November 2003 Sky from the Keeble Observatory
Earth's atmosphere is unique among the planets of our solar system.
Only on Earth do we find this much oxygen in the air, the product
of living green plants. Only on Earth do temperatures remain where
water can be found in all three phases . solid, liquid, vapor. Yet,
Earth's atmosphere is driven by the same underlying physical principles
which apply to other planets' atmospheres.
Our atmosphere is heated by the Sun. In the lowest part of the
atmosphere, a layer called the troposphere, sunlight warms the surface,
which then warms the air. In the tropics the sunlight strikes the
ground from high overhead, while as we move towards the poles, the
light arrives at a more oblique angle. The result is that the equatorial
regions are warmer than the poles. As we discussed last month, this
differential heating drives a global circulation of air which is
affected by the planet's rotation. We find three broad zones of
prevailing surface winds in northern and southern hemispheres .
blowing from the east near the equator and poles, from the west
in the middle latitudes.
Venus rotates very slowly, taking 243 of our days to turn once
on its axis. Unaffected by Coriolis forces, Venus' surface winds
blow from pole to equator. On gas giants Jupiter and Saturn, which
rotate in under 10 hours, the pole to equator circulation is broken
into many belts and zones of counter-circulating winds.
On Earth, as air rises from the warm surface, it is replaced by
air from the surrounding parcels. This region of inflowing air is
known as a low pressure center (marked by a big L on weather maps!).
The Coriolis effect causes the inflowing air to spiral counter-clockwise
around a low in the northern hemisphere, clockwise in the southern
hemisphere. This is one reason that hurricanes only rarely cross
the equator, i.e. the Coriolis forces would disrupt their overall
circulation. Another effect of the rising air is that it carries
water vapor into cooler air above the surface, where it condenses
to form clouds, which ultimately lead to precipitation.
Cool air aloft can descend to the surface, where it pushes the
surface air outward. The region of outflowing air is called a high
pressure center (H on the weather map). These winds spiral clockwise
in the north, counterclockwise in the south. The descending air
warms, so clouds tend not to form.
Both highs and lows are carried on the prevailing winds to bring
our varied weather patterns. A quick look at an image of Jupiter
reveals similar circulation patterns, with cyclones and anticyclones
being carried around the planet by swift wind patterns. Prominent
in the southern hemisphere on Jupiter is the great Red Spot, a high
pressure system which has persisted at least since the early 17th
century when it was first observed by Galileo.
We have to be a little careful in making analogies, however. Most
of the weather we observe on Jupiter is driven not by sunlight,
but by heat escaping from the interior of this massive planet. Also,
the composition of the air is very different. Our air is about
80% nitrogen, and 20% oxygen. Jupiter's atmosphere is about 70%
hydrogen and 30% helium, almost identical to the composition of
Lunar phases for November: Full Moon on the 8th at 7:14 pm; Last
Quarter on the 16th at 11:16 pm; New Moon at 6:00 pm on the 23rd;
First Quarter on the 30th at 1:17 pm.
This month's Full Moon will be accompanied by a total lunar eclipse!
The Moon moves into the penumbra (outer portion) of Earth's shadow
shortly after sunset. The umbral phase, where the Moon is in the
dark central part of the shadow begins a little after 6:00 pm. Totality,
lasting 11 minutes, will occur at about 8:20. The New Moon will
be the occasion for a total solar eclipse, however it will be visible
only from Antarctica.
Mars is high in the southeastern sky at sunset, and remains visible
until after midnight. Saturn rises at 9:00 pm at the beginning of
the month, and a bit earlier each evening, so that by month's end
it rises at about 7:00 pm. Jupiter rises at 2:00 am, so it's a better
pre-dawn target, though it too rises earlier each day. By the first
of December it will rise at midnight. Venus is visible to the southwest
just after sunset.
Looking overhead at mid-month, about two hours after sunset you
will find the Milky Way dividing the sky from northeast to southwest.
Deneb and Vega are the two bright stars to the west of zenith, with
Altair to the southwest marking out a rich triangle. Near Vega,
a clear night and steady hand with binoculars may enable you to
see the Ring Nebula. This is an example of what will happen to our
own Sun in another five billion years . as the fuel in the core
is used up, the outer layers will eventually be expelled into an
expanding planetary nebula. To the east of zenith, look for the
Andromeda Galaxy, a faint smudge to the naked eye. This is actually
a galaxy much like our own, over 100 billion stars yet dimmed by
its vast distance of some 2 million light years. Even though it
is the most distant object visible without a telescope, it is close
by on the cosmic scale of things.
Deneb marks the tail of the constellation Cygnus. The head
is a faint binary called Alberio, which is one of the most beautiful
pairs you'll see. With a small telescope, one star glows a brilliant
blue while the other is bright orange. Cygnus lies in the general
direction of the Sun's orbit around the center of the galaxy, which
lies in the direction of Sagittarius, which is just setting on the
southwest horizon. Just rising to the east is bright Aldebaran,
in the constellation Taurus. This is a harbinger of coming winter.
By month's end we will see Orion rising at this hour.
For your own monthly star chart, you can direct your web browser
You will find extensive descriptions of what's worth looking for,
and you can download and print a single copy for your personal use.