The five brightest planets visible from Earth have lined up
in plain sight to form a spectacular celestial array that
will not be seen again until 2040.

Through the next four weeks, Jupiter, Mars, Mercury, Saturn
and Venus will appear in the western sky, visible in the
evening with the naked eye.

"The five naked-eye planets are converging in one part of
the sky and from now until mid-May you can see all five at
one glance, which is pretty unusual," said John Mosley, an
astronomer at the Griffith Observatory in Los Angeles.

Over the weeks, the alignment will assume different shapes,
as the five planets take their orbital paths around the
sun. The planets orbit in the same plane, like grooves in a
phonograph record, only at different distances from the
sun.

Similar bunchings occur every 20 years or so, though they
are not always visible. The last they were this visible was
in 1940.

In 2004, they will appear together again in the night sky,
but will be spread over a much wider area, said J. Kelly
Beatty, executive editor of Sky & Telescope magazine. They
will not be as easy to spy at a single glance again until
2040.

Astronomers emphasize that there is no astronomical
significance to the pileup. It is, Mr. Beatty said, just a
"pretty coincidence."

http://www.nytimes.com/2002/04/18/science/18PLAN.html?ex=1020118669&ei=1&en=e909e888b258b40c