informal advice and influence — indeed, it would be a foolish government that didn’t take advantage of the Queen’s extraordinarily long experience — but if the government insists then it can always get its way.

This is not a bad a model for the relationship between government and head of state; it has now been exported across most of the democratic world. It doesn’t follow, however, that hereditary monarchy is a good way of selecting that head of state.

The plain fact is that Britain in recent times has been very lucky. Most monarchs have understood their place and done their job reasonably well.

In the two centuries since George III, none have seriously tried to extend the bounds of their constitutional power. The last monarch to have a different view of his duties to that of his government — Edward VIII — was content to abdicate rather than force the issue.

Even more importantly, monarchs over those two centuries have been of sound mind, and for even longer — in fact, ever since the accession of Mary I in 1553 — they have all been adults. But in the annals of monarchy that’s very unusual. Madness, child monarchs and disputed regencies are the rule as much as the exception.

Walter Bagehot famously remarked that “It has been said, not truly, but with a possible approximation to truth, ‘That in 1802 every hereditary monarch was insane’,” and went on to say that “The more we study the nature of cabinet government, the more we shall shrink from exposing at a vital instant its delicate machinery to a blow from a casual, incompetent, and perhaps semi-insane outsider.”

The most important thing to remember when framing systems of government is to expect the worst, not the best. If we assume that personnel will always be wise and virtuous, the task becomes much simpler. But when that assumption comes to grief, as sooner or later it inevitably will, we will wish that we had included more safeguards and trusted less to good fortune.

By all means celebrate the good fortune that has given us 60 years of a decent and competent monarch. But let’s not draw the wrong lesson about the usefulness of monarchy.