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FRIENDS OF THE PLANETARIUM NEWSLETTER – SEPTEMBER 2006
Thanks to those of you who have renewed your affiliation with Friends of the Planetarium.
Your receipt is enclosed with this newsletter if you haven’t already received it. Please
encourage your friends and colleagues to join. Remind them that their annual contribution is
tax deductible.
Spring has arrived, at least unofficially. New Zealand considers September 1 as being the
official first day. In actual fact there is no official “First Day of Spring” or for that matter, any
“Official” first day of any season! Astronomically, spring is defined as beginning at the
Vernal (Spring) Equinox and ending at the Summer Solstice. However, in popular parlance,
spring in the Southern hemisphere is comprised of the months of September, October and
November. When spring actually starts is a matter of the weather, and the buds and blossom
on the plants and trees.
The following lines about spring were written by Edward Fitzgerald (1809 - 1883).
Come, fill the Cup, and in the Fire of Spring
The Winter Garment of Repentance fling:
The Bird of Time has but a little way
To fly – and Lo! The Bird is on the Wing.
The Southern Hemisphere Spring Equinox is at 16 03 on September 23. Technically this is the
point on the celestial sphere where the celestial equator intersects the ecliptic. Put simply it is
when the Sun returns to the Southern Hemisphere, crossing the equator from the Northern
Hemisphere. At this time of year the length of the day increases by about 3 minutes per day,
which is the greatest increase for the year. However, because of the ellipticity of the orbit of
the Earth around the Sun, the Sunrise time is getting earlier at a faster rate (~2 minutes per
day), than is the Sunset time getting later (~1 minute per day).
In the night sky, Scorpius and Sagittarius are directly over our heads in the early evening. In
mythology, Scorpius was the scorpion whose sting killed Orion. In the sky, Orion still flees
from the Scorpion, for Orion sets below the horizon as Scorpius rises. The distinct curve of
stars that forms the tail and the stinger is known to the Maori as Te Matau o Maui, the Hook
of Maui, that was used to pull up the North Island out of the sea. For those of you with
binoculars or a small telescope, this area of the sky is rich with star clusters and nebulae. Just
below Shaula and Lesath, the two stars that make the Stinger, are M6 and M7, two lovely
open clusters that are easy binocular targets. Sagittarius looks like a teapot standing on its
handle, with the bottom of the teapot facing south. It is actually meant to resemble a centaur, a
mythical beast half man, half horse, with a bow and arrow raised and ready to fire. Sagittarius
was a friend of Orion so he is attempting to kill the scorpion with his arrow aimed at Antares,
the red supergiant star that represents the heart of the scorpion. On September 28, the Moon
will occult (pass in front of) Antares. Antares will disappear at 9:24 p.m. and reappear at
10:10 p.m.
Spring this year is not a very good season for planet viewing. Jupiter is the only planet easily
visible in the evening sky. It sits just west of Scorpius in the constellation of Libra. Binoculars
or a small telescope will show the four Galilean moons of Jupiter and, on a clear night, the
two dark equatorial cloud bands on the giant planet itself. Astronomers have been paying
close attention to these bands recently. You may be familiar with the great red spot, an intense
storm of high pressure more than twice the size of the Earth, which resides in one of the
bands. Late last year, a new spot formed which naturally enough was dubbed Red Junior.
These two have been approaching each other and it looked like they might merge into an even
larger spot. It seems now that they have passed by each other with a minimum of fuss.
The most important news regarding planets is the redefinition of the word planet by the
International Astronomical Union at its recent meeting in Prague. Up until now, a planet had
to satisfy only two conditions: it had to orbit a star and it had to be big enough that its own
gravity pulled it into a spherical shape. The IAU added a third qualification: the object must
have cleared out its own unique orbital path. This means that most asteroids could never
qualify as planets because many of them (1000’s in fact) share the same orbital path. It also
means that Pluto can no longer be considered a planet as its orbit crosses the orbit of Neptune,
albeit at a highly inclined angle. A new category of object called the dwarf planet was
introduced in which to place Pluto and a few others At the present time we now have eight
“classical” planets and three “dwarf” planets, these being Ceres, Pluto and 2003 UB313.
Ceres is the largest member of the asteroid belt between Mars and Jupiter and was often
referred to previously as a minor planet, a title that no longer exists. We hope that the IAU
will officially name 2003UB313 as Xena, now that it has dwarf planet status. Many other
objects between Pluto and Xena will be added to the list of dwarf planets in the months and
years to come but for now we have eight planets and three dwarf planets.
Meanwhile at Saturn, the Cassini spacecraft continues to send back dazzling new images of
Saturn and its moons. It appears to have taken images of numerous lakes containing liquid
methane or ethane on Titan, Saturn’s largest moon. The lakes were identified using Cassini’s
radar, which maps surface roughness. A rocky surface appears bright, a smooth surface
appears dark, and a featureless surface of a still liquid appears pitch black. Cassini team
members speculate that the lakes may be the source of the methane smog that engulfs the
large moon. The lakes are all located close to the poles of Titan and may answer questions
about the weather on Titan. The weather is certainly not very appealing: a perpetual thin
methane drizzle with scattered episodes of torrential downpour. Don’t ever complain again
about our weather!
This newsletter started with a discussion of spring and we will finish it with more news of
spring. Spring has sprung, literally, on the surface of Mars. Every spring brings violent
eruptions to the south polar ice cap of Mars. Jets of carbon dioxide gas erupting from the ice
cap as it warms carry dark sand and dust high aloft. The dark material falls back to the
surface, creating dark patches on the ice cap, which have long puzzled scientists. Deducing
the eruptions of carbon dioxide gas from under the warming ice cap solves the riddle of the
spots. It also reveals that this part of Mars is much more dynamically active than had been
expected for any part of the planet.
Enjoy the fine spring weather, everyone, and be grateful that we are not on Titan or Mars!