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I must have missed the original post but the reply by David Basey is spot on. The optical depth is important, given the rather thin atmosphere of Mars. Some types of cloud on Mars, like the north polar spiral clouds, are best seen at the morning terminator, whereas the orographic or mountain clouds are best seen in late afternoon and evening. Just occasionally, when the band of equatorial cloud is at its most dense it can be seen in white light or in RGB composite images. A good example is the recent image of Phil Miles from a few days ago posted in the current observations part of the Mars Section website. In blue light the cloud blots out most of the Syrtis Major at the central meridian but only a trace can be seen in the white light composite. Past observers from 100 years ago rarely used colour filters but by looking for comments upon the faintness or obscuration of the Syrtis on the CM we can infer that the ECB formed every Martian year: for example, Denning in 1903 was surprised to see a belt of white cloud cutting across the south part of the Syrtis. Thus the present informs the past, as it always does… or is it vice versa?
Although I cannot claim to have known Dudley well personally, I well recall visiting his shop in Golders Green before he moved it to the much more convenient Farringdon Road. In the early 1970s for a time there was a huge 8-inch Cooke refractor which somehow had been set up as a window display. I am told it was eventually bought by someone in Saudi Arabia. Later I used just such an instrument at Cambridge and was able to appreciate how good an 8-inch OG can be. I also recall buying an eyepiece holder from Dudley in the mid-1970s and getting him to take it out the back to machine it to fit a flat tube instead of a curved one. He was very quick on the lathe!
Yes, I shall miss that accent. I worked with Colin to help with an exhibition he was doing on the history of Mars exploration, in the year prior to Beagle II (2002) and then provided groundbased support while Beagle II was about to land. By chance (but not unexpectedly for the martian season) a regional dust storm arose in a nearby part of the planet in 2003 December, and caused Colin some anxious moments. Unable to get images from NASA or elsewhere in real time, he did announce that he had ‘phoned up Richard McKim’ for his Mars weather reports! It was easy for me to follow the planet from my garden, and then to give him a call. And of course I received images from around the world to add in to the picture. In the end, we shall never know for certain whether the dust storm or some other factor was the reason for the demise of Beagle II. Colin was a great British scientist and we are definitely the poorer for his untimely passing.
Richard
