The moon will make its closest approach to Earth on March 19, at the same time it becomes full. It will be the first time since 1993 that the moon has been full while making its closest approach.

The result will be slightly higher tides, and the full moon will be somewhat brighter than it normally is, by about 30 percent, and it will appear 14 percent larger as well.

Such 'supermoons' happen whenever the moon happens to be both full and at its closest approach to Earth, or perigee. (The point in the orbit furthest from Earth is called apogee). The moon orbits the Earth in an ellipse rather than a perfect circle and the orientation of the ellipse (the direction of its long axis) changes with respect to the Sun over the course of the year. So the distance the moon is from the Earth when it reaches full phase isn't always the same. Perigee and full moons only coincide once about every 18 years.

At perigee it will be 221,565 miles (356,575 kilometers) from Earth, just under an hour after it reaches full phase. On the East coast of the U.S. it won't be visible as it will happen at 3.p.m local time, before the moon rises, which will be at 7:23 p.m.

For observers in the U.K. the moon will have been above the horizon for an hour and a half (it rises at 6:23 p.m. local time) so they will have a chance to see it at the precise moment of perigee, though the difference between an average full moon and a supermoon is hard to see. In Asia, people in Beijing will see perigee as the moon is on its way down, as it will be about 3 a.m. March 20.

Such close approaches do not cause earthquakes or natural disasters, as the difference between perigee and apogee is about 26,000 miles (42,000 kilometers) or about 11 percent. Tides might be slightly higher, depending on the local geography. Tides tend to lag the actual full moon, though this also differs depending on location. The Bay of Fundy experiences the largest tides in the world, and will see 30-foot differences between high and low tide. Bit for most areas the changes will be minimal.

Scientists at NASA have stressed that previous perigees have not coincided closely with major earthquakes or tsunamis. The Indian Ocean tsunami occurred on Dec. 26, 2004, some 15 hours before the moon was full that month, but the moon was not approaching perigee at that time -- it was actually further away for that particular full moon. The earthquake in Japan occurred on March 11, and the moon was also not near full phase, nor was it particularly close. While the moon can create tides, those forces are too small to have much effect on the Earth's crust.