Saturn: Picture Gallery

Atmospheric colours Opposition surge

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 will be directly in line with the Earth, and thus effectively invisible, before opening up again to expose the north pole in 2018. The sequence then repeats in reverse, so the south pole will once more be on show in 2033.

This movie shows the change in the rings between 2004 and 2009, as they close up. Whether I shall be observing long enough to capture the full cycle remains to be seen!

The favourable orientation of the rings in 2004 gave 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).

In the upper image we see Saturn and its rings, 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 camera.

The middle 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 bottom 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.

This montage shows the effect on the ring-shadow features of the movement of the Earth in its orbit. Saturn was at its nearest to Earth ("at opposition") on 14th January 2005. At this moment the Sun, the Earth and Saturn are in a straight line, so the shadow of the globe of Saturn on its rings will be directly away from the Earth. When seen from a viewpoint slightly below the plane of the rings, the shadow will thus appear equally on the two sides of the planet: this can be seen in the third image. In the first two, taken before opposition (i.e. before the Earth has quite caught up with Saturn), the maximum shadow is on the right. On the bottom, taken after opposition (i.e. after the Earth has overtaken Saturn), it is on the left. Those with a geometrical mind will see that this proves that, relative to a north pole pointing "upwards", the Earth rotates around the sun in an anti-clockwise direction!

Two other effects can also be seen. Firstly, the shadow does not get much greater in the month between March & April: compare this with the months between January & February and February & March. This is because by this time the Earth has travelled nearly a quarter of the way round its orbit since closest approach and so is moving almost directly away from Saturn. The view we see of it will thus not change much, but it will get smaller as the Earth moves further way: this is the second effect, clearly noticeable in the February to April period.

Not only does the shadow of Saturn's globe fall directly away from Earth at the point of nearest approach but so do the mini-shadows thrown by each one of the countless rock and ice fragments making up the rings. The consequence of this is that the rings as a whole get considerably brighter as the mini-shadows are no longer falling on, and thus dimming, other fragments as seen from the Earth. This montage shows the effect during January 2005: each image (in black&white for clarity) is balanced so that the globe of the planet stays the same brightness. Nothing much happens until 11th but then the rings suddenly brighten until the 14th (closest approach), when they start to dim down again so that by 17th they are back to where they were on the 11th. The effect is called an "opposition surge" and happens for such a short period because it is very sensitive to the Sun-Earth-Saturn angle - any more than 0.5deg off a straight line and the "shadow hiding" is insufficient for any brightening to be noticed.

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) 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.

The composite picture shows that the track of Iapetus' orbit is clearly non-parallel to that of Titan's: this is caused by the tilt mentioned above. With Saturn in its current orientation a projection forward of the two tracks will just barely cross (remember the tracks are ellipses, not straight lines). This observation, plus a bit of geometry, gives an estimate of the orbital tilt of Iapetus as around 15degrees - very close to the true value of 14.7deg.

Back

	In the upper image we see Saturn and its rings, 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 camera. The middle 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 bottom 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.


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.