According to the National Retail Foundation, the most popular Halloween
costume last year was Spiderman. Next was "a princess," followed by
witches and vampires, SpongeBob, Barbie and Harry Potter.

Sounds about right. But if you read the complete list of top costumes,
you'll notice something missing: astronomers. There are no Sagans, no
Galileos, not even a Hubble.

And that's funny, because Halloween is an astronomical holiday.

It has to do with seasons: Halloween is a "cross-quarter date,"
approximately midway between an equinox and a solstice. There are four
cross-quarter dates throughout the year, and each is a minor holiday:
Groundhog Day (Feb. 2nd), May Day (May 1st), Lammas Day (Aug. 1st), and
Halloween (Oct. 31st).

In the 5^th^ century B.C., \"the Celts of the British Isles used
cross-quarter days to mark the beginnings of seasons," explains John
Mosley of the Griffith Observatory in Los Angeles. "Winter began with
Halloween. Halloween marked the transition between summer and winter,
light and dark \-- and life and death."

"On that one transitional night, according to folklore, those who had
died during the previous year returned for a final visit to their former
homes. People set out food and lit fires to aid them on their journey
\-- but remained on guard for mischief the spirits might do on the one
night when the dead returned to the land of the living.\"

In this way, something astronomical became something spooky. It's not
the first time. Have you heard that comets are bad omens? Or that a full
moon brings out werewolves? Astronomy and superstition are old friends.

This year Halloween has a new astronomical significance:

On Oct. 31^st^, the planet Mars is making its closest approach to Earth
for the next 13 years. (13 years? Cross your fingers.) Technically
speaking, the moment of closest approach occurs on Oct. 30^th^, a day
before Halloween, but the difference in distance between the 30^th^ and
the 31^st^ is too slight to matter.

Trick or Treaters will notice Mars rising in the east at sunset. It
looks like a pumpkin-colored star, so intense that people in
brightly-lit cities can see it. Some say it's blood red, but maybe
that's just Halloween talking.

Mars will soar overhead at midnight (as seen from North America) and
stay "up" all night long. Halloween 2005 is truly the night of Mars.

Because Mars is so close---only 69 million km away, which is nearby on
the vast scale of the solar system---it looks great through a backyard
telescope. Lately amateur astronomers have been watching dust storms
swirl around Mars. They've seen icy-blue clouds gather over the Martian
north pole, where it is winter. And they been sketching and
photographing strange dark markings that dapple the planet's surface.

So---nothing against Spiderman, mind you---you might wish to reconsider
your costume. Grab a telescope and be an astronomer! It *is* Halloween,
after all.

As eerie as it looks, Mars is not the scariest planet in the Halloween
sky. That would be Venus.

Venus pops into view even before the twilight sky fades to black.
Everybody notices

As eerie as it looks, however, Mars is not the scariest planet in the
sky on Halloween. That would be Venus.

Early trick-or-treaters will notice Venus even before sunset. Everybody
notices Venus. It pops out of the twilight long before the sunset sky
fades to black. Venus is 20 times brighter than Mars.

What makes it scary? Venus has a case of global warming run amok. Its
thick carbon dioxide atmosphere traps solar heat, warming Venus's
surface to a hellish 800 C. The atmosphere itself is crushing; Venus'
air pressure at "sea level" is 90 times greater than on Earth. Oh, and
those clouds floating overhead ... they're laced with sulfuric acid. If
you were teleported to Venus you'd be dissolved, crushed and melted (not
necessarily in that order).

Scary enough?

Mars, for comparison, only looks scary. It's a cold world with a thin
atmosphere. You'd need to dress up in a spacesuit to walk around
outside. But you'd be just fine.

Mars and Venus are both lovely to see through small telescopes. Venus
has phases, and at the moment it looks like a cloudy half-moon. Mars is
rusty red, cross-crossed by strange dark markings, and it has icy-blue
clouds over the north pole where it is winter.

NASA is taking advantage of the ca to send a robot-spacecraft to Mars:
the Mars R O. It left Earth in X and will arrive at M in 2006 to survey
the planet for future human explorers.