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The Discoveries of Galileo – Part 3: The Moon
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The Discoveries of Galileo – Part 3: The Moon
As mentioned in Part 1 of this series, Galileo first heard of an invention out of Holland in the year 1608 called a “spyglass” that magnified distant objects. So, Galileo used his talent to make his own spyglasses in the summer of 1609. He probably started out with a low magnification spyglass of around 3x or 4x and over the summer increased the magnification. He used a simple plano-convex lens for the objective and a plano-concave for the eyepiece to give him an upright image (figure 1).
Galileo pointed this higher power spyglass to the night sky and saw the magnified beauty and majesty of God’s universe as no human had ever seen before this moment in history. He observed the Moon, the Milky Way, stars, planets and sunspots. Each of these celestial objects were only visible with the naked eye and thus unavailable for detailed study aside from their movements across the heavens over time.
In the early 17th century the prevailing wisdom among natural philosophers and the Roman Catholic Church in Europe was based on the teachings of Aristotle and Ptolemy (circa 350 B.C. and 100 B.C. respectively) that all heavenly bodies rotated around the Earth and these bodies were perfect spheres. They believed that Earth was not a heavenly body and was imperfect. Besides an imperfect body couldn’t possibly exist in the realm of perfect bodies! These entrenched beliefs started to crumble the night Galileo pointed his spyglass to our closest celestial object, the Moon.
In the fall of 1609 Galileo began studying the Moon with his spyglass and he would sketch the features he saw. We need to understand that the magnification of his spyglass by this time was around 21x (figure 1) and it had a very narrow field of view such that he could only see about half the width of the Moon (figure 2). His telescope was also rather long at about a meter (39.4”) so it must have been very difficult for Galileo to track the Moon with his makeshift tripod.
From his booklet Sidereus Nuncius (Starry Messenger, published on March 13, 1610), Galileo wrote the following,
“Let us first speak of that surface of the moon which faces us. For greater clarity I distinguish two parts of this surface, a lighter and a darker; the lighter part seems to surround and to pervade the whole hemisphere, while the darker part discolors the moon’s surface like a kind of cloud, and it makes it appear covered with spots. Now these spots which are fairly dark and rather large are plain to everyone and have been seen throughout the ages; these I shall call the “large” or “ancient” spots, distinguishing them from others that are smaller in size but so numerous as to occur all over the lunar surface, especially the lighter part. The latter spots had never been seen by anyone before me.” – Discoveries and Opinions of Galileo, p. 31
The latter spots are of course what we know today to be impact craters which were not observable until Galileo’s invention of the higher power spyglass. The darker spots are the seas or maria which have been upfilled by volcanic activity covering over many ancient craters. Galileo continues in his description,
“From observations of these spots repeated many times I have been led to the opinion and conviction that the surface of the moon is not smooth, uniform, and precisely spherical as a great number of philosophers believe it (and the other heavenly bodies) to be, but is uneven, rough, and full of cavities and prominences, being not unlike the earth, relieved by chains of mountains and deep valleys. ” – Discoveries and Opinions of Galileo, p. 31
So, Galileo was able to clearly see the ever-changing shadows along the Moon’s terminator and determined that the Moon had mountains with “rough and jagged peaks” and valleys just like Earth (figure 3). From his nightly observations Galileo was able to correctly discern that the surface was not a smooth unblemished sphere.
These imperfections raised troubling questions for natural philosophers such as if the Moon is not a smooth unblemished sphere then could the Sun and planets also not be perfect spheres? And if these heavenly bodies can be imperfect then can the imperfect Earth be a heavenly body too and be movable? Galileo quickly published his observations in the Sidereus Nuncius pamphlet (Starry Messenger) with detailed drawings to support his findings of the imperfections of the Moon’s surface.
Galileo made two significant observations of the shadows along the terminator over a series of observations: 1) the length of the shadows varied with the incident angle of the sunlight and 2) some bright points seemed to “float” in the night side of the terminator. He accurately concluded that these floating spots were mountain peaks (see peaks A & B in figure 3). He knew the length of the shadows were directly related to incident angle of sunlight being blocked by the mountains just like what is observed on Earth. So, Galileo used geometry to calculate the height of many of the mountains on the Moon based on the shadows cast by them.
Lastly, Galileo wrote an extended commentary on the secondary light observed as a pale glow of the night side portion of the Moon or what we refer to today as earthshine (Discoveries pp. 41-45) as seen in figure 4. It’s of interest to note that Leonardo da Vinci was the first European to propose in 1510 that the “ghostly glow” of the Moon was caused by sunlight reflecting off the Earth. Using his spyglass observations, Galileo deduced that this secondary light was not caused by Venus, other stars, or sunlight transmitting through the Moon as some natural philosophers proposed but was from sunlight reflecting off the Earth. Understanding how much the full Moon brightens up our “darkest gloom of night” on Earth, he wrote:
“Now since the secondary light does not inherently belong to the moon, and is not received from any star or from the sun, and since the whole universe there is no other body left but the earth, what must we conclude? What is to be proposed? Surely we must assert that the lunar body… is illuminated by the earth.” - Discoveries p. 44.
In conclusion, Galileo was able to determine with his higher power spyglass that the surface of the Moon was imperfect and full of cavities, that the Moon had rough and jagged mountains of measurable heights like Earth, and that the “ghostly glow” of the unlit portion of the crescent Moon was caused by earthshine.
Next month I’ll dive into Galileo’s observation of Venus and his discovery that the green planet has phases like our Moon.
Ed LaBelle has been a member of Austin Astronomical Society since 2010. He has worked as an engineer in the semiconductor industry for 30 years and is the founder of the Psalm 19 Astronomy Society based in Austin, Texas USA. The Psalm 19 Astronomy Society has been a recognized society of the Astronomical League since 2018 and earned the League's Outreach Master Award in December 2018 for greater than 150 outreach hours.
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