Mercury, Venus, Saturn converge for close
encounter
Stick up your thumb and hold it at arm's length. It doesn't seem
very big, does it? But it is, big enough to hide three
planets.
This weekend Mercury, Venus and Saturn are going to crowd together
in a patch of sky no bigger than your thumb. Astronomers call it a
"conjunction" and it's going to be spectacular.
The show begins on Saturday evening, June 25th. Step outside and
look west toward the glow of the setting sun. Venus appears first,
a bright point of light not far above the horizon. As the sky
darkens, Saturn and Mercury pop into view. The three planets form a
eye-catching triangle about 1.5o long, easily hidden by your
thumb.
It gets better on Sunday evening, June 26th. The triangle shrinks
with Venus and Mercury only 0.5o apart. Now they fit behind your
pinky!
Monday evening, June 27th, is best of all. With Saturn nearby,
Venus and Mercury converge. At closest approach, the two planets
will be less than one-tenth of a degree apart. Such pairings of
bright planets are literally spellbinding.
If you go outside to see the show, take someone along. Here are
some fun facts you can share:
The closest planet to the sun, Mercury, is not the hottest. Venus
is. The surface temperature of Venus is 870 F (740 K), hot enough
to melt lead. The planet's thick carbon dioxide atmosphere traps
solar heat, leading to a runaway greenhouse effect. On Venus,
global warming has run amok.
Venus is so bright because the planet's clouds are wonderful
reflectors of sunlight. Unlike clouds on Earth, which are made of
water, clouds on Venus are made of sulfuric acid. They float atop
an atmosphere where the pressure reaches 90 times the air pressure
on Earth. If you went to Venus, you'd be crushed, smothered,
dissolved and melted--not necessarily in that order. Don't
go.
Mercury is only a little better. At noontime, the surface
temperature reaches 800 F (700 K). If you turn your kitchen's oven
to that setting, the pizza will burn to a smoking crisp. Radars on
Earth have pinged Mercury and found icy reflections near the
planet's poles. How can ice exist in such heat? NASA's MESSENGER
spacecraft is en route to Mercury now to investigate.
see captionHere's one way to trick an astronomer: Show them a
picture of Mercury and ask what it is. Many will answer "the Moon,"
because the Moon and Mercury look so much alike. But Mercury has
something that the Moon does not: long sinuous cliffs called
"lobate scarps." Some researchers think Mercury's scarps are like
wrinkles in a raisin, a sign of shrinkage. A shrinking planet?
Weird.
If you look at Venus or Mercury through a telescope, you won't be
impressed. Both are featureless, Venus because of its bland clouds,
Mercury because it is small and far away. Saturn is different. Even
a small telescope will show you Saturn's breathtaking rings.
Galileo Galilei discovered Saturn's rings almost 400 years ago, but
he didn't understand what he saw. A planet with rings was too much
even for Galileo. Scientists today are still reeling. Saturn's
rings are improbably thin. If you made a 1-meter-wide scale model
of Saturn, the rings would be 10,000 times thinner than a razor
blade. They're full of strange waves and spokes and grooves. And no
one knows where they came from.
One school of thought holds that Saturn's rings are debris from the
breakup of a tiny moon or asteroid only a few hundred million years
ago. As recently as the Age of Dinosaurs on Earth, Saturn might
have been a naked planet--no rings! Tiny moons orbiting among the
rings today appear to be stealing angular momentum, which, given
time, could cause the rings to collapse. Is Saturn like a flower,
temporarily in bloom?
That's one of many questions being investigated by NASA's Cassini
spacecraft, which has been orbiting Saturn since 2004. Cassini is
on a 4-year mission to study Saturn's moons (all 34 of them), rings
and weather. Every day the craft beams stunning images to Earth:
click here to see them.
A lot can happen behind your thumb. This weekend is a good time to
look.
From NASA
19 Jun 2005 - Western Sky
