Dried up

Residents of Venice, Italy, rely on boats to navigate the city's crisscrossing canals. Last January, many of these gondolas were grounded when the canals' water levels dropped to the lowest depths in over a decade. Why the dip? A period of fair weather overlapped with a special time in the moon's orbit.

Every day, the water levels on Earth's shores rise and fall in tides because of gravity from the moon and sun. Because the moon is closest to Earth, its pulling force is stronger than the sun's. As the moon revolves around Earth, it tugs on the oceans, causing a watery bulge. Coasts that lie in the bulged area experience high water levels, or high tides. Coasts in bulge-free zones, however, get low tides.

On January 10, the moon's pull was stronger than usual because it was in perigee, or the point in its orbit when it is closest to Earth. In addition, the sun and the moon were lined up, pulling in the same direction (see diagram, below). This heavy tug caused a higher-than-normal watery bulge and a bulge-free zone with superlow water levels. That situation is a spring tide.

Combined with the good weather, the low spring tide caused the water in Venitian canals to plunge 90 centimeters (35 inches) below the average level. Pierpaolo Camposrini of CORILA a research group studying the Venice lagoon system, says: "Areas of the city became inaccessible to larger boats."

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