The moon orbits the Earth every 29 1/2 days, so it reaches perigee more than once a month. The orbit of the moon changes slightly over time, so the distance between Earth and the moon also changes -- but only slightly, Williams said.
On March 19, it will probably be only about half a percent closer than it ever is every 18 years, he said, which is a "very, very small amount."
And though the gravitational effect of the moon causes the tides (when the moon is closer, the tides are slightly larger), he said there's "no scientific reason whatsoever" to expect that this supermoon will result in floods or other extreme conditions.
But, Williams said, on the night of March 19, you will want to peek up at the sky.
"Because it's a full moon at its closest approach, it's going to be big and really bright. It should be noticeably brighter than a normal full moon. I would suggest that you take the opportunity and go out at night," he said. "This is the biggest full moon that you will ever see. You will see this moon again, but this is as big as it gets."