As the year draws towards its close, so the splendid arch of the Milky Way—such a feature of the winter sky—lies ever lower across the horizon, carrying with it those stunning constellations Scorpius and Sagittarius.
For city dwellers, the sky overhead begins to seem almost empty. This visual desert is in part due to light pollution which brightens the sky and masks fainter stars. A recent calculation shows that almost any New Zealand town contributes more light to the night sky than did the whole of Tudor England!
But contributing to the sparseness of the stellar scene is the fact that the Galactic South Pole is now high in the sky, and our view is towards the depths of intergalactic space and away from the plane of our galaxy, where most stars are concentrated.
Three other galaxies can be seen with the naked eye. To the south are the two Magellanic Clouds, which look like small luminous clouds. They are small, irregular galaxies so closely associated with our own that they are being distorted and destroyed by our high-powered gravitational field.
Low in the northern sky lies the great Nebula in Andromeda, M31. Rather more than 22 million light years away, it appears as a ghostly elongated ellipse. Given its low surface brightness, M31 is best seen with low-magnification night glasses. Only photography can reveal anything more than a hint of the great disk of stars surrounding the centre.
All the other galaxies, including the two great concentrations of Fornax and Sculptor, need something larger than a small telescope to be seen. Such a telescope—say a Newtonian reflector of 150-200 mm aperture—is within the ability of any teenager to build if he or she buys the optical parts from a local enthusiast. Sir William Herschel, working with hand tools in the late 1700s, made a series of such telescopes with which he was to discover not only Uranus but hundreds of star clusters and some 2500 “nebulae,” many of which were galaxies.
As spring gives way to summer, the planet Saturn dominates the sky. Jupiter, Venus and Mercury, which have been such an eye-catching group in the evening sky, are now setting in the bright skyglow of sunset, and Mars is still an after-midnight object.
Saturn, first observed by Galileo with a small refractor of stunningly poor quality, immediately posed a problem. Whereas Galileo was able to see that Jupiter and Mars were spherical and thus akin to the Sun and Moon, Saturn looked different. Sometimes it appeared to him to have a large moon almost touching either side, and at other times to have “ears” attached. Over several years, he noticed that the “ears” diminished and finally disappeared.
Not until 1655 did Christiaan Huygens, with a much superior telescope, discover that the ears were rings. He also found Titan, Saturn’s largest moon. Observing that the orbit of Titan is inclined to the plane of the ecliptic, Huygens argued that the planet’s equator and the rings were similarly inclined. At times we see the rings edge on, when they appear invisible (being only tens of metres thick, though 74,000 km wide), while from oblique angles they appear massive. At present, the rings are presented to us at a modest and decreasing angle as we sweep towards our interception with the plane, so are near invisible.
Saturn, like Jupiter, is a gas giant—predominantly a ball of hydrogen and helium. The only solid material is a small core of water, ammonia and methane ices mixed with silicates and metallic oxides. Saturn is unique among the planets in that it is less dense than water.
Saturn holds pride of place for the number of its satellites-21 at the last count. The smallest are so tiny and irregular that the term “moon” is inappropriate, but the largest, Titan, with a radius of 2575 km, is 800 km larger than our moon. While our moon lacks an atmosphere, Titan is shrouded in red-brown clouds in an atmosphere of nitrogen and a small amount of methane.
Saturn itself presented the Voyager mission of 1980 with an embarrassment of information. Like Jupiter, the planet showed coloured bands at various latitudes, though the differences are subtle. These bands appear to be related to, but do not exactly correspond with, circumplanetary wind belts which blow with terrible ferocity. Between adjacent belts, relative wind speeds may be up to 500 metres per second. On earth the fiercest high altitude jet streams blow at 100 m/s, and are relatively short-lived events. Just how Saturnine meteorology works, what its energy sources are, and why the system is so stable are unknown.
A further puzzle is Saturn’s very strong magnetic field, more than a thousand times that of Earth’s and aligned with the planet’s spin axis—making the magnetic and geographical poles co-incident. Accepted theory requires the magnetic axis to be at a different angle from the axis of rotation. Somehow Saturn is breaking this “rule.”
Following the observation of the rings by Huygens, there was a popular theory that they were solid. One cleric even went so far as to calculate their probable population, taking both sides into consideration but overlooking the fact that the gravitational pull is towards the centre of the planet. Life on the north face of the Eiger!
In 1857, J. C. Maxwell demonstrated that the rings must be composed of small individual particles.
That the rings are not uniform has long been known. The Outer ring, which is separated from the prominent Bright ring by Cassini’s division, is visible in any half-decent telescope. Larger telescopes show that the Outer or A ring is divided into two by a dark gap, Encke’s division, and that inside the Bright or B ring is the faint Crepe ring which merges into the very faint D ring.
The Voyager missions not only showed these rings in breathtaking detail but revealed the presence of faint new rings. The classical rings proved to be mystifyingly complex, composed of thousands of mini-rings containing braids, spirals and even traces of radial structure.
Early next century the Cassini mission will arrive at Saturn. After delivering the probe vehicle Huygens to the surface of Titan, Cassini will go into orbit around Saturn to give us a detailed inspection of this loveliest of giants, and perhaps provide answers to the many questions we still have about this comparative neighbour.