Io@400 Part 2: Looking through the telescope in 1609

This week we are looking back at the discovery of Io and the other Galilean satellites by Galileo Galilei 400 years ago this Thursday.  Yesterday, we briefly examined the state of astronomy at the start of the 17th Century and the conflict between different world systems in play.  We also examined how Galileo first developed his telescopes based on a Dutch invention.  Today, in part two of our series of posts in commemoration of this significant event in the history of astronomy and science, we look back at Galileo's first scientific investigations with the telescope in the autumn of 1609.  We also look at Thomas Harriot's similar investigations that year.

These spots have never been observed by anyone before me; and from my observations of them, often repeated, I have been led to that opinion which I have expressed, namely, that I feel sure that the surface of the Moon is not perfectly smooth, free from inequalities and exactly spherical, as a large school of philosophers considers with the regard to the Moon and the other heavenly bodies, but that, on the contrary, it is full of inequalities, uneven, full of hollows and protuberances, just like the surface of the Earth itself, which is varied everywhere by lofty mountains and deep valleys.

- From Galileo's Sidereus Nuncius (trans. by Edward S. Carlos)

The Imperfect Moon

With a copy of his new telescope gifted to the Venetian Republic, Galileo then traveled to Florence in October 1609 to provide the Grand Duke of Tuscany, Cosimo II de' Medici, a demonstration of his 8x power telescope (which he created after receiving new lenses in mid-September 1609).  Cosimo II was a former pupil of Galileo and had become Grand Duke earlier in the year following the death of his father, Ferdinando I.  The visit was intended to smooth over relations with the ruler of his hometown after after he had given a copy of his telescope to a rival of Tuscany in north Italy in hopes that he might get employment at the University of Pisa.  During this visit, Galileo showed Cosimo the view of the Moon through his telescope.  This demonstration also provided support for Galileo's 1606 theory that the bright regions of the Moon contained tall mountains and the dark regions low plains.

Upon his return to Padua in late-October 1609, Galileo set to work creating a more-powerful telescope, this time with a 20x magnification, based on the lens blanks he had received a couple months earlier.  Galileo began his first detailed observations of the Moon, which he later published in Sidereus Nuncius, on December 1 (though other references suggest that he might have begun his first detailed observations in early October).  Again, he found shading on the Moon resulting from topography, hills and valleys that reminded Galileo of the hills and valleys of the Apennines of central Italy.  He correctly interpreted bright spots on the Moon's night-side as large mountains tall enough to reach into the sunlight, akin to sunlit mountains seen from the surrounding twilit plains.  He also correctly interpreted dark spots along the terminator, the dividing line between day and night, as valleys (in the case of the Moon, the floors of impact craters).  Over the next few weeks, Galileo continued to draw what he saw at the moon, noting sunlight slowly filling valley floors as well as the dark plains that seemed to be flat, devoid of topographic shading (though he noted that some were ringed by mountain chains).  These observations contradicted the current prevailing view of the Moon of perfect, smooth world.  The dark maria were attributed as simply being the result of differences in the density of the Moon.  Galileo's observations instead showed an imperfect world, more akin to the Earth with mountains he measured as four miles tall.  He even went so far as to suggest that the Lunar highlands were akin to terrestrial land areas and the maria were actually seas.

In his March 1610 treatise, Sidereus Nuncius, Galileo acknowledged a potential argument against the Moon having substantial terrain.  While he observed evidence for significant topography within the Moon's highlands, its limb appeared to be smooth even at 20x magnification.  With mountains as great as four miles in height, the limb should be lumpy.  Galileo proposed two explanations.  First, he suggested that limb appeared smooth because the mountains were more distant than the ones near the center of the disk and would appear smaller and foreshortened, akin to mountains in the far distance on Earth.  His other suggestion, which he would later accept as incorrect but one I am more familiar with on another moon, was that an atmosphere on the Moon would make the appearance of the limb indistinct, blurring features and topography as emission angle increases.  So in this case, he argued that the limb appeared smooth because we are seeing the edge of the Moon's atmosphere, not the physical horizon. Finally, in the Sidereus Nuncius, Galileo notes Earthshine on the nightside of the Moon, most visible when it is near new moon phase, as well as albedo markings on the dark maria (rays from craters such as Copernicus and Tycho).

During this period of late 1609, Galileo also observed background stars with his new "high" power telescope. He noted that his telescope did not magnify the appearance of stars, like the planets or the Moon.  Instead they seemed to brighten and become smaller.  His important discovery from these observations was the finding that telescopes seemed to bring out stars that were fainter than are visible with the naked eye.  For the Sidereus Nuncius, he highlighted three deep-sky features as demonstrations of this effect: the Orion constellation, the Pleiades star cluster, and the Milky Way.  In the first two cases, he noted that both areas of the sky had significantly more "invisible" stars than visible ones, charting 36 of these stars in addition to the six normally seen with the naked eye in the Pleiades cluster.  In the case of the Milky Way, he found that one what had previously been considered clouds were instead densely packed clusters of stars.  He found similar cases at other stellar "clouds", such as the Orion Nebula and the Beehive Cluster.

In early January 1610, Galileo wrote a letter to a friend in Florence detailing his discoveries to date with telescope, and prepared its conclusion on January 7, 1610 when he made an even more amazing discovery.

In England...

As I mentioned yesterday, news of the invention of the telescope in the Netherlands spread quickly across Europe, with 3x-powered scopes being demonstrated in the courts of Europe throughout 1609.  Word also spread to an English scientist named Thomas Harriot and in early 1609 he purchased a copy while he was working at the Syon House, the residence of the Earl of Northumberland (well, not at the time... the Earl was in prison at the time following the Gunpowder Plot of November 5, 1605).  Harriot recorded the first astronomical observations using a telescope in the summer of 1609, including several illustrations of the Moon.  However, these were not well known at the time as he failed to publish his observations, unlike other astronomers at the time like Galileo, Kepler, and Simon Marius.  These observations included detailed maps of the Moon that would not be matched for several more decades.  Later, he did publish a book on algebra and determined Snell's Law of Refraction independently.

For Tomorrow

One other astronomer who was using a telescope in 1609 and 1610 was Simon Marius.  Tomorrow, January 6, we will take a look at his claim that he was the discoverer of the four largest moons of Jupiter in December 1609.