Lunar exploration has always held a strange position in the history of exploration. For all of human history, people have been staring up at the Moon, and for centuries astronomers used telescopes to study the lunar surface. The telescopic surveying and mapping of the Moon by astronomers can (and should, I think) be considered a form of exploration. From this perspective, the Moon had been thoroughly explored far before the dawn of the Space Age. But on the other hand, because of the nature of the Moon’s orbit, the Moon also possessed some of the most mysterious and inaccessible terrain that ever taunted exploration-minded humans.
The Moon is tidally locked, meaning that only one side of the Moon ever faces the Earth. And so for all those millennia of Moon-gazing, there was an entire half of our natural satellite that no human had ever seen before. We would only get our first look at the end of the 1950s, and it would take even longer for us to complete a full map of the Moon. Here is a visual history of how we did it, designed to guide you through the process, even if you aren’t yet familiar with any lunar features.
First, let’s look at the near side of the Moon. The dark parts are the maria, Latin for “seas.” The brighter parts are, generally, called the highlands.
The maria provide helpful landmarks for understanding how the far side of the Moon was revealed, and I have highlighted a few helpful maria below. In red you can see the Sea of Tranquility, probably the most famous mare, since that’s where Apollo 11 landed. But the other ones I have marked will be the most helpful landmarks for this history. We will especially be tracking Grimaldi, really a crater on the western limb (edge) of the Moon with a mare floor. Over on the eastern limb, highlighted in green, is the Mare Humboldtianum. I’ve highlighted Mare Crisium in blue, since it is large and distinctive. But mainly keep your eye on Grimaldi and Humboldtianum.
Oddly, even though the moon is tidally locked, we have been able to see small portions of the far side, due to a wobbling motion called lunar libration. You can see libration happening in the animation below. You might be able to notice that Humboldtianum actually disappears over the limb at certain times. Because of libration, we have actually been able to see more than fifty percent of the lunar surface for a long time. In the 17th century, astronomers began using telescopes to study the extent of libration, and to extend lunar maps slightly.1
