Tuesday, December 5, 2017

La Bella Luna: The Children of Earth Love You, Awesome Moon

This is the picture of the Full Moon at present in the Wikipedia article on the Moon. Here is additional information: Full Moon photograph taken Oct. 22, 2010 from Madison, Alabama, USA. Photographed with a Celestron 9.25 Schmidt-Cassegrain telescope. Acquired with a Canon EOS Rebel T1i (EOS 500D), 20 images stacked to reduce noise. 200 ISO 1/640 sec. Photo by Gregory H. Revera. Source here. Large image here.

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There was a Full Moon on Dec. 3rd -- specifically, the "Full Cold Moon" -- that was also the one and only "supermoon" of 2017. As the Space.com article states:


Supermoons happen when a full moon approximately coincides with the moon's perigee, or a point in its orbit at which it is closest to Earth. This makes the moon appear up to 14 percent larger and 30 percent brighter than usual. The moon becomes totally full at 10:47 a.m. EST (1547 GMT) on Sunday (Dec. 3). It will officially reach perigee less than 24 hours later on Monday (Dec. 4) at 3:45 a.m. EST (0845 GMT), when it is 222,135 miles (357,492 kilometers) away from Earth.

Source here.

The CWG featured two entries with photographs of this Supermoon (links embedded):



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So why does the Moon keep the same face (side) to the Earth?

Because the Moon is far less massive than the Earth and has therefore become tidally locked owing to a torqueing effect of the Earth on the Moon that has caused the Moon's orbital rotation rate about the Earth and its axial rotation rates to tbe the same -- or rather, ALMOST the same.


The Moon is not fully tidally locked, but instead there is a minor libration (see animated gif below) that allows us to see about 59% of the lunar surface over the course of its phases. In terms of size and mass, while the Moon is roughly one-quarter the size of Earth, it is only about 1.2% of its mass (since it is poor in heavy metals such as iron and nickel).

Caption: Simulated views of the Moon over one month, demonstrating librations in latitude and longitude. Also visible are the different phases, and the variation in visual size caused by the variable distance from Earth; source article here (yes, Wikipedia again).

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The side facing the Earth looks markedly different than the far ("dark") side as a result of this tidal locking with the darkened "seas" -- ancient lava beds -- still visible. Each sea ("mare") has a name, of course.

The Full Moon with the main maria (that's "mare" plural) labeled along with the really large craters (e.g., Tycho, Copernicus, Kepler).

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The two sides look very different with the maria mostly missing on the far side. The far side is fully exposed to space and has had far more meteor bombardment over the billions of years of the Moon's existence. But the reason for the lack of maria is because the crust of the far side is much thicker -- so the lava was not able to emerge during meteor bombardment. Why the crust is so different between the two faces is unknown. Here is a Phil Plait piece from July 2014 giving some ideas on the matter.

Here is the near and far sides of the Moon, respectively, in a side-by-side composite images.

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Sunlit views of the far side -- which occurs when the Moon is in new phase -- are visible in the following two glorious lunar photobombing image series taken about a year apart. Both were captured by the Earth Polychromatic Imaging Camera (EPIC) camera aboard the Deep Space Climate Observatory (DSCOVR) satellite located at a fixed point 1 million miles from Earth.

Yes, this animated image is "for real" (although, necessarily, processed somewhat).

As explained by NASA:

EPIC maintains a constant view of the fully illuminated Earth as it rotates, providing scientific observations of ozone, vegetation, cloud height and aerosols in the atmosphere. Once EPIC begins regular observations next month, the camera will provide a series of Earth images allowing study of daily variations over the entire globe. About twice a year the camera will capture the moon and Earth together as the orbit of DSCOVR crosses the orbital plane of the moon.

These images were taken between 3:50 p.m. and 8:45 p.m. EDT on July 16, showing the moon moving over the Pacific Ocean near North America. The North Pole is in the upper left corner of the image, reflecting the orbital tilt of Earth from the vantage point of the spacecraft.

The second awesome photobombing happened on July 5, 2016 (again when the Moon was in new phase since the Sun is almost directly between behind the satellite and fully illuminated the Earth):


To be clear, the Moon orbits the Earth in the same direction that the Earth rotates -- only the Earth rotates much faster. This is why the Moon appears to rise in the east and set in the west even as it "retrogrades" to the east from new to full and back to new.

You can read about the second EPIC capture of the Moon passing in front of the Earth from its perspective here. In both the animated gifs, you can see how featureless is the far side of the Moon (although in both that one circle mare called by the Russians the "Sea of Moscow" is visible). 

As for the Moon's many stunning surface features ...

Above: Close up of the enormous Tycho Crater with the rays emanating from it. The crater is about 85-km across.

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Close up image of the Tycho Crater floor as seen by the Lunar Reconnaissance Orbiter. What's so noteworthy about Tycho is how young it is -- estimated at 108 million years versus 3.9 billion for the ancient lunar maria.

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As posited in the Giant-Impact Hypothesis, if indeed there was a colossal collision between the "proto-Earth" and another rocky Mars-sized body ("Theia") occurred in the early Solar System to form the Earth-Moon system, the far more massive Earth quickly torqued the newborn Moon so that it quickly became tidally locked to our planet. In this scenario, the Moon was able to form because the disk of material -- similar to Earth's mantle -- was just beyond the Earth's Roche limit.) 


In the angular momentum exchange, the Moon receded to its current position while the Earth gradually spun down to its current 24 hours. The Moon continues to recede from the Earth at about 1 inch a year while lunar tidal drag slows the Earth's rotation at about 2 milliseconds per century. (These rates were much faster when the Moon was much closer.)

The ocean tides on the early Earth would have been stupendous since the height factors as the inverse of the cube of the distance separating the bodies. (To be clear, there are both ocean and land tides. Obviously, ocean tides are far greater.)


As a result of the tidal locking, if you were on the Earth-facing side of the Moon, the Earth would never move in the sky -- except for a slight oscillation that is the flip side of the aforementioned libration. However, you would see the Earth spin on its axis over 24 hours, so you would see all parts of the Earth. (That should be obvious since there is no place on Earth where you can never see the Moon.)

Close up of Mare Crisium and the limb of the Moon.

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One feature of the Moon that has long intrigued me is the roughly circular mare on the upper limb of the Moon (in about the 2 o'clock position). That is Mare Crisium (Sea of Crises) -- see image directly above. Slightly oval in shape, its east-west diameter is variously put between 350 and 450 miles. This means it is a big bigger (but not by too much) than the State of Iowa.

Mare Crisium in the 6-day old Moon; image by J. Ferreira.

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Another part of the Moon that intrigues me is the mountainous boundary between Mare Imbrium (Sea of Showers (Rains)) and Mare Serenitatis (Sea of Serenity).

Those two enormous maria form the right and left "eyes," respectively, of the "Man in the Moon" pareidolic image (with Mare Tranquillitatis (Sea of Tranquility) making up the rest of the left eye.

Left: Iconic image from the movie Le Voyage dans la Lune (1902).

But it is the huge mountain ranges known as Montes Caucasus (the Caucasus Mountains) and Montes Apenninus (the Apennine Mountains) that particularly intrigue me. For one thing, they were probably formed with the impactor that created Mare Imbrium. The Montes Apenninus also contain some of the highest mountains on the Moon -- with Mons Huygens (Mount Huygens) the highest mountain (though not the highest point) on the Moon at approximately 18,000-feet above the surrounding plain.

A close-up of Montes Apenninus (lower end of the arc) and Montes Caucasus (upper end) with a gap between them. To the west is Mare Imbrium and to the east Mare Serenitatis.

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When the Moon is in a waxing crescent phase -- around 40 to 45 percent -- the topography of these two ranges is such that if you look really closely with the naked eye, you can actually seen a slight perturbation (wrinkle) in the terminator line as result of the peaks of these mountains (in particular, the Montes Apenninus) being bathed in lunar morning sunlight while the valleys are still in darkness.

Here is a nice image taken by Mike O'Hara ("Space Mike") with the features along the terminator line labeled. This is a waxing crescent Moon 42 percent illuminated. You can see how those mountain ranges -- not to mention the western crater walls of the Albategnius and Klein craters -- are illuminated in sunlight.

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As for Montes Caucasus, here is what the Apollo 15 astronauts photographed while on revolution 35 at 106 km above the lunar surface on December 31, 1970:


Above: This is an oblique view from south-to-north. There are three craters that caught my attention because I think I see them in the picture above of the boundary of the Mare Imbrium and Mare Serenitatis. First off, though, here are the three craters (numbered below):


Above: The same view of Montes Caucasus facing north with the three craters noted (1, 2, and 3). They are located as follows: Just west of Montes Caucuses (1), on the western flank of the mountain range (2), and in the middle of the northern edge of the range (3).


Above: The yellow region is my best estimate of the region encompassed in the Apollo 15 image), although perhaps the far arc is a bit closer. My main point is that the three craters in the image directly above -- 1, 2, and 3 -- are quite possibly same as those in the Apollo 15 flyover view that I numbered as 1, 2, 3, respectively.

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Finally, here is another fascinating feature on the Earth-facing side of the Moon: A crater called Aristarchus that is the brightest (highest albedo) object on this face:

A close-up of Aristarchus and the lunar limb.

Aristarchus is 25 miles (40-km) across and reaches a depth of 2.3 miles (3.7-km).

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According to the Keeper of All Knowledge (which won't stop begging for money this month):

Aristarchus is bright because it is a young formation, approximately 450 million years old, and the solar wind has not yet had time to darken the excavated material by the process of space weathering. The impact occurred following the creation of the ray crater Copernicus, but before the appearance of Tycho. Due to its prominent rays, Aristarchus is mapped as part of the Copernican System.

A close up of Aristarchus seen during the Apollo 15 flyover.


OK, I'm going to wrap up this entry but not before I say the following:

The Moon very likely has played an important role in the evolution of life on our planet. For one thing, the presence of the Moon stabilizes the tilt of Earth between 24.5 and 21 degrees -- allowing for stable climate zones (e.g., the Amazon Rain Forest doesn't end up in the polar night region).

Conversely, the Moon creates ocean and land tides that over the course of our planet's lifetime have kept it active in both a volcanic / plate tectonic sense and a biological mixing sense. This was especially true when the Moon was much closer (and the tidal effects correspondingly greater).

The Full Cold Moon as seen through an array of blue Christmas lights, December 3, 2017;
Image by George Jiang and reposted in this CWG entry.

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It is probably not an overstatement to say that without the Moon, Earth might not have developed life, at least not in the incredibly diverse and complex way it has. Also, given how the Moon likely formed, it is probably rare for a rocky planet such as Earth to have a "twin" so near in size so close to it.


All in all, the having the Moon is a very, very, very good thing for Earth. And we love the Moon.

--Regulus

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