Review: “A Fundamental Survey of the Moon”

Published in 1965 by McGraw-Hill, it weighs in at 138 pages plus a short but excellent glossary, and index. Old school, so no editing errors noted.

Folks on the LEAG mailing list recently received news that one of the early pioneers of modern Moon science, Ralph Baldwin, had passed away. He is most noted for two prior works, “The Face of the Moon” in 1949, and “The Measure of the Moon” in 1963. As neither of those tomes are yet in the Lunar Library, I’ve decided to pay my respects with a review of his one book that is in the Lunar Library.

“A Fundamental Survey of the Moon” begins with a preface wherein the author summarizes the work as a comprehensive view of what was known up at that time, and more importantly notes that sufficient historical background has been given to understand the conclusions and assumptions about the Moon. In many regards, this makes the work also an exposition on the application of the scientific method to the Moon, rather than just a straight conveyance of facts.

Mr. Baldwin is regarded as a bit of a “gentleman scholar”. He was trained as an astronomer, but worked in the family business at Oliver Machinery Co. in Grand Rapids, MI. In his first book, he made the case that most of the craters we see on the Moon couldn’t have been volcanic, even if everyone assumed they were. We just don’t find volcanic craters that big on Earth, nor anywhere even approaching the size of those found on the Moon. There were also physical features more akin to those one would find in an impact rather than in a volcanic eruption. Problem was, everyone assumed that the craters were all volcanic, so his book was largely ignored in the scientific community. As folks started to look more closely at the Moon, however, his conjectures proved increasingly accurate. The race into space, and to the Moon, was unveiling surprise after surprise about our little sister, and more and more assumptions were being thrown in the rubbish bin. Mr. Baldwin didn’t work from assumptions, though. He worked from basic scientific principles.

In chapter one, he introduces historical perspectives. The author explains how knowing the resolving power of telescopes by diffraction of light tells us that most terrestrial telescopes are limited to seeing objects larger than about half a mile across, and then lays out other gaps in knowledge regarding the nature of the Moon. Ancient names are dredged up: Democritus, Aristotle, Aristarchus, Hipparchus, and Ptolemy, many of whose discoveries were almost lost to time under the burden of commonly held assumptions over the next 14 centuries. The story then picks up with Brahe, Kepler, Newton and Galileo, who laid the groundwork for our growing knowledge of the Moon until the modern era.

The next chapter looks at the law of gravitation and the orbit of the Moon. We step back to Tycho Brahe, who spent years and years meticulously tracking the position of the planets. He spent his entire life trying to make the data fit within the ‘circles within circles’ perfection demanded by the church. The frustration must have been terrible. His assistant, Kepler, decided to analyze the data and see what it told him. To this day, orbital mechanics is based on Kepler’s 3 Laws of Planetary Motion, though with much refinement. No one understood why they worked, but they did, and it took Newton to make the connection with the Law of Gravitation. Putting together Kepler’s Laws of Planetary Motion with Newton’s Laws of Motion gives us the formula:

f=GMm/r²

where G is the Constant of Gravitation, M is the mass of the big object, m is the mass of the smaller object, and r is the radius from the center of mass (CoM) of the big object to the CoM of the smaller object.

At first the formula didn’t work for the Earth and Moon. It was off by about 20%, which was no good. Turns out that everyone’s assumption in 1665 about the length of a degree on the great circle of Earth was 60 miles (which affects your calculation of the radius). Oops. In 1671, the Frenchman Picard pointed out that it was actually 69 miles. So Newton went back and reworked the formula, and bingo the observation matched the theory. Score one for the scientific method team.

The author then goes on to explain how it is determined where the Moon is (and will be) in its orbit around the Earth, and notes some of the difficulties particular to the Moon. He cites E. W. Brown’s book “Tables of the Moon”, which filled over 360 magazine-sized pages with calculations of the Moon’s motion. It notes over 155 terms in the expression of the Moon’s longitude whose coefficients are more than 0°0’0″0.1, and over 500 with coefficients less than 0°0’0″0.1. That’s a lot of calculating. A tidbit that was unearthed during the exercise was that the perigee of the Moon’s orbit advances, while the ascending node (where the Moon passes up through the plane of the ecliptic) was regressing. This is complicated by the fact that the line of apsides (aka the major axis along the long part of the ellipse, half of which, the semimajor axis, is used in orbital calculationa) is advancing, but not continuously. Oh, and the angle of inclination of the Moon’s orbital plane to the plane of the ecliptic actually varies between 4°59′ and 5°18′, and the eccentricity moves between about 1/15th and 1/23rd, only averaging about 1/18th. There’s other stuff too, like the tides, the Earth’s fat middle, and the Moon’s receeding from the Earth at about 3cm or so per year.

Speaking of tides, the next chapter takes a closer look at them. The author walks through the theory of the geometry of it, and then explains why it’s not quite that simple. He also notes an ingenious experiment by Michelson in 1913 to measure the tide-raising forces, and explains how all of the bodies of the Solar system have their own particular affect on the tides.

From tides, the next chapter moves on to tidal friction and the shape of the Moon. The author notes the difficulty of extrapolating orbital positions in the future or distant past. We have records of eclipses from the Bible, and the Chinese have records dating back thousands of years. We can calculate when an eclipse should have occured on any particular date in history, but it always ends up earlier than when the records said it happened. Deduction? The Earth was rotating a wee bit faster back then. Turns out the Earth is slowing down by about 1 second per day per 120,000 years, so back in New Testament times the day was about 1/60th shorter than today. One of the more esoteric consequences of this is that the orientation of the Earth in its rotation has shifted a bit.

We then look at how the affect of tidal motions effects the slowing down of the Earth’s rotation, and where that energy gets dissipated to (the Moon) and the effects thereof, such as slowly inching farther and farther away from Earth. Nothing to worry about though. What’ll happen is that eventually the same side of the Moon will face the same side of the Earth, it’ll be about 550,000 km away, and the day/month will be 47 days long. This is estimated to take about 50 billion years. Given that the Earth and Moon have only been around about 4.5 billion years, I don’t think we have to dwell on it.

Now if the Moon is getting farther away, that means in the past it was closer in. Hard to tell how much closer in with current data, but it was probably in close enough that there would have been tidal effects on the shape of the Moon. This seem to bear out as the Moon is a triaxial ellipsoid, meaning the length of the axis that goes through the center of the face of the Moon is different from the one going across which is also different from the one going up and down. Using some simple math for the moment of inertia, we discover that the Moon seems to have a fat bulge facing towards Earth.

A slight aside. I was at the Lunar & Planetary Science Conference a couple years back, and at the poster session some of the young guys from JAXA showed me a 3-D ‘printed’ Moon created using laser altimetry data from Kaguya. The surprise was when you held it sideways and the Aitken Basin just pops out as a big old slice out of the rear end of the Moon. Absolutely stunning, and something that isn’t conveyed in most globes or videos. Something that goes a long way towards explaining the apparent ‘bulge’ facing towards Earth.

To try to get a better sense of this, the next topic is contour maps of the visible face of the Moon. The author explains the difficulty of putting them together with the tools of the time, but once done they further demonstrate significant variations from a normal sphere.

baldwincontourmapmoon.jpg

Then, and now…

kaguyatopographymoon.jpg

Kaguya topographical map of the Moon

The question of where the Moon might have gotten started, which affects the nature of the bulge, is explored in the next chapter. The author explores a number of the ‘Moon sloshed off from a molten Earth’ (or fission) hypotheses, most of which break down for angular momentum reasons. There’re the ‘Moon and Earth formed in the same region of space’ (or coaccretion) theories, which break down mainly for density reasons, and the ‘Moon formed elsewhere and wandered into the Earth’s gravity well’ (or capture) theories. Of which the latter is closest to the current ‘Big Whack’ theory of cataclysmic impact by a Mars-sized planetoid named Thea, which sloshed off a good part of itself and the Earth’s crust into space.

Chapter six looks at the major surface features of the Moon, detailing the different features of increasingly larger craters, and their effects outside the crater. We learn of features associated with the impacts that later became the maria, and then of the maria themselves, features like rilles and wrinkle ridges and mountain ranges and chain craters, as well as what little was known of the far side.

Next up is “What caused the Moon’s craters”, where the author walks through the reasoning leading to the conclusion that the Moon’s craters were formed by impact. Terrestrial astroblemes and crater-like structures are examined, as well as the results of studies of explosive cratering. Relationships are drawn from the data, and then compared with Lunar observations to see if the patterns fit. Serendipitously, they do.

In the next chapter the author describes the mechanical process of forming a crater. Lots of descriptions of violent processes tearing at the surface of the Moon, scarring it again and again. This is followed by a chapter on the formation of the dark maria, wave after wave of lava flowing across the Lunar surface.

Once the nature of the surface is established, it’s only natural to wonder how hot it is, and the next chapter looks at the thermal cycling experienced during the long Lunar ‘day’. The book concerns itself primarily with the brutal variations seen in the more equatorial latitudes. Thankfully our knowledge base has progressed significantly over the succeeding forty-five years, and folks are looking at setting up shop at the Lunar poles, where the low angle of the sunlight is thought to create an ambient surface temperature of about -40° (F or C, I can never remember which), which is much easier on the engineering requirements for the machinery.

Now that we have a sense of the temperatures at the Lunar surface, what is it like in other regards? The author looks at reflection spectra, polarization, light backscattering and other indirect methods that scientists were limited to at the time. Temperatures aren’t the only thing that changes on the Moon’s surface, and in chapter 12 the author considers phenomenæ like transient Lunar events (TLEs), particularly Alphonsus and Aristarchus. Given that these TLEs are usually assumed to be gases venting from the Lunar interior, the next chapter looks at the vanishingly thin Lunar atmosphere. He calls it an exosphere, but I’ve also seen it refered to as a collisionless gas (where the molecules rarely if ever bump into each other). Chapter 14 looks at the mechanics of Solar and Lunar eclipses, and the book finishes off with an update regarding the Ranger photographs of the Moon.

So, all in all a comprehensive overview for the time, much of which is still relevant. The book is also a useful reminder of the kinds of scientific thinking that help expand the frontiers of knowledge. It’s written in an engaging style that makes it easy to digest, even when dealing with some college level mathematics and calculus. I’ve been trying to think of a more modern equivalent, and probably the two closest works are Chuck Wood’s “The Modern Moon”, and Paul Spudis’ “The Once and Future Moon”, which is also the name of his blog. Dr. Spudis pays his respects in “A Founding Father of Lunar Science“.

I enjoyed reading it again for this review, and while I would like to give it top marks I do have to recognize that a fair amount of the material is well outdated, enough so that it’s probably best read by someone with a fair degree of current Moon knowledge already; the state of the art has changed significantly in the last forty-five years. So I’ll go with a waxing three-quarter Moon for “A Fundamental Survey of the Moon”.

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