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Dark as Basalt: The Moon and Recent Eclipse
Michael J. Urban, MEM-1910
Recently, many of us received a golden opportunity to wit- ness a relatively rare total (or partial) solar eclipse last summer on August 21, 2017. If favorably situated across a particular swath of the continental United States, and offered clear skies for viewing, you too may have observed a seemingly unnatural darkening of the mid-day sky when the moon cast its narrow shadow upon you and your surroundings as it passed directly in front of the sun. Although the event lasted over an hour for most, the memory of it may well last a lifetime. Such a striking display of astronomical wonder may not only give us pause to contemplate the more aesthetic qualities of nature, but also cause us to take more pointed notice of our closest celestially neighbor – the moon. [See the references (Hoang, 2017) for an animation of the eclipse.] Let’s take a few moments to inspect this seldom considered sky-bound visage now that it’s back in full view.
Moon Impressions
Many ancient and contemporary stargazers doubtless have been mesmerized and inspired by the moon’s prominence and cyclically varying appearance in the sky. Sketches by Galileo when he turned his early spyglass toward the moon provided some of the first evidence of the moon’s terrestrial nature and revealed a place not altogether unlike our Earth. After many decades of close scrutiny, and several in-person visits to the moon, we now know even more about just how intimately this alien world is connected to us.
During the Apollo Missions of the 1960s and 1970s, twelve American astronauts set foot on the moon, and bounced around its surface owing to the much lower gravity. Many samples of moon rocks were collected and returned to Earth, provid- ing details about its composition. A prevailing theory for the moon’s formation, advanced by Hartmann and Davis (circa 1975), describes a large Mars-sized object impacting Earth at just the right angle to blast part of Earth’s crust and mantle into orbit to mix with the remains of the impactor itself (Spudis, 2003; Planetary Science Institute, 2017). Over a short period of time, the material coalesced into the natural satellite we see and know today. Evidence for this Giant-Impact Hypothesis includes the compositional similarity of the moon to the Earth’s crust or mantle, and the relative absence of volatile elements (Wood, 1999). More recently, Canup (2012) provided a modified version of the idea, suggesting that a collision between two larger bodies (both maybe 5 times more massive than Mars) created the Earth and moon simultaneously.
Our nearest neighbor is tide-locked with us, meaning the moon keeps the same face always pointed toward the Earth, because its rotation (axis spin) rate equals that of its revolu- tion (orbit). In truth, we can see just a bit more than half of
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the moon – not at once, but over time – due to a wobble, or rocking, of the moon in both the latitudinal and longitudinal directions. The phenomenon is known as lunar libration and it enables us to see approximately 59% of the nearest face of the moon. Prior to 1966, and the first orbit of the moon by the Russian Luna 10 probe, nobody had ever seen the backside of the moon (Bell, 2017). Because of tidal interactions with the Earth, our moon is receding further from us with each passing orbit it completes (centimeters per year), while at the same time the gravitational drag slows the rotation of the Earth.
You will occasionally hear people refer to a dark side of the moon, and there is most certainly a dark side of the moon, since (excepting eclipses) one half of the moon is always basking in sunlight (refer to Figure 1), and the other side is in darkness. What these same people sometimes mistakenly say is that this “dark side” of the moon is always the same side. This is a misconception, since during the new moon phase, the familiar side of the moon, facing the Earth, is dark; whereas during the full moon phase, it is the side we never see, facing away from the Earth, which is dark. To take a slightly different approach to this issue, the length of a day (one rotation) on the moon is equal to 27 days on Earth; therefore, for any given location on the moon, sunset occurs approximately two weeks (or 13.5 days) after sunrise. The entire moon receives some sunlight during its monthly rotation, just as the Earth does (neglecting seasonal variations).
Geologic Context
The composition of the moon is complex, and varies locally, just as it does on Earth. However, just as locations on Earth can be described as being either continental crust or ocean crust, parts of the moon may be described as one of two primary crustal types too: the maria or the highlands. The lunar highlands are older and more heavily cratered than the younger maria. The highlands are composed of lighter colored, lower density, anorthosites (plagioclase feldspar), whereas the maria lowlands are darker and denser basalts (Wood, 1999). The moon has no appreciable atmosphere. And, although the moon appears bright white in the sky, its actual albedo (reflectivity) is quite low – ranging from about 15% in the highlands to as little as 5% in the maria (Spudis, 2003). Contrasting this with the 30% albedo of the Earth itself, and the 90% of fresh snow, invites one to imagine what gathering crops might be like under the light of a harvest moon covered in ice or clouds -- some 30-40% brighter!
An interesting consequence of the formation of the moon via a giant impact is the probability of a molten crust that would have stratified by density, leaving the lighter-weight and light- er-colored feldspars at the top (i.e., lunar highlands) (Wood,
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