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When I was observing the Moon yesterday, I asked myself:

What would happen if a meteoroid impacted the Moon on the more clear part of the Moon which we can see, leaving a crater of the size like the region of the upper left part of the moon when observing it?

a) How would the impact look? Would you be able to spot the aproaching object seconds before impact with the naked eye? Could you observe with the naked eye debris of the Moon pushed into the space?

b) Would a impact on that scale directly have some impact on Earth? I.e. would the Moon lose by this event enough mass to cause a notable change to our tides?

And first of all: would this impact cause any possible danger for us, humans, like debris from the Moon colliding with earth?

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    $\begingroup$ Note that you are talking more about maria, not really craters. en.wikipedia.org/wiki/Lunar_mare I think for the size object that can create such features on impact, there would be significant worries that it might hit Earth instead. There aren't many free-roaming objects left in the solar system of that magnitude size, however. The debris ejected would likely be in small pieces, so would pose no danger to humans or Earth except possibly to astronauts in orbit. $\endgroup$ – a CVn Apr 18 '16 at 7:34
  • $\begingroup$ Ah, so this thinks aren't actually even resulted from impacts? Ok, but lets take my question anyway the way I asked it. $\endgroup$ – Zaibis Apr 18 '16 at 7:48
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    $\begingroup$ I read once, long ago, that during the Middle Ages, some monks in England were looking up at the moon and saw a brilliant flash at the edge of the moon followed by a wave that seemed to blur the face of the moon for a few seconds. I have never been able to find a reference to this event on the Internet. $\endgroup$ – Howard Miller Apr 18 '16 at 14:02
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    $\begingroup$ According to the medieval chronicles of a Canterbury monk, 12th-century sky watchers got an eyeful one June evening, as the new crescent moon appeared to ignite and convulse, spewing molten rock into space: scientificamerican.com/article/medieval-lunar-impact-the $\endgroup$ – Thucydides Apr 18 '16 at 17:25
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The dark spot on the upper left side of the moon, the 'Ocean of Storms', formed when the moon was still partially molten and is the result of lava flows, possibly released by a massive impact with a small planetoid or a collision with a second, smaller moon that formed alongside the main one.

In order to create a similar feature today, you'd have to impact the Moon with enough energy to melt a large section of the crust. An ordinary asteroid or comet impact couldn't do that and we've got pretty good tabs on the large objects in our solar system, so you'd probably be talking about either an extrasolar rogue planet or a distant Kuiper Belt object with an abnormally eccentric orbit that passes through the inner solar system every few million years or so.

An impact on that scale nowadays would certainly be visible from Earth - it is likely that we'd be able to see approaching planetoid and the collision with the naked eye and watch a chunk of the moon turn to lava on impact. At a minimum, it would form a haze of dust around the moon that would last for some time.

As for the effect on Earth, it depends on how fast the object would be moving and how much, if any, material would reach escape velocity. Most likely, we wouldn't be affected by debris unless the moon was practically blasted apart.

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The Lunar Prospector probe crashed on purpose, to form the pre-named Shoemaker Crater. Lots of telescopes were ready to observe as it went down (pun intended).

What was it like? It's not hypothetical.

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    $\begingroup$ But I'm actually talking about a crater as big as 1/5 of the plain eye observable moon. Interessting info anyway. $\endgroup$ – Zaibis Apr 18 '16 at 7:31

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