January 2009

Welcome to the latest newsletter which, we hope, you will enjoy reading it.
There are currently wonderful, almost overhead, views of the upside-down Orion constellation. To the right of Orion, in line with the three stars forming the ‘belt’, is Sirius (in Canis Major), the brightest star in the skies. The bright star to the left of Orion is red Aldebaran, part of Taurus.

The Christmas social was an enjoyable, informal event. In addition to the tasty snacks and drinks brought by members who attended, everyone won chocolates at the musical quiz, which was kindly organized by Steve Kleyn.

The committee will be meeting soon, following the Christmas break. An update on progress will be included in the February newsletter.

Meetings will be held at 7 pm on Thursdays on the following days during the first half of the year:
29 January (AGM and How to read star maps (Steve Kleyn)
26 February
26 March
30 April
26 May
25 June

Meeting dates for the remainder or 2009 will be given in the February newsletter. Details of presenters at these, and later, meetings in the year will be circulated when they have been confirmed.

1: Close to home? A star, called S2, has been observed over its complete 15.8-year-long orbit around the Milky Way’s central black hole. The star approached the black hole to within one light day, which is only about five times the distance between the planet Neptune and the Sun,

Astronomers have crunched 16 years of data to make the most detailed observations yet of the stars orbiting the centre of our galaxy, bolstering the case that a monstrous black hole lurks there. The observations began in 1992, in Chile, at the European Southern Observatory’s (ESO) 3.5-metre New Technology Telescope. It was then pursued at ESO’s Very Large Telescope (VLT), an array of four 8-metre telescopes. A project called GRAVITY will combine light from the four telescopes at the VLT.

According to Frank Eisenhauer, principal investigator of the GRAVITY project, this should improve the accuracy of the observations by a factor of 100 and “has the potential to directly test Einstein’s general relativity in the presently unexplored region close to a black hole”.

2. The “Antikythera mechanism” discovery, reported in a past newsletter, has now been closely studied and deciphered and reveals a mechanism that almost equals a modern computer programme of the solar system’s complex motion,s although it was “invented” something like 1,000 years ago.

Enclosed in a wooden box, the user turned a handle on the side, winding backwards or forwards in time to see the positions of heavenly bodies at any chosen moment. On the front of the box, a large bronze dial with revolving pointers showed the relative position in the sky of the sun, moon and probably the five known planets, along with the date. A rotating black-and-white ball displayed the phase of the moon. Around the dial were inscriptions detailing the risings and settings of the stars at different times of the year. On the back were two spiral dials, each with an extendable pointer. Once the pointer reached the end of a spiral, it could be lifted by hand and reset to the beginning – like the stylus on a record player. The top dial showed a repeating 19-year calendar used to track the motions of the sun and moon. This timescale was chosen because 235 lunar months fit almost exactly into 19 solar years. The bottom dial was used to predict eclipses, and showed the 223 months of an 18-year cycle over which eclipse patterns repeat. Inscriptions marked the months in which to expect a lunar or solar eclipse, as well as its exact time and duration. Incredibly, all of this was achieved by intermeshing bronze gearwheels, which multiplied the speed of rotation by precise mathematical ratios depending on the number of teeth on each wheel. Turning the handle drove a “mean sun”, or date pointer, which revolved around the sky once per year. Three pairs of gearwheels then multiplied that speed of rotation by 235/19, to calculate the mean motion of the moon.

Beyond that, things got more complicated. For example, the moon’s speed as seen from Earth is not constant. The moon has an elliptical orbit, so it is sometimes closer to us (when it moves faster) and sometimes further away (when it slows down). The alignment of this ellipse rotates around Earth about once every 9 years. The idea of an ellipse would have been blasphemy to the ancient Greeks – they were convinced that celestial orbits, which they saw as divine, involved only perfect circles. Instead, in the second century BC the astronomer Hipparchus came up with a theory to account for the moon’s varying speed by superimposing one circular motion onto another with a different centre. The gears inside the Antikythera mechanism precisely model this theory. One gearwheel sits on top of another, but on a slightly different axis. A pin sticks up from the bottom wheel into a slot in the wheel above. As the bottom wheel turns it drives the top wheel round, but because the two wheels have different centres, the pin slides back and forth in the slot, causing the speed of the top wheel to vary, even though the speed of the bottom wheel is constant. This pin-and-slot mechanism was carried around on a much larger turntable, with one rotation equalling 9 years, to model the shifting axis of the moon’s orbit. This combined motion was then super-imposed onto the mean speed of the lunar pointer, so that it matched the speed of the actual moon. The gearing for the sun and planets is lost, but the Antikythera mechanism almost certainly modelled these too. The planets’ motions appear particularly erratic to us because they orbit the sun and not Earth. The Greeks accounted for this by superimposing small epicycles onto larger circular orbits. There is evidence that the Antikythera mechanism calculated these using what is still known today as epicyclic (or “planetary”) gearing – small wheels riding around on bigger wheels.

3. The James Webb Space Telescope, scheduled for launch in 2013, is already taking an incredible journey right here on Earth. The 18 mirror segments that will ultimately form the Webb telescope’s huge primary mirror will be trucked from pit stop to pit stop across the USA for careful processing and polishing. They will visit seven states, some several times.

Beryllium powder is poured into a big, flat can, heat and pressure applied, and the residual gas is removed to create a large slab called a mirror billet. Next, they split the billet in half Oreo-cookie-style to form two mirror blanks (no cream!). These mirror blanks are the largest ever produced in beryllium.
Workers in Alabama machine the back of each blank into a honeycomb structure to make the blanks lighter without reducing stiffness. The machined ribs are less than 1 millimeter thick. “This precision machining/etching removes 92 percent of a blank’s mass,” says Lee Feinberg of the Goddard Space Flight Center. “Mass is critical in launching space missions.” Next, a California company grinds and polishes the segments to a very smooth and exact shape and optically tests them at room temperature.

Because it is an infrared telescope, the JWST is designed to pick up the heat of faint, awesomely distant stars and galaxies. To do that it has to be kept extremely cold. It will operate in space at about -238 deg Celsius (-396 deg Fahrenheit, 35K). Super-cold testing is done in Alabama.

Once the mirror segments are polished to precision, gold is evaporated over them. All 18 segments finally meet at Goddard Space Flight Center. Here, they’re mounted on structures that will ultimately hold them in place and let them perform as if they were part of a single giant hexagonal mirror. Next the telescope is fully assembled and attached to the instrument module, and the whole unit is acoustic and vibration tested. Final cryogenic testing takes place at Johnson Space Center, in the same vacuum chamber that tested the Apollo lunar lander. Integrated with the spacecraft and sunshield at Northrop Grumman in California, it will lift-off from Kourou, French Guiana, on an Ariane 5 rocket.

4. Saturn surprise. Throughout 2008, the rings of Saturn were visible, but, on Christmas day, there were……no rings! As Saturn orbits the sun, once every 14 to 15 years) its rings turn edge-on to Earth. Because the rings are so thin, they can actually disappear when viewed through a backyard telescope. At present the opening angle is a paper-thin 0.8o. Viewed from the side, the normally wide and bright rings have become a shadowy line bisecting Saturn’s two hemispheres. Astronomers call the phenomenon a “ring plane crossing.”

As Galileo predicted, although not knowing why, after ‘losing’ Saturn’s rings shortly after discovering them, “They’ll be back,” However, the opening angle won’t be precisely 0o until Sept. 4, 2009, and Saturn will be so close to the sun, no one will be able to see the rings wink out. The best time to look is now. In January 2009, the rings will open up again slightly, a temporary reversal caused by the orbital motions of Earth and Saturn. The next ring plane crossing that’s easy to watch won’t come until 2038.

5. Black holes and galaxy size. Evidence suggests that black holes and galaxies grow together, with black holes enlarging by gobbling up material around them. Nearby galaxies all seem to follow a relationship that they have central bulges of stars – shaped like the yolk in a fried egg – about 700 times as massive as the gargantuan black holes at their hearts. The relationship holds for a wide range of galaxy sizes and ages.

However, four galaxies in the early universe have been found that violate this previously observed relationship between the mass of a galaxy and that of the colossal black hole at its centre. This suggests that supermassive black holes may have matured long before the galaxies that surround them instead of growing in lockstep with each other.

This month, our overview of the plants features beautiful Saturn.

Apart from, perhaps, a close up view of the moon, there is no more beautiful and awe-inspiring sight than the rings of Saturn, viewed through a telescope.

The second largest planet in the Solar System was named after the Roman go of farmers. It takes 29.46 Earth years to orbit the Sun (more than double that of Jupiter) and it has an axial tilt of 26.7 degrees. Hence, over a period of years, it will be seen from Earth either tilted with its northern pole towards Earth, edge-on (when you can barely see the rings at all,) or its southern pole towards us. Saturn is 95x larger than Earth and can fit 764 Earths inside it. Like Jupiter it is mainly made up of hydrogen and helium with an inner core of iron.

The famous ring system is huge, with a diameter from the nearest ring to the outer edge of the furthest ring is approximately 65,000 km’s. The ring structure can also be up to 30km’s deep in places. First discovered by Galileo, it is mainly dirty ice, some as big as boulders and some just grains of dust. The ring structure is in six bands. Innermost is the C ring, then the B and A rings (reflecting their order of discovery rather than their relative positions). The noticeable gap of much less dense particles between the A and B rings forms the Cassini Division. More recently, the Encke gap was identified towards the outer edge of the A ring. Other tiny almost gossamer rings have also been found at the outer edges of the system; the E, F and G rings. The D ring lies closest to the planet, inside the C ring.

Saturn’s 34 known moons include:
• Titan – the second largest moon in the Solar System has a diameter of 5,150 km and orbits 1.22 million kms from the planet. Its atmosphere has some similarities to Earth in its early days. In 2005 the Cassini spacecraft released its Huygens probe (named after Dutch Christiaan Huygens) through the atmosphere on to the planet. It found that there was a thick layer of methane rain falling on to the surface where it forms rivers and lakes and evaporates into thick clouds of smog.
• Rhea – (diameter 1,528 km)
• Iapetus – (diameter 1,436 km)
• Enceladus – (diameter 512 km).
• Mimas – has a bluish hue and a massive crater which takes up about a third of the entire planets land mass (diameter 418 km)
• Janus – (diameter 194 km).
• Prometheus – orbits inside the F rings (diameter 148km).
Reference: www.aerospaceguide.net
John Saunders (Chairman) 028 314 0543
Steve Kleyn (Technical Advisor) 028 312 2802
Pierre de Villiers (Treasurer) 028 313 0109
Irene Saunders (Secretary) 028 314 0543
Pierre Hugo 028 312 1639
Jenny Morris 071 350 5560

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