Previously most of my lunar observing had been limited to the spectacular array of impact craters. Yet most of these contain details that passed me by until I was told to really look, and they only scratch the surface (so to speak) of what the Moon has to offer. Now I’ve surveyed 90% of the sights listed in the Lunar 100 list, what were the highlights?
Discovering Lunar domes and dark halo craters has lead me to consider lunar vulcanism more closely. The latter were completely new to me despite being easily visible under bright illumination. These are long extinct volcanoes or vents and probably a result of the impacts in the early life of the Moon.
Which leads me to the lunar rilles (rimae). I love trying to resolve those thins dark or bright lines, depending on the direction of the sunlight. They’re a real observing challenge though they range enormously in both complexity and size, some being much more easily spotted than others. Faults lines or collapsed lava tubes? They have different origins to investigate.
Likewise I’ve developed quite a soft spot for crater chains (catenae). The Davy Crater Chain is listed amongst the 100, but I’d never noticed it before attempting the list. I’ve discovered that there are several more of these structures which are the result of the Earth tearing a passing asteroid or comet apart. Remember Shoemaker/Levy? That’s what we’re talking about, albeit on a smaller scale.
This is the small scale detail so far and as much as I love this the Moon isn’t the Moon without those vast impact basins that for the Maria. They’re not hard to see, even with the naked eye they’re obvious, so what’s to learn?
Firstly I certainly know where to find more impact basins than before. I also enjoy trying to trace the multiple rings of mountains (I think of them as ripples) around many of them. The area around the Mare Vaporum is a favourite now because of its variety. Evidence of volcanism and the deep scouring by debris from the Imbrim impact, one of the biggest, is really obvious.
With a telescope and the right illumination the Imbrium lavas are covered in dorsa (wrinkle ridges). Initially I thought these were evidence of cooling lava flows. By working out the direction of slope as the illumination changed I hoped I could trace their origin. But I couldn’t because these are genuine ridges and not sloping ways from any centre. This is what happens when the marial lava cools and the impact basin sinks under its weight: the surface really wrinkles! The detail at modest magnification can be impressive, especially since it all disappears when the Sun is high.
This project started out as another observing list to tick off, but I’ve come to realise that misses the point of the Lunar 100. To get the most out of it you have to observe the features and then think about what they’re telling you about the geology and history of the Moon. More than any previous casual sessions, this project has lead me to identify features from continuing observation and research. I have a far better feel for the complexity of our nearest celestial neighbour.