The Wall: Eternal Day
Page 30
When lit obliquely, a concentrated pattern of furrows is visible. These ridges are located about 50 kilometers from the mare’s edge and form a partially broken inner ring. The most massive furrow, the 300-kilometer-long Dorsum Oppel, extends in an arc up from the filled-in Yerkes Crater (36 kilometers wide) along the northwestern periphery of the mare. In the northern section, the Dorsum is crossed by several ridgelines coming from the edge of the mare.
To the northeast, there are smaller furrows such as the Dorsa Tetyaev group (150 kilometers long) or the Dorsa Harker (200 kilometers). When the sun climbs higher, its rays play across the marbled surface of the Mare Crisium in varying colors, for example, in the light from the bright impact crater, Proclus (28 kilometers across, to the west). At noon other such craters appear, including Picard, Peirce, and Greaves.
A few days after the full moon, the Mare Crisium begins to throw shadows on its western edge, and its eastern extremities disappear into the dark of the night-day boundary. The mountains to the west gleam in the sunlight. On the eastern edge there is a noticeable gap through which flows of lava once blazed their way. Among other things, they formed the Mare Anguis (‘Serpent Sea’), one of the smallest lunar maria. This appears as an irregular, 200-kilometer-long mark.
From the southeastern edge, a mountainous promontory juts into the Mare Crisium—the Promontorium Agarum. There is much more to see in this sea and its surrounding areas during the two weeks per month in which it is lit—a great object of study.
Tip: The Mare Crisium is particularly impressive when you observe it through a telescope or binoculars. In the morning sun it appears as a mighty, flooded crater. An additional moon filter increases the contrast and suppresses disruptive light reflections.
Mare Humboldtianum
During favorable libration, this sea is a fascinating object to study.
Let’s turn to the extreme edge of the moon and take a look at the Mare Humboldtianum—a sea only just visible from the Earth. It lies in the northeastern part of the satellite, a dark spot about 270 kilometers in diameter. This sea is on the near side, but its eastern spurs reach 90 degrees of longitude.
As the moon takes precisely the same amount of time to turn on its axis as it does to circle the Earth, the same side is always facing us. This should lead us to conclude that the Mare Humboldtianum would always be visible when lit by the sun, but this is not the case due to libration, the phenomenon mentioned previously.
Libration has various causes, but the main effect is triggered by the moon’s elliptical course combined with its monthly axial spin. Libration sometimes causes landscape features on the far side to come into view, and sometimes areas on the near side disappear temporarily from the focus of the telescope. These affected features include the Mare Humboldtianum.
When libration is unfavorable (when the southwestern section of the moon is easily visible), the Mare Humboldtianum shifts toward and over the northeastern edge until you can no longer see it. Favorable libration, on the other hand, brings the northeastern spurs into view if the lighting is good. The sea can then be easily made out through binoculars. This is the case in early March, which provides a good opportunity to see the smaller, lesser-known seas on our satellite with more precision.
But let’s return to describing the Mare Humboldtianum. It is a dark patch of solidified lava, with a diameter of 273 kilometers, in the center of a large, old impact crater itself measuring 650 kilometers, the eastern spurs of which stretch a long way around the far side of the moon. The impact took place around 3.8 billion years ago.
Later impacts furrowed the Mare Humboldtianum. Smaller ones threw up rocks that display different colors depending on the light. On the northeastern flank of the sea, the 200-kilometer Bel’kovich Crater has created a breach. The name Mare Humboldtianum was coined in 1837 by the German astronomer Johann Mädler, in honor of his compatriot Alexander von Humboldt. This explorer’s indomitable spirit, which led him at the end of the 18th and beginning of the 19th century to the discovery of previously uncharted parts of the Earth, was likely an inspiration for Mädler, whose moon globe was in turn praised by Humboldt. Another lunar crater was later named after Mädler (as was a crater on Mars). From above, the Mare Humboldtianum resembles a wide sickle. It was first photographed in October of 1959 by the Soviet Luna 3 probe.
Favorable libration and a waxing moon tip the northeastern region into view for three to 13 days. That’s when the Mare Humboldtianum is especially easy to see.
If you want to observe the Mare Humboldtianum in the evening, the time of maximum libration is the best. The dark spot on the northeastern (top right) edge is easy to detect with binoculars. A telescope shows even more detail, even when the sun is high and the lunar landscape relief is not casting long shadows.
Tip: Get this sea in your sights when the libration pulls the northeast edge of the satellite toward us during a waxing moon. An additional moon filter increases the contrast and suppresses disruptive light reflections.
Mare Undarum
Right on the moon’s eastern edge are a few seas that are only entirely visible during favorable libration.
We previously took a look at the Mare Crisium (the ‘Sea of Crises’), a large, oval plain on the moon’s northeastern edge—a lunar sea of sufficient size to be able to make it out, with the naked eye, as a dull patch. You can imagine the Mare Crisium as a large impact crater whose interior was flooded with lava that flowed sometime after the bombardment of the crust.
The libration, which was explained earlier in the tour, causes the Mare Crisium to tilt into clear view. In any case, the Sea of Crises lies entirely on the side of the moon facing us, so you can observe it any time the sun is in the right position. There are groups of smaller seas to the east of the Mare Crisium. None of these formations can be described as having a crisp, oval outline. They are more irregular patches in a variety of shapes.
Roughly 100 kilometers southeast of the Sea of Crises lies the Mare Undarum (Sea of Waves). It consists of a number of lava-filled craters. Like the Mare Crisium, the Mare Undarum is also always on the side of the moon facing the Earth. So, it remains visible regardless of the libration phase. The outline of the Mare Undarum is irregular. It measures about 100 kilometers from north to south and 200 from east to west. The only effect the libration has is that the appearance of this sea, as seen from the Earth, is more or less squashed.
Further to the east of the Mare Crisium lie two other seas of considerable size. They lie directly on the 90th eastern degree of longitude, which forms the border between the visible and the hidden sides of the moon. These are the Mare Marginis (Sea of the Edge) and the Mare Smythii (Sea of Smyth).
You can find the Mare Marginis east of the Mare Crisium. It has an irregular outline, is 360 kilometers wide, and lies in the libration zone. During strong oscillation this Sea disappears almost completely from view. Normally, however, you can make it out from a waxing half-moon up to a full moon. The Mare Smythii (named after a British astronaut) also lies in this region. It measures around 200 kilometers in diameter. Its silhouette is almost circular, but quite faintly defined.
During favorable libration the Sea of Smyth presents itself to an Earth-bound observer to its full extent, but the oblique angle makes it seem narrower than it is. Like the Sea of the Edge, it consists of several lava-filled craters and sometimes disappears beyond the edge.
Mare Imbrium
The Mare Imbrium is the much-visited eye of the ‘Man in the Moon.’
One of the first science fiction films appeared in 1902. ‘A Trip to the Moon’ was created by cinema pioneer Georges Méliès, based on two Jules Verne novels on related topics. One scene shows the distressed face of the ‘Man in the Moon,’ who has an oversized projectile in his right eye (the left side for the viewer). According to Jules Verne, the capsule was fired from a giant cannon. When we look up today, we are lucky to see the eye unscathed—the Mare Imbrium, or Sea of Rains.
Like all large lunar seas,
the Mare Imbrium is also an impact basin—a crater formed in the early days of the solar system when a celestial body hit the moon’s surface. Scientists surmise that a 250-kilometer protoplanet impacted here 3.9 billion years ago. The collision left behind a 1,200-kilometer-wide pit that today is known as the Mare Imbrium.
The Sea of Rains looks impressive, even through binoculars. Manned and unmanned spacecraft have landed there, and it has become apparent that the Mare Imbrium is indeed a fascinating place. The vast plain of frozen lava is strewn with countless craters. To the east rise the Apennine Mountains; to the north is the Plato Crater, with its dark floor; and in the northwest, its edge bulges out to form Sinus Iridum ( Bay of Rainbows). When the sun’s rays hit at a sharp angle, you can see that the seemingly smooth surface is as wrinkled as unpressed linen.
Over the years, several large missions have selected Mare Imbrium as a landing site due to the fascinating and diversified geology. The Russian Luna 17 probe landed there in 1970. On board was the Lunokhod lunar vehicle, the first rover to explore a celestial body.
A year later, Apollo 15 landed near the Hadley Rille, a 1.6-kilometer-long, winding moon valley. Dave Scott and James Irwin wound through here with the Lunar Roving Vehicle (LRV). It was to be another 40 years before another vehicle left its tracks in the dust of the Mare Imbrium: the Yutu (jade hare) rover, set down by the Chinese Chang’e 3 probe. This rover functioned for 31 months and transmitted numerous spectacular images.
The Sea of Rains lies in darkness until the terminator (the night-day boundary) wanders across the peaks of the Apennine Mountains to the east of the plain. A few days later, the Mare Imbrium is half-illuminated, and another few days later, it lies in full sunlight.
This is the best time to observe the most prominent craters in the eastern half—Archimedes and its smaller northern neighbors Autolycus and Aristillus—and the many ridges and furrows. The Sea of Rains presents itself in its best light for ten days, until the terminator roams back over the surface and the region disappears again into the darkness of the lunar night.
Tip: Observe the Mare Imbrium when the terminator has just reached it. Then you can see all the small craters and details.
Mare Orientale
An impressive structure that we only rarely get to see is the Mare Orientale.
Most of the landscapes and places of interest that we have visited on our tour here can be quite clearly made out—deep craters, long, jagged mountain ranges, extensive plains, and so on. The place that we are now concerned with is a bit more challenging. To be precise, you can only observe it from the Earth for a few days.
The Mare Orientale, with its surrounding mountains, forms one of the largest and most imposing structures on the moon. If it lay on the side facing the Earth, it would dominate the face of our satellite and would have found its expression in diverse religions and cultures. But the original crater was blasted out so far to the west that today’s plain mostly hides from view. Only if the libration—the oscillation of the moon on its axis—is particularly strong, can we sometimes peek around its western edge. Then a truly breathtaking landscape is revealed.
Only the amazed, wide eyes of the Apollo astronauts and the clicking cameras of the rovers have glimpsed the Mare Orientale in its full splendor. One of the youngest impact structures was spread out before them, blasted out almost four billion years ago by an asteroid measuring over 60 kilometers.
The catastrophic hit left a 327-kilometer crater behind, which later filled with lava. Then it cooled to form a vast plain. If you include the concentric rings of the surrounding mountain ranges, you have a structure measuring over 900 kilometers in diameter. If you imagine that the Mare Orientale lay on the near side of the moon, our satellite would have a kind of giant cyclops eye staring down at us. Now, imagine that feature as it would look during a lunar eclipse, when the scene is drenched in blood-red light. It’s interesting to ponder how priests from various religions would have interpreted such a spectacle.
However, the opportunities to glimpse the sea are few and far between, and the visibility window is narrow. If you ever get the chance to see the Mare Orientale, you should grab it. If the moon’s libration turns it in such a way that its western side comes far enough around toward us, the ‘Eastern Sea’ reveals itself with all its encircling mountain ranges. But it would help if you had good binoculars, or even better, a telescope. Even then, it’s not much more than a region of stark contrasts of light and shadow. But the discovery is still exciting every time.
Where exactly should you look? The easiest way to find the Mare Orientale is to imagine the lunar disc as an analog clock face. The Eastern Sea will appear tilted. When the libration is strongest, point your binoculars or telescope at the 8 o’clock position—the dark Grimaldi Crater. To the left and below it, at the very edge, you will see something like a number of dark lines that resemble scratches on the surface—that’s the Mare Orientale with its numerous mountains.
Of course, the stronger the magnification settings, the more details and landscape features reveal themselves—but even with maximum zoom the circular outlines are not noticeable. And yet, that’s not so important. What counts is that you’ve come face to face with something that is generally hidden from view. You can enjoy the fact that you are looking at a structure that most humans have never seen.
Tip: Get hold of a contrast-increasing moon filter if you can. That way the details will appear more clearly.
Mare Tranquillitatis
The ‘Sea of Tranquility’ is one of the most famous regions of the moon for two reasons: it’s easy to locate, and the historic Apollo 11 landing took place there. The sea—in reality, a dark basalt plain—was formed about four billion years ago. At that time there was still volcanism on the moon, before our satellite cooled and went to sleep, so to speak.
The Mare Tranquillitatis has a total diameter of almost 870 kilometers. To illustrate, that’s roughly the distance from Indianapolis to Washington D.C. as the crow flies. When the region lies in sunlight it’s not difficult to detect the Sea of Tranquility. You just need good binoculars.
It’s best to view the region at times when it’s brightly lit. At those times the pale gray surroundings are distinguishable from the dark basalt floor. To find it, you can orient yourself on nearby formations, for example, the Mare Serenitatis (Sea of Serenity), which looks like a snowman. The Mare Nectaris (Sea of Nectar) also borders the Sea of Tranquility, but it’s significantly smaller. The Mare Tranquillitatis is quite dark. This makes it difficult to distinguish individual formations, but the shadows, especially around the edges, display the most differentiated colors.
We assume that the region originated in the so-called pre-Nectarian period, that is, a period from 4.5 to 3.9 billion years ago. If we fast-forward to the present, we see a strip of land that has been scarred by countless minor impacts. There are ridges, notches, and volcanic channels from geologically active times. But when walking around, the feeling is said to be like moving around on a layer of powder, since fine dust covers the moon.
The region became world-famous on May 20, 1969, when Neil Armstrong and Buzz Aldrin landed there in the Eagle lunar module from Apollo 11. When Armstrong climbed out, it was the first time in history that a human had set foot on a foreign celestial body. The landing site was given the name of Tranquility Base. The two of them walked around for six hours, collected ground samples, and took photos. They planted the U.S. flag, and when they departed, they left their ladder behind, bearing a stainless-steel plaque that reads, “We came in peace for all mankind.” It displays the signature of President Nixon, among others.
It’s fun to track down these places and the landing sites of the other Apollo missions. All you need is a decent pair of binoculars that can zoom in on the craters on the surface. To find the precise touchdown point of Apollo 11, it’s best to search for the nearby Theophilus Crater. Once you’ve located that, draw an imaginary line up toward Mare Tranquillitatis to where the small but dis
tinct Moltke Crater is located. Directly northwest of there is the landing site. If the telescope and the viewer’s eyes allow it, you can also spy three more very small craters: Aldrin, Armstrong, and Collins—the third named after Michael Collins, the pilot of the Apollo 11 Command Module.
Even before the legendary moon landing, the Sea of Tranquility had received a visit from the Earth. On February 20, 1965, the Ranger 8 NASA probe performed a (planned) crash landing, having taken 7,137 photos of the lunar surface during its nosedive.
The Surveyor 5 probe managed a soft landing two years later, although it braked too late and ended up 30 kilometers away from the planned landing site.
Therefore Mare Tranquillitatis is one of the most well-researched regions of the moon, and inspires the imagination of people all over the globe, which shows up in numerous books and song lyrics.
Tip: The Apollo 11 landing site is easiest to find in direct sunlight. Then the small craters around it are also visible.
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Glossary of Acronyms
CERN – Conseil Européen pour la Recherche Nucléaire (European Council for Nuclear Research)
CIA – (U.S.) Central Intelligence Agency
CSA – Canadian Space Agency
EEG – ElectroEncephaloGram or ElectroEncephaloGraph
ELT – Extremely Large Telescope