byungkyupark's photostream

A composite picture showing the various positions of the ball in the minute preceeding 1 p.m. The ball is raised to the top at 1 minute before the mark (it was at midpoint during the last 5 minutes); one of the picture was taken the moment the ball was dropping (compared to the GPS time, it was off by either 1 second or a fraction of a second), and the final one taken 2 seconds after 1 p.m., when the ball had already dropped.

The guide explained how this method was used to synchronize the clocks on ships with the land-based clock at the Greenwich Observatory (not too unlike the modern GPS, system, where electromagnetic waves---radio-frequency, not visible---are still used to synchronize the GPS satellites occasionally to the standard time kept on the ground).

While we wait for the ball to drop at the Observatory, marking 1 p.m., Greenwich Mean Time (or BST, during summer).

On the tour to Greenwich; the boat I was on this time had no deck on the top, so it was a bit more difficult to get good photos.

Photo of the Tower Bridge as we were passing through; I think this is the north end of the bridge, taken from the east of the bridge.

As the back of the boat was the only place to get glass-free photos on both sides of the boat, a few fellow passengers were there as well.