SKYLIGHTS

Astronomy news for the week starting Friday, December 25, 1998.

A Merry Christmas and a Happy Holiday Season to all. We begin the last week of 1998 with the Moon almost in its first quarter, the phase reached on Saturday, the 26th, Boxing Day for those in Canada and the UK. Our companion then waxes through its gibbous phase to reach full just in time to celebrate the New Year on Friday, January 1, among the bright stars of Gemini. On Christmas night, the Moon will appear to the east of Jupiter, and on the night of Sunday the 27th to the east of the giant planet's close cousin Saturn. Later in the week, the night of Wednesday, the 30th, the Moon will make a close pass to the star Aldebaran, rising in North America to the east of the star.

Returning to Saturn, the ringed planet becomes "stationary" this week on Sunday the 27th, as it halts its retrograde or westerly motion against the stars and begins once again its normal movement to the west against the dim stars near the Aries-Pisces border.

Jupiter and Saturn are now close together in the sky, and getting ever closer as Jupiter, now to the west of Saturn, moves the faster. Just how close are the two great planets physically? Though Saturn has only a third the mass of Jupiter, the two are remarkably alike. The lower mass of Saturn produces a smaller gravitational field. As a result, the planet balloons outward, making it almost (but not quite) as large as Jupiter, both about 10 times the diameter of Earth. Each spins with a period of about 10 Earth-hours, each has banded ammonia clouds that parallel the equator, each has equatorial winds that blow to the east at high velocity. Each also has numerous Moons (Jupiter 16, Saturn more than 18), and though Saturn's are by far the more glorious, each has rings made of orbiting debris. More profoundly, each has a similar construction. Made mostly of hydrogen and helium, the pressures inside the planets are so great that the hydrogen first turns to a liquid state and then into a metallic liquid. Buried deep inside are cores that are probably made of the heavier elements out of which the Earth is constructed. While there are great differences in the details, the two planets are vastly different from the two outer bodies Uranus and Neptune, which contain less hydrogen and more heavy stuff and whose winds blow in the opposite direction.