Saturn: Picture Gallery

Saturn is perhaps the most dramatic planet in the solar system, with its extensive system of rings giving it a truly splendid appearance. Its surface features are quite muted however, meaning that for amateur astronomers the rings and the moons give the greatest interest.

The Rings

The rings of Saturn lie at an angle to its orbit but maintain a fixed orientation in space as Saturn goes round the Sun (in 29.5yrs). This means that, as seen from the Earth, the tilt of the rings will vary at each nearest approach. In 2003/4, when I started observations, the rings were almost "fully open", giving a good view of the south pole of the planet. The rings then began to slowly close up so that in 2009/10 they were directly in line with the Earth, and thus effectively invisible. They then opened up again, but with the north pole on show this time: the maximum tilt will be in 2018. The sequence will then repeat in reverse, so the south pole will once more be on show in 2033/4.

This animation shows the aspect of the rings at each opposition (nearest approach) between 2004 and 2016, as they close up and then begin to open out again. The situation in 2017 & 2018 was very similar to that in 2016, but as Saturn was by then very low in the sky at opposition it was difficult to obtain usable images.

The period around Saturn's nearest approach in 2003/4 was just about optimal for observation because of the coincidence of several favourable factors - a) the rings were fully-open, giving an excellent opportunity to see any bands on the planet and also to observe the shadow cast on the rings by the globe of Saturn (and vice versa); b) Saturn was only just past perihelion (its closest distance to the Sun) and the Earth was at aphelion (its furthest distance from the Sun, and thus closest to Saturn), meaning that Saturn had its greatest possible visual size (21 arc-seconds); c) because opposition was in the winter, the ecliptic (the apparent path of the Sun across the heavens, near to which the planets are always to be found) was high in the sky at midnight, giving Saturn a considerable altitude (60deg) thus lifting it above most atmospheric disturbance.

As the years passed, Saturn's increasing distance from the Earth slowly reduced its size which, together with the closure of the rings, decreased its brightness to a minimum in 2010: even though the rings then opened out again the distance factor meant the overall brightness, though increasing, was still down on its value in 2003. Also, as the date of opposition slowly moved towards the summer (successive oppositions happen at intervals of 1yr 13days, on average) the elevation of the ecliptic decreased, reducing Saturn's altitude considerably - it was down to 17deg by 2016, leading to worsening atmospheric conditions: this made the obtaining of sharp, detailed, images increasingly difficult. Things will continue to get worse until the fully-open position (with the north pole in view) with Saturn at aphelion and the Earth at perihelion in summer 2018, but will then begin to improve as Saturn gets closer to the Earth and higher in the sky again: the next fully-open position with the south pole on view (in 2033/4) will in fact be almost as good as in 2003/4. This is because 28 opposition periods is not only almost exactly equal to 29yrs but is also very close to one orbital period of Saturn i.e. 28 oppositions after the one in 2003 the calendar date of the opposition, and hence the elevation of the ecliptic, will be back to where it started; the Earth will be at aphelion again, and Saturn will be (almost) at the same position relative to its perihelion. [Note that this numerical relationship is not a coincidence but rather a simple mathematical consequence of the duration of the interval between successive oppositions of any planet further from the Sun than the Earth. To find out why, click here.]

	In the upper image we see Saturn and its rings in late 2003, looking at the the south pole of the planet and thus with a viewpoint from under the rings. A faint equatorial band is clearly visible, with perhaps a slight darkening of the polar region also. The dark area on the lowest sector of the rings is the shadow of the globe of the planet and the dark rim at the bottom of the upper ring sector is the ring's shadow on the planet. The rings themselves have a darker outer ring and a lighter inner one: this is a real effect, not something that the imaging has thrown up. It is, however, not possible to see Cassini's Division between the outer A ring and the inner B ring as it is narrower than the resolving power of the telescope/camera combination.
	The second image, taken in 2008, shows more of the entire globe of the planet as the rings close up. The somewhat purple shading of the northern hemisphere is a true colour difference, thought to be caused by changes in the chemical composition of Saturn's atmosphere as this area receives increasing amounts of sunlight.
	In the third image, taken just a year later (early in 2009), the rings are almost "edge-on" to Earth so the entire globe can be seen - the purple shading is now very obvious. The rings appear to have shrunk considerably: this is because only the brighter (inner) rings can be seen at this angle. Their shadow across the planet is still clear, however.
	In the final image, from 2011, the rings are noticeably opened up again (but in the opposite sense), revealing more detail of the northern hemisphere. A dark band is now visible, dividing the blue/green polar regions from the redder equatorial belt. Confusingly, the band looks just like the lower shading which is, in fact, the rings crossing Saturn's globe!

	In late 2010 a large "storm" was observed in the northern hemisphere which lasted a surprisingly long time. It was still there when I was taking images in April 2011 and can be seen in this slightly enhanced close-up view. Such storms are thought to consist of clouds of ammonia crystals, triggered by "spring" arriving as the northern hemisphere begins to tilt towards the Sun.

Saturn's moons

Of the 48 named moons of Saturn, four are visible with a moderate telescope. Titan, easily seen in even a small instrument, is actually larger than the planet Mercury and has a considerable atmosphere: the European Space Agency's Cassini-Huygens spacecraft landed there in January 2005 and returned spectacular views of the terrain and land surface (shown at left). Rhea, Tethys & Dione are all markedly dimmer than Titan but will usually be visible with my reflector. Because Saturn's axis of rotation is inclined to its orbit (by about the same amount as the Earth), the moons are not always in a straight line, like those of Jupiter, but instead appear to move in ellipses around the planet. This makes determining their orbits somewhat more of a challenge, but one I decided to accept! Things do get a little complicated though so I've put details of my project to plot their orbits on a separate page. Click here to read all about it.

The direction of rotation of the moons around Saturn (clockwise in these views, because the south pole is tilted towards us) is the same as Saturn itself rotates and so the orbits are deemed "direct". If they had gone round in the opposite sense (as some very small moons do), it would be called "retrograde". As it is almost impossible for a moon created at the same time as its mother planet to orbit in the opposite direction to the planet's own rotation, the presumption is that "retrograde" moons have been captured by the planet's gravity at a later date.

Here we have two images of Saturn and its major moons, taken 13days apart. Saturn itself is clearly considerably overexposed, in order to capture the much fainter moons. Titan, the brightest moon in each image, takes almost 16days to go round Saturn so has only completed just over 3/4 of an orbit. The other three are much faster, so have all completed several orbits. Tethys moves so fast you can clearly see the difference over one night's observing! Click on any moon to get further details.

And now a movie to give a better idea of how the moons orbit Saturn than can be shown by a series of "stills". This is Titan, in a sequence derived from observations on several different nights, circling its mother planet in 16days.

At the 2005 opposition several factors came together to make it possible to see a fifth moon, Iapetus. Iapetus is a strange moon as one half of it is much darker than the other half. The reason for this is not entirely clear currently but it does mean that when the "light" side is facing Earth it is much brighter than when the "dark" side is. Luckily, in mid-January 2005 the light side was facing us. Secondly, although Iapetus is actually twice as far from Saturn as Titan, because its orbital plane is at quite an angle to those of the other major moons it appears to pass very close to Titan at the points where the orbits seem to come together. Remarkably, at the exact date in January when Saturn was at its closest to Earth and so everything was at its brightest and thus most likely to be seen, not only was Iapetus near to one of these points, but so was Titan. Titan could thus be used to help identify Iapetus among the noise & "digital clutter" on the webcam images.

On 12th January, Iapetus is seen above Titan to the left of the image. Rhea, Dione & Tethys are much closer to Saturn. Click on any moon to get further details.	By the 13th, Titan has just overtaken Iapetus while the nearer moons have completed significant portions of their much faster orbits.	A composite of the two previous shots.