October 2003 Sky from the Keeble Observatory
I know that we've all had enough of hurricanes and tornadoes. But, the recent demise
of Galileo, intentionally crashed into Jupiter on the 21st of September to ensure
that it would not contaminate Europa, reminded me that wind systems seem the norm
for all planets with atmospheres.
If Earth did not rotate, the wind patterns would be simple. Hot air would rise at
the sub solar point, and be replaced by cooler air coming in from the cool shaded
side of the planet. The sub solar point would travel around the tropics once a year,
so the equatorial regions would be generally warmer than the poles. The overall
pattern would have surface winds blowing from pole to equator, with upper altitude
winds blowing the other way. We actually find this Hadley circulation near the surface
on slowly rotating Venus, which has a dense carbon-dioxide atmosphere.
For our atmosphere, the rotation of the Earth once each day gives rise to an additional
forcing term, called the Coriolis effect, which causes moving parcels of air to
turn to the right in the northern hemisphere, and to the left south of the equator.
This gives rise to the characteristic clockwise circulation around northern high-pressure
systems, and counterclockwise around the lows (the H an L that you see on weather
maps). Coriolis forces also break the Hadley circulation into equatorial and polar
belts where the surface winds blow from the east, and mid-latitudes where the prevailing
winds are from the west. The complex interplay of prevailing winds and circulating
highs and lows give rise to much of the weather we have come recently to respect.
Next month I'll say more about Earth's weather, and how it compares with that on
the other planets in our solar system.
Lunar phases for 2003 October: First Quarter on the 2nd, at 3:10 pm; Full on the
10th, at 3:29 am; Last Quarter on the 18th, at 8:32 am; New Moon on the 25th, at
8:51 am. All of these times are Eastern Daylight Time. We return to Standard Time
(fall back) on the 26th - to celebrate, the Moon gives us another First Quarter
on the 31st, this one at 11:26 pm EST.
Mercury is a morning star at the beginning of the month, but moves back into the
glare of the Sun by mid-month. At sunset, you may notice a bright star low to the
southwest; that's Venus. Mars remains a bright red presence to the south, not as
spectacular as early September, but still impressive. Jupiter and Saturn rise late
(Saturn about an hour after midnight, Jupiter several hours later) and are better
Cygnus is almost directly overhead about two hours after sunset, marking the plane
of the Milky Way, which bisects the sky from northeast to southwest. The bright
star alpha Cygni (Deneb) is closest to zenith, with Albireo to the southwest. Vega
toward the west, and Altair to the south mark with Deneb the Winter Triangle. Sweeping
binoculars from Vega towards Arcturus to the west-northwest will take you through
the constellation Hercules. On a clear, moonless night you may notice the diffuse
globular cluster in that constellation. Globular clusters are compact groups of
up to a million old stars, collectively orbiting the Galaxy. A similar binocular
sweep from Deneb toward the east-northeast will take you through the constellation
Andromeda and allow you to view the Andromeda Galaxy. This object is not part of
our Galaxy, rather is part of the Local Group; it lies some two million light years
distant. Ursa Major is low to the north-northwest, probably too low on the horizon
to see through the haze and ground clutter. From Deneb to the southeast, your view
will take you out of the plane of the Galaxy, into regions sparsely populated by
nearby stars. These constellations seem widely spaced and empty when compared to
the rich star fields along the plane of the Milky Way.
For your own monthly star chart, you can direct your web browser to
http://www.skymaps.com. You will find extensive descriptions of what's worth
looking for, and you can download and print a single copy for your personal use.