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Saturn Through the Telescope

by Martin J Powell

Saturn is arguably the most beautiful of all the planets to view through a telescope; indeed, many say it is the most beautiful telescopic object in the entire night sky. It is certainly one of the first objects that beginner astronomers turn their telescopes upon.

Saturn as it appears through the eyepiece of a small telescope (Copyright Martin J Powell, 2011)

 

A Telescopic View of Saturn obtained by pointing a video-camera through the eyepiece of an 8-inch reflecting telescope in January 2004, some two weeks after the planet's opposition at the end of 2003. The rippling effect simulates how the Earth's turbulent atmosphere (the 'seeing conditions') affects the clarity and steadiness of the telescopic image.

Even small telescopes at 50x magnification will reveal the planet's impressive ring system - the finest of all the outer gas giants - although larger telescopes are required to see the ring's individual regions. The rings are not always on view, however; about every 14 years the Earth passes through the planet's ring plane, causing them to almost disappear from view (for more details, see the Saturn orbit diagram).

The rings are classed into three main regions: A (the outermost), B (the brightest) and C (innermost). Ring B is the most easily seen, the inner region being slightly darker than the outer region. Separating rings A and B is the Cassini Division, which can be seen in 75mm (3 in) telescopes when seeing conditions are good. Within ring A, the much fainter Encke's Division typically needs larger instruments to resolve. Ring C (also known as the Crpe Ring) is faint and difficult to see, requiring large telescopes. Several other rings were discovered by the Pioneer, Voyager and Cassini spacecraft; these rings are, of course, too faint to be detected from the Earth.

Like Jupiter, the darker bands running across Saturn's globe are known as belts and the lighter bands are referred to as zones. Saturn's globe is relatively featureless, with only a couple of low-contrast cloud belts visible for much of the time; the absence of distinct features is partly due to a layer of haze in the planet's upper atmosphere obscuring the view of the weather patterns beneath. Observing the planet in twilight (hence reducing the glare against the background) - or using blue filters - can sometimes help to enhance the planet's cloud features.

White spots or white ovals occasionally appear on Saturn and whenever they do, they are usually short-lived phenomena, lasting from only a few days to a few months. A prominent white spot appeared in 1933 (discovered by British comic actor and amateur astronomer Will Hay) and another in late 2010 (discovered by Australian amateur Anthony Wesley) became the most widely-observed spot in Saturn's observational history.

There is some evidence to suggest that, for any particular region of the planet, Saturn's Great White Spots (GSWs) re-appear about every 57 years (just under two Saturnian years). GSWs were recorded in Saturn's Equatorial Zone (EZ) in 1876, 1933 and 1990. In the Northern Tropical Zone (NTropZ) GSWs were sighted in 1903 and 1960. The 2010 GSW also seems to have had a precursor - in the same region of the planet - in 1953. Thus we might expect to see a GSW appear in Saturn's Northern hemisphere in 2017, i.e. 57 years after the 1960 event.

Whenever white spots appear, many amateurs attempt to determine their rotation period around the planet. In order to do this, the time at which the spot crosses the planet's central meridian is noted on several occasions (for more details on how a planet's central meridian longitude is defined, see the page on Mars Through the Telescope). The larger spots are usually observed over a prolonged period, during which time they typically spread out in longitude, eventually encircling the planet before fading out of view.

The view of Saturn through the telescope (above) reveals what a typical small-to-medium-sized telescope is likely to see under average observing conditions. The image was recorded through an astronomical telescope, so the image is inverted (South-up when viewed from the Northern hemisphere). Hence in this view from early 2004, we see the planet's Southern hemisphere tipped in our direction (i.e. the planet's South Pole is at the top). The rings are almost fully presented (the ring opening angle or tilt is about -26) and since it is soon after opposition, the shadow of the globe on the rings is cast directly behind the planet (this is in contrast to the situation at eastern or western quadrature, when the shadow appears slightly to one side, e.g. see the section on the superior planets' movements in the night sky). A faint, slightly darker cloud belt (the Southern Equatorial Belt or SEB) can just be discerned crossing the globe. Ring A is clearly darker than B and they are easily separable, however the seeing conditions are too poor to discern the Cassini Division between them. The innermost region of Ring B appears darker as it approaches the Crpe Ring (Ring C) which in this case is barely discernible.

Since the 1950s, an interesting effect called the opposition effect or the opposition surge has been identified among numerous Solar System bodies. In the case of Saturn, it is sometimes referred to as the Seeliger effect (after German astronomer Hugo von Seeliger, who first noted it in the nineteenth century). The Seeliger effect is a sudden brightening of the planet's rings in the hours around the moment of opposition, i.e. when the planet is directly opposite the Sun in the Earth's sky. Naturally, the effect is most marked when the planet's rings are wide open to view. The Seeliger effect is not normally accounted for in Solar System modelling (e.g. when calculating a planet's opposition magnitude) because it is somewhat ephemeral and is not always observed. It is believed to be caused by the "coherent backscatter of light from the icy particles in the ring system as the phase angle approaches zero".

Several moons of Saturn can be seen through telescopes, the brightest of them being Titan - for more details, see the current Position of Saturn in the Night Sky page.

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Copyright  Martin J Powell  2011


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