Strip Maps

Another method for visualising the appearance of a planet as more of it rotates into view is the strip map technique, i.e., a long image strip with a vertical height between, say, 60 degrees south and 60 degrees north and a horizontal width of up to 360 degrees. An example, by Damian Peach, is shown in Figure 13.13. Obviously, it is impractical to include the polar regions in such a strip as the higher latitudes get increasingly distorted if they are represented as a strip map. Even at 60 degrees latitude the circumference of a globe is only half that at the equator. Astronomical software packages such as Christian Buil's IRIS can be used to convert images of planetary discs into strip maps.

Finally, I would like to end this chapter with one of the most stunning images of Jupiter I have ever seen. Figure 13.14 was taken by Damian Peach using a 235-mm aperture SCT. At first glance it looks like a Hubble space telescope image. Amazing!

Figure 13.13. A stripmap of Jupiter covering 360 degrees of longitude, compiled from images taken over two five-hour periods on the nights of January 28 and 29, 2003. Celestron 11 image with a ToUcam Pro webcam. Image: Damian Peach.

Figure 13.14. This absolutely stunning image of Jupiter was secured by Damian Peach, from Barbados, on April 23, 2005. A Celestron C9.25 and Lumenera video camera were used for this composite of 3 x 1,000 frames at 17 frames per second.

CHAPTER FOURTEEN

Imaging Saturn j

Without a doubt, Saturn is the must see planet that every one should view, through a telescope, in their lifetime. Most people, at some time, do actually look at the Moon, either with binoculars or a small telescope. However, to the naked eye, Saturn just appears as a bright star and is not obviously a planet. So unless you know where it is, you are stuffed! However, once you find it, the sight is mesmerizing. On first glance it seems just so incredible that a planet can have rings actually going round it: like something out of a science fiction film!

Saturn orbits the Sun at an average distance of 1427 million kilometers or roughly 9.4 times the distance of the Earth from the Sun. A photon of light takes 8.3 minutes to travel from the Sun to the Earth, but 80 minutes to get to distant Saturn. Even when Saturn is at its closest to us its light has taken over an hour to reach us.

Although the globe of Jupiter (equatorial diameter 142,880 kilometers) is almost 20% larger than the globe of Saturn (equatorial diameter 120,536 kilometers), Saturn's visible ring system spans an incredible 274,000 kilometers, that is 70% of the distance between the Earth and our own Moon. Put another way, it would take just under a second for light to travel across the ring system. Saturn's rings are the largest apparently single structure orbiting the Sun, although, in reality they are not a solid structure at all.

Saturn takes 29.4 years to orbit the Sun and during that time we view the northern hemisphere and the northern face of the rings and then the southern hemisphere and the southern face of the rings. This, of course, is because Saturn's rotation axis is tilted with respect to the plane of the solar system (called the ecliptic). The tilt is 26° 44' or nearly 27° and thanks to this we are able to have a glorious view of the ring system. If Saturn had the relatively low axial tilt of Jupiter (a mere 3°) we would be denied the most spectacular sight through a telescope: Saturn with its rings wide open. Needless to say, as Saturn moves around in its orbit and the rings appear to close up, there comes a point where the rings are seen edge-on. In fact, a sequence of events takes place, because, while the Sun passes through the ring-plane in one go, illuminating first one side of the rings and then the other, the Earth, moving around in its orbit can peep just above and just below the ring plane several times before the rings really start to open up again. The sequence of events witnessed can be fascinating, although, since spacecraft visited Saturn, there is (probably) little scientific value to be derived. The tilt of Saturn's rings and the parallax created as Earth orbits the Sun is also the reason why Saturn's rings do not quite open and close smoothly; they can pause for a few months in the opening/closing process depending on where the Earth is relative to Saturn.

Because the orbit of Saturn is not a perfect circle, the time interval between successive edge-on events is not equal. We see the southern face of the rings for 13 years, 9 months and the northern face for 15 years, 9 months. While the southern ring face is fully exposed, Saturn reaches perihelion in its orbit and comes to opposition at a high northerly declination, as occurred on December 31, 2003. That is good news for high northern latitude observers like me. The rings will next be edge-on (with respect to the Sun) on August 10, 2009, May 5, 2025, and January 22, 2039. Saturn comes to opposition two weeks later every year.

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