* Astronomy

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.


Expand
Saturn, Venus and Mercury converging at sunset on June 19, 2005. The three planets will get much closer together in the days ahead. Image credit: Jimmy Westlake of Yampa, Colorado.

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 will next pop into view. The three planets form an eye-catching triangle about 1.5o long, easily hidden by your outstretched 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.

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 sulphuric 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 baked tatties 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. It will also investigate Mercury's scarps which 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.