SKYLIGHTS
Astronomy news for the week starting Friday, November 13, 1998.
We begin the week with the crescent Moon waning toward full, making
a lovely sight to the east before dawn. Our companion passes
through its new phase around midnight the night of Wednesday the
18th, and by next Friday night will be visible as a thin crescent
in western evening twilight. Giant Jupiter gets into the act this
week too, in fact today, Friday the 13th, as it stops its
retrograde westerly motion among the stars and begins once again to
move in its normal easterly direction. Retrograde is only an
apparent motion caused by the Earth passing between the planet and
the Sun, and the Earth has moved far enough along in its path for
us to see Jupiter's real motion. Now in eastern Aquarius, the
planet will head toward the next constellation of the zodiac,
Pisces.
The week belongs not to Jupiter or the Moon, however, but to the
famed Leonid meteor shower, rather meteor storm, which occurs only
once every 33 or so years on or about November 17 when the Earth
passes through, or close to, a concentration of debris flaked off
Comet 55P/Temple-Tuttle (the "P" standing for "periodic"). The
comet has a 33 year period around the Sun, the stuff that causes
the storm following in a thin ribbon behind it. Since the comet
returned to our vicinity last January, we have a chance to see the
storm once again. As the Earth plows through the cloud, the
particles -- "meteoroids" -- hit the atmosphere at 70 kilometers
per second, and nearly 100 kilometers up heat to some 6000 degrees
centigrade, leaving a streak of electrified air behind them, the
meteor easily visible on the ground. The particles are small,
ranging from under a millimeter for those that produce the faintest
meteors, to rather large pieces maybe up to a meter across that
produce beautiful fireballs. All the debris is so fragile, though,
that none ever hits the ground.
The stream of meteoroids is moving such that they seem to come out
of the direction of the constellation Leo, and hence are called the
"Leonid" shower. They can be spectacular. Meteor fall rates in
1833 are estimated at 100,000 per hour, the storm in 1799 not far
behind. The 1866-1867 events (seen both years) were modest, while
less was seen in 1900 and 1901, and nothing in 1933. But in 1967
it returned in wild proportions, with a rate for an hour or so
close to that of 1833.
No one knows what will be seen. The best predictions are that we
will hit the stream around 2 PM the afternoon of Tuesday the 17th
in daylight in North America, making the meteors impossible to see
here. However, there is so much uncertainty about the structure of
the stream, that we may well see something the morning of Tuesday
the 17th or Wednesday the 18th. The best time for viewing is after
Leo rises, between about 2 AM and dawn. We may see nothing, we may
be delighted with a great sky show, or find something in between.
The only way to know, of course, is to find a dark location away
from artificial lighting and look.
