The Eiffel Tower Grows Taller in Summer

When Gustave Eiffel designed his iconic tower for the 1889 World's Fair in Paris, he chose wrought iron as his primary material, a decision that makes the structure a living example of physics. The tower is composed of over 18,000 pieces of this puddled iron. Like most metals, iron is subject to thermal expansion; when heated, its particles gain energy and move apart, causing the material to expand. On hot Parisian summer days, the entire 300-meter-tall iron lattice absorbs heat, causing it to increase in height by as much as 15 centimeters. Conversely, during colder winter months, the metal contracts, and the tower shrinks back to its normal height.

This seasonal fluctuation in size was not an unforeseen quirk but a known property of the metal that Eiffel and his engineers accounted for in the revolutionary design. The open-lattice structure is not rigid but flexible enough to handle these temperature-induced changes without sustaining damage. This principle is also at work on a daily basis. As the sun moves across the sky, it heats only one side of the tower at a time. This uneven heating causes the sun-facing side to expand more than the side in the shade, resulting in the tower leaning slightly away from the sun. This movement means the top of the tower traces a small circular curve in the sky over the course of a summer day.