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And Then, There Was One

If you've been watching the skies over the past few months, no doubt you noticed that recently, there have been lots and lots of planets to see. Just last month, there were four visible in the early evening sky, all at the same time.


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This month, all of a sudden, three of them are gone. Venus, Mars, and Saturn set so early now that they are completely lost in the glow of sunset. The only planet left is mighty Jupiter, high in the west, just beneath the feet of Leo the Lion.

Fortunately, Jupiter is a fascinating telescopic target - even a good pair of binoculars will reveal four of its largest moons. Larger equipment can show off Jupiter's cloud bands - in the eyepiece, Jupiter looks as if it has faint stripes!

When will we see the others again? Venus will be the first to return, toward the end of June, but it will appear in the early morning just before sunrise. Saturn will join it in early August, and Mars won't reappear at all until December! Unfortunately, fewer people will notice them because experience tells us that most people prefer their astronomy at more convenient times than before dawn.

I'm Ready For My Close-Up, Mr. Cassini

Saturn may be impossible to see in the night sky right now, but that's no reason to ignore it. Especially now, when the Cassini spacecraft is on final approach to the ringed planet. The first significant date will be June 11, when Cassini flies past Saturn's moon Phoebe. Phoebe stands out as an oddball among Saturn's moons. It's unusually dark, and it orbits Saturn in the opposite direction from all the others. There's a good possibility that Phoebe was once a wayward asteroid that was captured by Saturn's gravity.

Another big date is July 1, when Cassini enters orbit around Saturn. Once in orbit, Cassini will be able to stay in the Saturnian system and study the planet, the rings and more than thirty moons for at least four years. The last spacecraft to visit Saturn, Voyager 2, flew past the planet, with only a few days to record as much data and take as many photographs as possible before moving on to Uranus.

Later this year, in December, Cassini will release a probe called Huygens which, in January, will land on the surface of Saturn's largest moon, Titan, for a look beneath its thick, smoggy atmosphere. Titan's atmosphere reminds many scientists of what they think Earth's atmosphere may have been like when our planet was young.

To keep an eye on what Cassini is up to, visit the Cassini home page at saturn.jpl.nasa.gov.

Transit of Venus

Venus is also lost the Sun's glow this month, but there is actually one way to see it - a very unusual way. On the morning of June 8, Venus will pass directly in front of the Sun as seen from Earth. This is an extremely rare event, called a transit. The last time a transit of Venus occurred was in 1882; meaning that nobody alive today has ever witnessed one. Consequently, astronomers around the world plan to be watching.

Way back in 1716, astronomer Sir Edmond Halley realized that if enough people around the world observed and recorded timings and details of a transit, one could accurately calculate the distance from the Sun to the Earth, which at the time wasn't known. Halley did not live to see it happen, but astronomers took his advice and travelled far and wide to make observations of the transits of 1761 and 1769. Even Captain James Cook stopped off the coast of Tahiti in 1769 so that researchers on board could make critical measurements.

When the data from the two transits were compiled and calculations made, the results showed that the Earth-Sun distance was around 153 million kilometers (95 million miles), which is very close to the true value of 149 million kilometers (93 million miles) we know today.

These days, there's not much that astronomers can discover watching a transit. Still, many are looking forward to the chance to witness such a rare event. Due to the geometry of Earth's and Venus' orbits, transits of Venus happen in pairs. Another one will occur eight years from now, in 2012. After that, it will be 105 years before another pair occurs.

Unfortunately, observers in Tennessee won't get to see the entire transit of 2004. As the Sun rises that morning, around 5:30 am, the transit will already be in progress, and Venus will be a small dark dot on the face of the Sun. Venus will slowly slide across the solar disk until about 6:25 am, when the transit ends. The circumstances are more favorable for Tennessee in 2012 when the transit begins in the mid-afternoon. We won't be able to see the entire transit, but it should still be an exciting event.

Viewing a transit of the Sun demands the same level of safety precautions required for viewing the Sun on any day. You never want to look directly at the Sun. You certainly do not want to look at the Sun through a telescope or binoculars. Doing so risks permanent eye damage. For observing and safety tips, fascinating stories from history, and a LOT more about the event, visit www.transitofvenus.org.

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