Monday, August 27, 2007
Small solar system denizen gets big chance to shine
John Goss
John Goss is chairman of the Mid-East Region of the Astronomical League and a former president of the Roanoke Valley Astronomical Society.
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Blog
Not all creatures that swim in the sea are fish. Likewise, not all bodies that orbit our sun are planets.
The next few nights offer the curious sky watcher an opportunity to spot one of these other celestial creatures, the large asteroid Vesta.
On Jan. 1, 1801, the Italian astronomer Father Giuseppe Piazzi stumbled across an unknown object. At first he thought it was a new comet, but after calculations showed that it lurked in an empty region between the orbits of Mars and Jupiter, he concluded it was a new planet. Piazzi named it Ceres.
Oddly, Ceres appeared starlike even in the most powerful telescopes of the day. Therefore, its real diameter must have been very small indeed, perhaps just a few hundred miles across -- less than a tenth of the Earth's diameter. It was so small that, if it was placed at our moon's distance, it would exhibit a disk less than one-fourth the size of our moon.
Matters became more confusing over the next three years when three more starlike bodies were discovered, all located in a similar orbit as Ceres. The fourth and brightest of these objects, a 300-mile-wide oblong rock later called Vesta, was discovered by physician and amateur astronomer Heinrich Olbers in March 1807.
At first, these small objects were thought of as being individual planets. As the years passed, more of them were discovered, forcing astronomers to create a new classification. Because they always appeared as pinpoints of light, they fell into the category of "asteroid," which means "starlike."
In the past 200 years, many tens of thousands of rocky asteroids much smaller than Vesta have been found, most of them orbiting in the same expanse where Vesta resides. Astronomers believe that they are accreted debris created 4.6 billion years ago when a small planet was prevented from forming because of the intense gravitational tug of nearby Jupiter.
Vesta occasionally is bright enough to find with the unaided eye, but just barely. Three months ago, it was close enough to the Earth that, under ideal viewing conditions, a sharp-eyed stargazer could have spotted it amid the many dim background stars. Now our Earth pulls away from Vesta, making it no longer a naked-eye sight.
It can still easily be seen through binoculars as an inconspicuous "star" slowly drifting eastward just above Jupiter. Think of the size difference between these two worlds: Vesta may be three times closer, but it is 300 times smaller than the giant planet.
Beginning tonight and continuing over the next week, plot Vesta's movement relative to Jupiter and the distant stars of Psi and Omega Ophiuchi. Sure enough, it swims in the celestial sea just as Olbers observed it 200 years ago.
Tuesday morning another sight awaits, one much closer to home. Our nearest neighbor in space, the moon, lies on the opposite side of the sky as the sun while also sitting on the ecliptic. This results in the year's second total lunar eclipse.
Starting at 4:52 a.m. our planet's shadow slowly envelopes the bright full moon, eventually engulfing it. By 5:52, the moon becomes fully eclipsed. Unfortunately, totality may be difficult to see because of the approaching sunrise. As the eclipsed moon sinks in the western sky, morning twilight brightens in the east, washing out the already dimmed moon. How close to sunrise can you still discern the setting moon?
