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{ Practical astronomy | Astronomy | The Moon }

The Moon


The Moon is extremely rich in surface detail. The large, dark, often circular, and almost crater-less areas are maria (singular mare, Latin for "sea"). Although they seem almost as flat as the Earth's oceans, the surface is solid. The maria are huge impact basins that filled up with lava from the lunar interior. In contrast, the bright areas riddled with craters are called highlands. These areas are indeed higher than the maria; the surface is older and has suffered more meteorite impacts after their formation.

Lunar craters are very varied. They are not volcanoes, but result from the impact of meteorites. The largest are 250 km in diameter, while the smallest you can image are as small as your equipment allows. Old craters have craters within them, large craters have central mountains and terraced rims; some of them have huge "rays" of ejecta.

The rims of maria create mountain ranges. There are steps of perhaps 100 m height in maria, which can be seen when close to the terminator (the boundary between light and shadow). In fact, all features are better observed not far from the terminator, as it is mostly the play of light and shadow that allows us to see these features.

the Moon
The Moon on 2001-10-27.

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The Moon deserves the use of high resolution, i.e. a camera behind a telescope, in prime focus or using eyepiece projection for extra magnification. I find it difficult to focus a dSLR in this situation; it would need a zoomed live-view feature to help focussing. A webcam has live-view "built in", actually on the laptop screen. While this helps getting a good image, the field of view is very small. Atmospheric seeing should limit resolution to one to three arc seconds, corresponding to between 1.5 and 5 km on the Moon. By selecting better than average frames and stacking a number of these, images can usually be unsharp-masked and may then show detail at the 1 or 2 km level.

Ideally, the telescope should be tracking while you take images. This makes it easier to compose the images, and it may also help a little if the exposures are a bit long. When taking stacks of images, tracking should be at lunar speed to avoid a significant drift of the field of view across the Moon during the sequence of frames.

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