Exploring the Far Side of the Moon: A Visual History – Inverting Vision

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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.

Needs no introduction. From NASA

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.

ORANGE: Grimaldi Crater, RED: Sea of Tranquility (Mare Tranquillitatis) , BLUE: Mare Crisium, GREEN: Mare 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

Lunar Libration, from Wikimedia

William Gilbert and Galileo were some of the first astronomers to detect and measure libration, and to attempt mapping the Moon in detail.2 Over time, astronomers began depicting the effects of libration on their maps, which can be seen very well in Johannes Hevelius‘ 1647 map from his Selenographia. The slivers on the northern and southern edges of the map show which areas come in and out of view through libration, and you can see that the effect is slightly more pronounced near the poles. You might even be able to find Humboldtianum on this map.

Hevelius’ map of the Moon (1647), from Wikimedia

Hevelius was using some traditional astronomical measuring devices, but he was also using telescopes of his own construction.3

Hevelius’ Telescope, from Wikimedia

Hevelius’ rooftop observatory in Danzig/Gdansk, from Wikimedia

We also see libration depicted on the map below, made by Giovanni Battista Riccioli in collaboration with Francesco Maria Grimaldi in 1651. Riccioli’s naming scheme was one of the most influential, and our maps today retain many of his names. You can see Mare Tranquillitatus and Mare Crisium on this map, for example. You can also see that Riccioli named the crater Grimaldi after his illustrator. He also named a nearby crater after himself, and yet another in the vicinity after Hevelius. Humboldtianum, however, is named Zoroaster on Riccioli’s map.

Giovanni Battista Riccioli’s map, rendered by Francesco Maria Grimaldi (1651), from Wikimedia

Below, you can see the effects of libration on the visibility of Zoroaster/Humboldtianum. It’s helpful to note the positions of Aristotle, Hercules, Atlas, and Endymion. They can help you orient yourself around the northeastern limb.

A close-up of Riccioli’s Map, showing craters from Aristotle to Mercurius. Herclus, Atlas, Endymion form a distinctive pattern. In the libration zone is a dark spot called Zoroaster.

The same area, showing that Zoroaster was renamed to Mare Humboldtianum. Note Hercules, Atlas, Endymion. Screenshot from Virtual Moon Atlas.

Zoroaster/Humboldtianum appears

Zoroaster/Humboldtianum disappears due to libration

This view was the one that astronomers had for the next...