What Actually Happens To Big Ben When The Clocks Change?

Twice a year, the time on the Elizabeth Tower’s Great Clock has to be changed, so that millions of Londoners know precisely when to get home to catch the soaps. Alright, most of us just check the time on our smartphones nowadays, but tradition still dictates that the Great Clock has to be changed, adjusting for the changes between British Summer Time (BST) and Greenwich Mean Time (GMT). But just how does this happen?

The responsibility for changing the clocks lies with the very regal sounding Palace of Westminster Clockmakers, who each spring and autumn must scale the tower to add or remove the necessary hour. The process is so precise that not a minute can be spared, and begins well in advance of the clocks actually springing forward.

Big Ben clocks change

It all begins at around 9pm local time, when the clockmakers silence the strike mechanism and reset it for 2am BST or GMT (depending on whether the clock is going forward or back), which will keep the hour chime silent until then.

So, by this stage Ben’s clock is still telling local time, but just isn’t announcing it to the world. Next, it’s time for the dial lights in the clock face to be switched off, leaving the Great Clock in the dark until a few minutes before the big moment. This is mostly to ensure that the public don’t think Ben – or worse, time itself – is having a freakout when the hands start spinning.

Once the lights are off, it’s time to move the clock hands to the new midnight (again, this can be BST or GMT depending on the season), taking great care not to go a moment too far, or the whole process will have to begin again. There’s a bit of a time crunch here, because during the movement of the hands, the gravity escapement – which regulates the clock’s accuracy – has to be released, and if it’s not back in time, the clock won’t be running smoothly.

In the time leading up to the new midnight, the team carries out maintenance on the clock and its mechanisms. At the precise midnight moment, the going train is started, and the clock starts ticking once more; this means that the Great Clock will actually be telling the wrong time for a little while, though thankfully no-one is able to see it. For the next two hours (until the new 2am arrives and all your electronic clocks automatically change), the clock runs without any chimes, which allows the team to monitor the accuracy and make any adjustments as needed, until Big Ben’s clock is running perfectly.

With fourteen minutes to go until 2am, there are no quarter or hour chimes left until the big reveal, so both mechanisms are switched back on (again, renovations prevent this from happening at present). Lastly, with a couple of minutes to spare, the dial lights are switched back on, in the order of the West clock face first, and then South, East, and North. When the clock strikes 2am, everything should now be in perfect working order! If that’s all proving hard to visualise, it’s summarised here in a handy video:

Did you think that was it for the Clockmakers? Of course it’s not! They have to adjust the time on a mere 2000 other clocks, dotted about the Palace of Westminster and the parliamentary buildings. Theirs is not an easy job of course, so be thankful most of your time-telling devices are going to do it automatically!

The clocks go back by one hour on Sunday, October 30 at 2am.

Also published on Medium.