SKYLIGHTS

Astronomy news for the week starting Friday, October 29, 1999.

The Moon, now in its waning gibbous phase, passes through third quarter to waning crescent on the last day of the month, Sunday October 31st, about dawn when the Moon stands high to the south on the meridian. In the third century BC, the great Greek astronomer Aristarchus of Samos tried to use the exact moment of the quarter to measure the distance of the Sun relative to that of the Moon, deriving a factor of 20. Though he was off by a factor of 20 (the true ratio is 400), he successfully showed that the Sun is indeed much farther away than the Moon. As it wanes toward new, the Moon will bracket brilliant Venus, appearing to the west of the planet the morning of Wednesday, November 3, and to the east of it the morning of Thursday, November 4.

Venus itself makes news this week, as it passes greatest western elongation from the Sun on Saturday, October 30th, when it is a full 46 degrees to the west of the Sun and as high in the east as it will get this year during the dawn hours, the great distance making the planet visible in the fully dark early morning sky. Even the most casual observers are noticing it, many wondering what the great "star" might be. More than any other body Venus is referred to as the "morning" or "evening star" regardless of its planetary status. It is a good time to reflect on Venus's nature. Close to the Sun, its thick carbon dioxide atmosphere (100 times the pressure of Earth's) has raised the surface temperature to 470 degrees Centigrade (870 F), softening the rocks and allowing great ancient volcanism. All the water is driven from the surface, as well as a great deal of sulfur, causing the beauty of Venus's thick clouds to be produced by droplets of sulfuric acid. It would not be a nice place to vacation.

But then neither would Jupiter, which now rises gloriously bright in the evening just about the time of sunset, Saturn following close behind. Both planets are made mostly of hydrogen and helium surrounding deep "rocky" cores of some sort. Beneath the colorful clouds, Jupiter's deep circulating hydrogen, which is in the liquid state, produces the strongest planetary magnetic field in the Solar System.

In the early evening we see a transition from the traditional summer constellations to those of fall. Lining up on the meridian around 8 PM are dim Cepheus near the pole, Pegasus high to the south, Aquarius below with his distinctive the 4-star "Water Jar," the first magnitude star Fomalhaut in Pisces Austrinus, the Southern Fish, and for those far enough to the south, the quite- marvelous modern constellation Grus, the Crane, looking for all the world like a great bird stalking the sky.