Eiffel Tower Alignment Display
Yes, it is true that the Eiffel Tower sways in the wind, and here is the proof! This real-time display on the first platform of the Eiffel Tower shows the movements of the tower away from the vertical due to various factors, primarily wind, sunlight, and a natural resonance in the tower.
In a stiff wind, the summit of the tower can move to one side under the wind pressure. The record was set in 1971 when a 160-kph wind moved the tower by a full 15 centimetres at the summit (although a windstorm in 1999 may have broken that record—I haven’t yet been able to verify this). The tower was designed to withstand movements ten times larger, however, so it is never in any danger. In strong winds, it is possible to feel the tower gently swaying, at least at the summit and (to a much lesser extent) on the second platform.
The tower may also lean to one side under the influence of sunlight. The sunlight heats one side of the tower and it expands, tilting the tower. Here again, the magnitude of the movement is never more than eight to twelve centimetres. This is a tilt, not a sway, and the tower remains tilted until the sun goes down. It follows a very regular pattern throughout the day, if the sun is shining.
One other movement is a very slow, gentle sway rather like a seiche in a lake. It takes place over a period of about three minutes and is so slight that it is not perceptible.
Anyway, the movements of the tower are measured in real time with laser-based alignment systems. In this rather busy photograph, you see a display that shows the current deviation of the tower from the vertical. A laser dot projected on the circular screen at the top of the display shows the deviation of the summit; in this photograph, the tower was leaning about five centimetres almost due east, under the influence of sunlight shining on the west side of the tower. The concentric rings on the screen are in one-centimetre increments (greatly magnified); there is a drawing of a pencil on the right side of the screen (hard to see here) that shows the magnitude of the movement compared to a pencil.
Another display (not visible here) shows the wobbling of the tower in real time. The movement is constant and amounts to a few millimetres. When the wind gusts, you can see the tower move a centimeter or two to one side.
In the background of this photograph, you can see some of the structural iron that makes up the tower. Notice how thin and delicate it is—much more so than you would expect for a tower this size. If all the metal in the tower were melted into a single cube, the cube would only be 10 metres on a side. The ironwork you see here just goes up and up, into the second platform. Beyond the railing in the background is a 19-story drop to the ground below.
By the way, this photo is a bit out of date, as the alignment display has been recently changed. I haven't had a chance to take a picture of the new version. In the new version, a computer display projected onto the screen you see here shows fake clouds and a little image of the summit of the tower that moves to show the tower's movements. It's inferior to the old display and it's hard to see even when you are looking at it in person; I preferred the old laser display shown in this photo. Anyway, I'll try to get a photo of the newer display one of these days.
See also my Eiffel Tower FAQ for more information on this landmark.
Click directly on the photo to see a smaller version (half this size). Photographed on September 3, 1999.