The Lunar Limb

On a final note, before we leave lunar orbit, I would like to say a few words about observing the lunar limb regions. It is often said that there is little point observing the Moon when it is full, simply because there are no long shadows and the finest details are always glimpsed when the Sun is very low over the formation being studied. Thus, at sunrise and sunset, maximum contrast is seen. At full Moon, the regions on virtually all the visible lunar disc are experiencing the Sun well above the lunar horizon. However, for features at the lunar limb, the opposite is the case. Close to full Moon these limb regions are experiencing sunrise or sunset conditions and this is the ideal time to bag some images showing just how rugged the lunar limb really is. Of course, the features will look incredibly foreshortened, but they will also give a far greater impression of being in a low orbit lunar spacecraft, skimming over the surface and seeing the curvature of the Moon. Some of the regions on the Moon's southern limb are incredibly rugged and, if you catch a favorable illumination and libration, the amount by which the Moon's limb deviates from a true circle can be quite staggering to behold, making the Moon resemble the rocky world it really is, rather than a perfect sphere with a few craters on it. For those who remember the 1970s BBC creatures "The Clangers" (knitted, woollen Moon creatures who lived on the Moon with a soup dragon!), the limb of the Moon always reminds me of the "Clanger" Moon where it is especially rugged.

Figure 10.28. An incredibly high-resolution image of Arzachel taken with a 250-mm f/6.3 Orion Optics Newtonian and an ATik 1HS webcam at f/38. Image captured on March 18, 2005. Image: Jamie Cooper.

Perhaps the most spectacular event is when a bright star grazes the rugged lunar limb causing the star to wink on and off behind the lunar mountains. Such events can easily be recorded with a webcam, especially when the star is of naked-eye brightness.

Of course, given a very favorable lunar libration and a favorable illumination, a rare glimpse of features normally regarded as being on the far side of the Moon can be secured. Perhaps the most famous feature in this category is the Mare Orientale, the center of which is situated at 20° S, 95° W. This mare is the bullseye at the center of a huge multi-ring basin. Surrounding the Mare Orientale is a giant circle of mountains called Montes Rook. At their eastern edge these mountains reach round to the near side, to 85° west in fact. But there is yet another ring of mountains surrounding the Mare Orientale at an even greater radius: the Montes Cordillera. These reach round to within 80° west longitude, i.e., 10° onto the near side at 20° south. With lunar librations capable of tilting the Moon by up to 10°, it is clear that the Cordillera Mountains and some way beyond can, indeed, be glimpsed from Earth, and under extreme conditions the Mare Orientale itself can just be glimpsed. Locating the region takes a bit of familiarity with the lunar surface though. The best marker to the area on this side is the dark, flooded basin Grimaldi located at 69° west and 5° south. Move about three Grimaldi lengths south from Grimaldi itself and then go to the nearest part of the lunar limb and that is where you will see the Cordillera Mountains and more, if a favorable libra-tion is in place. How do you find out when a favorable libration will happen? Well, most decent planetarium PC packages, like The Sky or Guide 8.0, display the lunar libration amount and position angle (north = 0°; east = 90°; south = 180°; west = 270°) in the lunar information text that comes up when the Moon is selected or clicked on. Failing that, the most comprehensive astronomy data books, like the Handbook of the British Astronomical Association, list the extreme libration values every two weeks.

CHAPTER ELEVEN

Imaging Mercury and Venus

At first glance it might seem that there is little to be gained from imaging the two inner planets. Apart from the fact that they are only ever at a decent altitude in twilight, there would appear to be little to actually image. In addition, when at their closest to Earth they both become tiny crescents with most of their globes in total darkness. Needless to say, when imaging Mercury or Venus, great care must be taken to ensure that the Sun does not enter the telescope field. Venus is not difficult to find in broad daylight, if you know where to look, but this can be a risky business. The inexperienced amateur should never point a telescope anywhere near the Sun until a full appreciation of the risks and a great deal of experience is acquired. It is all too easy to damage one's eyesight and, unlike a webcam, an eye cannot be replaced. For information on safe imaging of the Sun and of solar transits of Mercury and Venus, please see the chapter on solar imaging. On this cautionary theme, as soon as the Sun has set, Mercury and Venus can be safely imaged without any worries about eye injuries. Atmospheric seeing conditions are usually very poor in daytime, due to solar heating. They are usually very poor an hour or two after sunset, too, as the Earth's atmosphere cools. However, during the first hour after sunset a stable period sometimes exists. This can be a good time to take highresolution images. A declination circle or a reliable "go to" telescope are invaluable in these situations, where planets need to be found in a bright, twilight sky.

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