As you watch coverage of Sandy as the storm approaches the East Coast, you've probably heard mention that Monday's full moon may worsen the effects of the storm. But why? The answer lies in the positioning of the Sun, Moon and Earth and the resulting spring tide.
Tides are caused by the moon, specifically the gravitational attraction between the Earth and moon. The oceans bulge out in the direction of the moon and another bulge forms on the opposite side as the Earth itself is pulled towards the moon (away from the water on the opposite side). The sun's gravity also contributes to this pulling and sloshing about of the oceans, though the effect is about 45% smaller because of the distance between the Earth and Sun. Around the time of each full moon, the Sun, Earth and moon align,which maximizes gravitational effects and produces unusually high and low tides known as spring tides. Not spring in the seasonal sense but spring as in the tide leap quickly up the shoreline. None of this is unusual, it happens about every 14 days with the full and new moons. The timing of Monday's full moon unfortunately aligns with the arrival of Sandy.
The height of the high tide is also affected by the changing distance between the Earth and moon. Fortunately the moon is currently near apogee, the furthest point in from Earth. A huge storm coming ashore during at high tide, during a new moon at perigee the tidal surge could be even higher. The Sun, moon and Earth line up creating a proxigean spring tides about every 1.5 years. The last one to occur was May 6 of this year.