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,
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
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.