Friday, June 30, 2017

Lunar 100 Target 1 - The Moon

Photo Details: Celestron 127SLT, ZWO ASI290NC, composite from video
07-May-2017 Larger version configured as 4k wallpaper is here

Welcome! This series of posts will document my efforts to explore, photograph and learn about the Lunar 100. This is a list of significant features on the moon that tell a story about the moon's geology, and history. This list was published by Charles A. Wood in a Sky and Telescope article and was his effort to produce a list similar to the deep sky object Messier List for lunar observers. 

The list is sorted by difficulty, with the easiest to find at the top. However, it is not possible to observe all the objects all the time, and certainly not optimal to photograph them. The best time to photograph a lunar feature is when it is near the terminator - the line separating dark and light - since that sets up less direct lighting. Much like a portrait of a person is less than flattering when shot with the pop-up flash on a camera, and far better with off-axis studio light, lunar features are much more prominent and aesthetically pleasing when shot near the terminator. As a result, I will likely present this list out of order, as I shoot them successfully. My goal will be to present a good photograph of each item and a few paragraphs describing their significance. The original list only gives a few words describing the significance of each, so I will learn a lot doing the research. I hope you find it interesting. 

The first entry in the list is simply the moon itself. (The list is sorted by order of difficulty from easiest to most difficult) The Lunar 100 list describes the significance of the moon as simply "large satellite".

The distance from the moon to earth varies through it's orbit, but averages 238,000 miles/383,000 km. By an odd coincidence, the disk of the full moon in the sky is the same size as the disk of the sun - this is why a solar eclipse can blot out all of the sun but the much wider corona. The moon has effectively no atmosphere, and a mass of 1/80th that of Earth.

The moon is tidally locked with the Earth. This means that it takes the same time to complete 1 revolution about it's axis of rotation (1 moon day) as it does to complete a revolution around the Earth. As a result, the same side of the moon is always facing towards Earth (with minor variance due to libration) An excellent animation showing how this works is here - I had trouble getting it conceptually, but it makes sense instantly when you see it in motion. The far side of the moon is thus never visible from Earth - our only imagery comes from orbiting spacecraft.

The moon's near side is pockmarked with craters from impacts with rocks from space. The far side is even more badly marked. It's enough to make one appreciate Earth's atmosphere even more than before.

There are two related theories as to how the moon was formed. The composition of lunar material is very similar to that found on Earth. The most generally accepted theory is that the moon formed from the ejected material from a glancing impact with an object about the size of mars four and a half billion years ago. The ejected material orbited the Earth and then slowly coalesced in to the moon.

However, simulations of this type of event show that a configuration of moon and orbit only rarely result from such a large single impact. An alternative theory indicate that numerous smaller impacts ejected material into orbit, which formed rings that condensed into smaller moons, and then into the moon. An excellent article on these two theories was published by Sky and Telescope magazine.

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