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Weekend: Close Encounter With Saturn's Largest Moon ; The Night Sky for October From the Birmingham Astronomical Society

Posted on: Saturday, 25 September 2004, 06:00 CDT

Two very different moons will be under the spotlight towards the end of this month. Our own will be the focus of attention on the 28th with a total lunar eclipse in the early hours of the morning. Around the same time but about 900 million miles away, the Cassini spacecraft will make its first flyby of Titan, the largest of Saturn's 31 satellites.

At over three thousand miles in diameter, Titan is the second largest moon in the Solar System. (The biggest is Ganymede, one of Jupiter's inner moons.) Titan is also the only one to possess a substantial atmosphere, swathed as it is in a thick, impenetrable cloud of dense gases. Previous Infrared and Radio surveys have allowed only tantalising glimpses of exposed icy bedrock and low valleys containing possible methane lakes.

Cassini's close encounter this month should provide the best images since the Voyager missions in the eighties. Yet more information is expected early next year when the probe, Huygens, is due to land on the surface.

Observing moons can tell us a lot about their planets. Discovering how diverse a world can be increases our knowledge of all the various processes that make each one so different.

Understanding the origins of our own Moon helps us to determine the turbulent beginnings of our own planet and its subsequent history. Current thinking describes a Mars-sized object colliding with a young Earth around 4 billion years ago. Chunks of material were blasted away into space but captured in orbit around the resulting combined and reformed planet Earth. These pieces of planetary debris gradually grouped together under gravitational influence to form the Moon as we know it today. Samples obtained from the Apollo Lunar Missions contained similar composite rock formations to those found on Earth. This was crucial evidence for the theory that both Earth and Moon had evolved from a common source.

Make an early start on the morning of the 28th. If you have a clear view to the south-west, you will see our own lunar companion slip under Earth's shadow. A total lunar eclipse like this occurs when the Sun, Earth and Moon are lined up in such a way that the Moon's ability to reflect sunlight is blocked by the Earth's shadow. Totality begins at 3:23 am and will last for about 1 hour 20 minutes. (It is likely to be quite a bright eclipse, however, due to the distortion of sunlight through our relatively polluted atmosphere.) Switching to our planetary neighbours, successful observing this month will mean setting your alarm clocks!

Mercury is poorly placed for observation and is in conjunction with the Sun on the 5th. Mars was in solar conjunction last month and so is also not visible Shining Venus is still a bright morning object in the east and will be 4 degrees south of a waning crescent Moon on the 10th.

Jupiter is also fairly elusive just now, being very low in the sky. You may catch a glimpse on the 12th as it will be less than 2 degrees south of a still crescent Moon. Look over to the eastern horizon about an hour before sunrise.

Saturn rises just after midnight by midmonth and will be 5 degrees south of the Moon on the 7th. With its rings well open it will make a fine sight in a telescope. Close by lie the heavenly twins, Castor and Pollux in Gemini. Uranus and Neptune are the only planets observable in the early evening October sky. Find them over in the south/south-east in the constellations of Aquarius for Uranus and Capricornus for Neptune.

Pluto lies low in the south-west as darkness falls and so will be difficult to see.

The Orionids are the main meteor shower this month. Made up of debris from Halley's comet, observers may see up to a dozen of these meteors in an hour given good 'seeing' conditions. Peak period will be on the morning of the 21st so look over towards Orion after midnight on the 20th and see how many you can spot.

Constellation of the month is Pegasus, the winged horse of Greek mythology. It rides high in the southeast at around 10 pm.

Its main feature is a large square of second magnitude stars aptly called 'The Great Square of Pegasus'. Ironically, of the four stars at each corner, only three belong to Pegasus. The brightest of the four (top left) is actually part of the Andromeda constellation.

The star at the top right of the square, Scheat, is a red giant that varies slightly in brightness and is classed as an irregular semi-variable.

The Square itself contains few stars but the constellation overall boasts a large number of deep sky objects. Notably, clusters of galaxies. Unfortunately, they are too faint to be seen with amateur equipment.

However, the bright globular cluster, M15, can easily be resolved through an average telescope. M15 is notable for containing a high number of variable stars and pulsars in its densely packed central region.

Pegasus is also home to the first planet to be discovered outside of our Solar System. It was found circling a faint star 50 light years away. The planet has about half the mass of Jupiter and takes just 4.2 days to orbit the star.

While browsing Pegasus, try to pick out M31, the Andromeda Galaxy in the near-neighbour constellation of Andromeda. It should be visible to the naked eye as a fuzzy patch on a clear night. Using a star-map, find Cassiopeia, the 'W' shaped constellation. Then sweep downwards from the bottom right-hand star of the 'W' towards Pegasus. Careful observation of this area should allow you to pinpoint our sister galaxy ; a mere 2. 3 million light years away!

Over to the east is Perseus. Scan its length to see myriads of beautiful star fields. Following on is bright Capella, lead star in Auriga but actually consisting of a pair of yellow giants 42 light years away.

If you can stay up until after midnight, you will see Orion and the Pleiades appear over the horizon. A sign that winter's constellations are on the march.

Check out our website at: www.birmingham-astronomy.co.uk

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