September 2009 Sky from the Keeble Observatory
Chicken Little may have been right. Fortunately, the sky fell on Jupiter, not Earth.
Last July 19th Anthony Wesley, an amateur astronomer in Australia was
surprised to notice a dark blemish near Jupiter’s south pole. It reminded him of
the impact sites observed for Comet Shoemaker-Levy 9 almost exactly 15 years earlier
… but those events were anticipated many months in advance as the shattered comet
was tracked to its ultimate ending. This came without warning, so there was no army
of observatories already aligned to study it.
NASA’s Infrared Telescope Facility in Hawaii quickly confirmed the impact site,
which glowed much hotter than the surrounding clouds. In visible light, the impact
showed a dark, irregular patch roughly the size of our own Earth – about 8,000 miles
across. The Hubble Space Telescope returned its first images from the newly installed
Wide Field Camera 3, which both confirmed the impact and demonstrated that the new
camera was working properly.
What did this? Could it happen here?
While it’s hard to be absolutely certain, the fact that it was not seen prior to
impact suggests that it was a modest “asteroid” rather than an active comet, though
this object was probably icy based on its being in the outer solar system. Estimates
place its size at several hundred meters in diameter, with a total mass in excess
of 4 million metric tons. For comparison, the meteor which caused the 1908 Tunguska
event, which flattened over 700 square miles of forest in Siberia, has been estimated
at no more than a hundred meters in diameter (some models place it much smaller,
closer to 20 – 50 meters). The asteroid or comet which led to the extinction of
the dinosaurs 65 million years ago must have been about 10 kilometers in diameter.
Meteor Crater in Arizona was produced by a 50 meter rock.
(Oh, yes, I guess something like this could happen here!) Lest we worry overly
much, we should note that Earth is struck by dozens of 1-meter size meteoroids per
year, which may release the equivalent explosive power of a small nuclear weapon.
These are essentially harmless because they hit over empty ocean and detonate high
in the atmosphere. If one really wants to be paranoid, try a web search for “Apophis.”
More about this object next month!
Lunar phases for September: Full Moon on the 4th at 12:03 pm;
Last Quarter on the 11th, at 10:16 pm; New Moon on the 18th,
at 2:44 pm; First Quarter on the 26th, at 12:50 am.
Predawn planet watchers can look for Venus to rise about 2 hours before the Sun,
trailing Mars which rises almost two hours earlier. By month’s end, Venus will move
closer to the Sun, and will be joined in the predawn skies by Saturn and Mercury.
Mars will climb higher.
Early evening planet watchers will find Jupiter low on the east-southeast horizon,
keeping it visible most of the night. Mercury and Saturn are low to the west and
west-southwest, both about 10 degrees above the horizon at sunset. They’ll both
disappear in the Sun’s glare, to emerge in the morning by the time we see October
on the calendar.
Our overhead view, two hours after sunset at midmonth, finds that Vega, which was
at zenith at this time last month, is now high to the northwest. Deneb in Cygnus
is now to the northeast, and to the south is Altair. This triangle of bright stars
reminds us that we are close to the autumnal equinox, when the Sun crosses the equator
moving south and into the coming winter season. This year the Equinox falls at 5:18
pm on the 22nd. Hercules, which was nearly overhead last month is now
almost halfway to the western horizon. Low to the east-northeast, in an otherwise
unspectacular patch of sky, you may notice the faint glowing patch of the Andromeda
Galaxy, returning to the night skies.
There is a steady rhythm of changing constellations with the seasons which has gone
well noticed but largely unchanged since the beginning of human society. What has
changed is the timing and direction to the constellations. As Earth rotates daily
on its axis, we note that it also “precesses” slowly, with the direction to the
North Celestial Pole shifting through a giant circle 47 degrees in diameter every
26,000 years. Over one lifetime it’s not noticeable, but the “precession of the
equinoxes” was known to the ancients in Babylon!