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 the Sun.
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
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
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 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.