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The Discoveries of Galileo – Part 5: The Milky Way, Orion, and Asterisms
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This is the final article on the Discoveries of Galileo from 1609 to 1612. His discoveries of Jupiter, sunspots, the Moon, and Venus were covered in parts one through four of this series. This article will cover Galileo’s observations of the Milky Way, the constellation Orion and star clusters or what we term today as asterisms.
I have touched on the prevailing European natural philosophy (the precursor to modern science) of the 17th century and the Roman Catholic Church’s involvement in censoring any ideas believed to be heretical i.e. counter to the Church’s interpretation of scripture. The Greek philosopher Aristotle (384 – 322 BC) had proposed in the 4th century BC that the Earth was the center of the universe and all the celestial objects rotated around the Earth on fixed spheres. The Sun’s sphere rotated in 24 hours, the Moon’s in 28 days, and the stars in one year. The planets each had their own spheres of differing time period. The other idea from Aristotle was that the heavens and all celestial objects were perfect and unblemished. Claudius Ptolemy (c. AD 100 – c. 170) of Alexandria (a Greek city in what is now Egypt) wrote a treatise of his celestial observations that was published as the Almagest around 150 AD. This was the most influential manual on astronomy that held reign for about 1500 years. In this multi-book tome Ptolemy documented 1022 stars in 48 constellations (Britannica.com).
Keying off Aristotle’s idea of the heavens being perfect, many natural philosophers believed that all of the stars had been documented by Ptolemy and no new stars existed. The Milky Way was an enigma for the prevailing cosmology up to the discovery of the telescope. It appears to the naked eye as a diffuse irregular band of light high in the heavens. The name comes from the Greek word "'galactos' which literally means 'the milky thing in the sky'"1 For Aristotle, the Milky Way was the point in the universe where the celestial spheres came into contact with the terrestrial spheres.2 Galileo shattered the belief of there being no new stars when he discovered the moons around Jupiter and again when he pointed his spyglass to the Milky Way.
Galileo wrote in the March 1610 pamphlet Sidereus Nuncius (The Starry Messenger) the following, “…I have observed the nature and the material of the Milky Way. With the aid of the telescope this has been scrutinized so directly and with such ocular certainty that all the disputes which have vexed philosophers through so many ages have been resolved,…The galaxy is, in fact, nothing but a congeries of innumerable stars grouped together in clusters.” (Discoveries, p.49). And just like that, Galileo shattered the prevailing belief that the number of stars was fixed and unchangeable. Not only are the numbers of stars not fixed but they are a disordered jumble and innumerable. Galileo continued, “Upon whatever part of it the telescope is directed, vast crowds of stars is immediately presented to view.”
The band of stars were so numerous that Galileo didn’t bother making any detailed drawings of the stars. And one cannot blame him considering the low magnification and narrow field of view of his rudimentary telescope (about 21x magnification and 0.9 degrees). Galileo expressed how daunting the task was to capture stars with such a narrow field of view when observing the constellation Orion. Galileo wrote, “…I had intended to depict the entire constellation of Orion, but I was overwhelmed by the vast quantity of stars and by limitations of time, so I have deferred this to another occasion. There are more than five hundred new stars distributed among the old ones within limits of one or two degrees of arc” (Discoveries, p. 47).
It’s interesting to note that Galileo didn’t draw or describe the Great Orion Nebula which is one of the winter sky’s most interesting naked-eye white fuzzy objects. The large nebula should have been quite noticeable and awe inspiring to look at magnified for the first time. I have an antique refractor telescope with a magnification of about 17x which would have been similar to what Galileo was using when he looked at Orion in February 1610. I took it out in mid-November to look at the nebula and could easily see that the large white fuzzy object was some type of celestial cloud. So, it puzzles me that Galileo didn’t sketch or mention the nebulosity. Historians suspect that Galileo may not have trusted what he was seeing when looking at the nebula due to a combination of poor-quality optics and narrow field of view.
However, Galileo does mention other “nebulas” when writing about the Milky Way in Starry Messenger. He wrote, “And what is more remarkable, the stars which have been called “nebulous” by every astronomer up to this time turn out to be groups of very small stars arranged in a wonderful manner.” (Discoveries, p. 49-50). He then includes a sketch of the nebula he called the Head of Orion which had twenty-one stars (the asterism around Meissa - λ Ori) and the nebula Praesepe “which is not a single star but a mass of more than forty starlets.” (Figure 2, Beehive Cluster (M44), Discoveries, p. 50)
Galileo also peered at the Seven Sisters or Pleiades and found numerous stars there visible with his spyglass. He wrote, “… I have depicted the six stars of Taurus known as the Pleiades (I say six, inasmuch as the seventh is hardly ever visible) which lie within very narrow limits in the sky. Near them are more than forty others, invisible, no one of which is much more than half a degree away from the original six. I have shown thirty six of these in the diagram…” (Figure 3, Discoveries, p. 48-49).
In summary for this series, Galileo is rightfully considered to be the father of our modern scientific method. In his pursuit of knowledge, he developed an experimental method to test for assumptions such as how bodies fall or float on water. Thus, Galileo used observational astronomy to shed new light on our solar system and helped to dethrone the ancient Greek beliefs about the universe. He showed that celestial objects such as the Moon and Sun were not perfect spheres, that Jupiter had moons, that Venus orbited around the Sun, and that there were innumerably more stars than those visible to the naked eye.
Galileo was arrogant and often condescending in his writings which got him in trouble with peers and the Church. He was unconventional in that he preferred to publish his discoveries in the local Italian vernacular instead of the accepted Latin. This singular action by Galileo brought the field of scientific inquiry from the elite ivory towers down to the common people. In the end, Galileo paved the way for the acceptance of the Copernican heliocentric theory within a few decades after his death in 1642.
Ed LaBelle has been a member of AAS 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.
The Discoveries of Galileo – Part 1: Jupiter https://www.cloudynights.com/articles/cat/articles/the-discoveries-of-galileo-%e2%80%93-part-1-jupiter-r3277
The Discoveries of Galileo – Part 2: Sunspots and Their Movements https://www.cloudynights.com/articles/cat/articles/the-discoveries-of-galileo-%e2%80%93-part-2-sunspots-and-their-movements-r3281
The Discoveries of Galileo – Part 3: The Moon https://www.cloudynights.com/articles/cat/articles/the-discoveries-of-galileo-%e2%80%93-part-3-the-moon-r3287
The Discoveries of Galileo – Part 4: Venus https://www.cloudynights.com/articles/cat/articles/the-discoveries-of-galileo-%e2%80%93-part-4-venus-r3292
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