By Deborah Zabarenko

WASHINGTON (Reuters) - The five so-called naked eye planets -- Mercury, Venus, Mars, Jupiter and Saturn -- will appear to clump together later this month in a sight that might not be seen again for a century.

But don't call this a planetary alignment. And even though this is fairly rare, there is nothing for earthlings to worry about, astronomer Geoff Chester of the U.S. Naval Observatory said on Tuesday.

The grouping of the five will begin to be visible with unaided eyes around April 20, with the planets clustered closest by around May 4, Chester said by telephone.

"It is an opportunity to see all five of the naked eye planets in the same part of the sky at the same time and that does not happen very often," Chester said.

It could be 50 or 100 years before this happens again, he said.

The display should be easy to see in most parts of the inhabited world, weather permitting, though those at extreme northern and southern latitudes may need binoculars or a small telescope, and Mercury could still be hard to spot, he said.

A similar grouping of the same five planets, plus the moon, occurred on May 5, 2000, accompanied by dire predictions of extraordinary tides and other cataclysms. Earth survived.

However, that cluster occurred on the opposite side of the Sun from Earth, and the Sun's light was so bright the planets could not be seen from Earth.

This time, most humans should have a good view, Chester said.

What they should be able to see will be Mercury, Venus, Mars and Saturn all grouped within the span of 10 degrees of the sky, or about the width of a fist held at arm's length.

Jupiter will be a bit higher in the sky, about three fist-widths away, but still quite close.

The cluster will start to dissipate around May 12, Chester said, when Mercury will dip closer to the horizon and become less visible.

This grouping is only the planets people can see without help, and does not count as an alignment, according to Chester, though others have used that term.

Chester considers a true alignment to be when all the planets are on the same side of the sun, and grouped within about 90 degrees -- nine fist-widths -- of each other, or closer. This occurs once in several hundred years.

The closest known planetary alignment in the last two millennia occurred on April 11, 1128, before most planets were identified as such, including Earth, according to Chester.