Mark McLellan outlines the star-gazing prospects for September and
looks into the mysterious rings of Saturn.
FROM where I was standing, the Perseid meteor shower developed into
nothing more than a drizzle, which was unfortunate considering the good
viewing conditions. Maybe the November Leonids will be more rewarding.
This month, expect good viewing conditions for stargazing on Monday when
the new moon occurs. The full moon will be on Monday September 19.
PLANETS THIS MONTH
Many of the gems which sparkled well through the summer are moving out
of view. Although the skies are getting darker again, Venus is now close
to the sun and will not be visible to us by the end of the month.
Jupiter will also be lost to view around the same time.
So what do we have to look forward to this winter in planet
observation? The two main planets which will dominate the sky over the
next few months will be Mars and Saturn. Mars will be in Cancer moving
into Leo and will approach evening viewing over the next few months.
Saturn is in Aquarius and reached opposition on Thursday. Don't be
alarmed to find a diminished ring system because every 15 years they
appear edge-on to our line of sight. Next year they will appear almost
invisible to us. More about Saturn later.
CONSTELLATIONS THIS MONTH
In mythology, Pegasus was the winged horse and it can be found as a
large star group in the southern sky. There are four main stars that
make up the Square of Pegasus. They are Scheat, Markab and Algenib, but
the fourth star Sirrah or Alpha Andromedae (see diagram) is now a member
of the neighbouring group Andromeda. There are few galaxies here but the
main attraction is the globular cluster M15.
The south-western sky is the home of Aquila (The Eagle) and you will
notice the bright star Altair, which shines relatively brightly at
magnitude 0.77. This star lies close to us in comparison with others and
is only 10 times more luminous than our own sun. Tarazed is just above
Altair and is red in colour. There are two open clusters NGC 6709 and
NGC 6755 which are relatively easy to observe. Any star guide will give
the locations.
SATURN'S FAMILY
Saturn has 18 known moons and this is the largest family a planet in
our solar system has. It was not until the Voyager probes encountered
the planet that more information was sent back. The rings were to reveal
orbiting satellites within them and it is likely that more moons are
lurking in that region. What would a view from one of those moons be
like? Hopefully the illustration will answer this.
Among the moons of Saturn are examples typical of our own moon and
others with remarkable features. Starting with the outermost moon and
working in toward Saturn they include the following:
* Phoebe has a very icy surface coated with dark material and lies at
a mean distance of 12,954,000 km from Saturn. It has similar features to
that of a comet and it is believed that Saturn may have grasped this
nomadic object with its gravitational tug. Scientists believe Phoebe is
the remains of such a Comet or even a stray asteroid.
* Iapetus is like Phoebe in blackness but one half is as white as
snow. It is 3,651,300 km from Saturn.
* Hyperion is an irregular chunk of rock which tumbles erratically
within the orbit of Saturn and has a mean distance of 1,481,100 km from
the planet.
* The most fascinating of all the moons has to be Titan, which has an
atmosphere of mainly nitrogen and is shrouded in orange cloud.
Scientists believe it may be similar to the Earth billions of years ago
with a surface of solids and liquids. This moon is the largest in the
family and it is also larger than Mercury and Pluto. So although it is
planet-sized it does orbit Saturn and not the Sun. It is therefore
classed as a moon. We may know more about this moon when the spacecraft
Cassini drops a probe into the cloudtops of Titan in the future.
* After Titan there is a significant gap in the satellite orbits. We
encounter Rhea which is the outermost of this group of 14 moons. These
are composed mainly of ice and are cratered like our own moon --
Enceladus is the only exception. It appears that a fresh layer of snow
falls regularly from volcanoes, leaving it smooth, unlike the others.
There are some remarkable features worth noting on some of the
satellites in the inner orbits. Mimas has a giant crater and looks like
a cross between an eyeball and The Death Star. This is the largest known
crater in the solar system. From what we know of these moons it is clear
that they have suffered continual batterings from other debris and it
may be that the rings of Saturn are the remains left by collisions which
happened billions of years ago. The extent of this disturbance may be
evident by two moons which orbit at the same distances from Saturn.
These are Janus and Epimetheus, which are irregular in shape. .
These are only a few of the satellites that orbit Saturn and there is
a lot more interest in this small area of our solar system. A visit to
your local library will fill in further information. As for observation,
we are fortunate that the next few years will reveal the moons rather
than the ring system. You will be likely to spot Iapetus, Rhea, Dione
and Tethys with small telescopes.
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