I am delighted that David Levy is now doing articles in Astronomy magazine. His current article, “A great’s life” (Aug. '09) , is an appropriate one for the IYA2009 as it is about Galileo.

There are some nits, nothing major, that some may think are fair points, though others will see these comments as either wrong or too pedantic.

Per the article…

Here is a really small nit… Galileo was not 78 but 77 when he died. However, he was only about 6 weeks shy of being 78, so 78 is probably more accurate for some, but not for others.

He says Galileo was often ill with arthritis and rheumatism, though I suspect it was something else in addition to these, perhaps.

For sunspot viewing, in addition to what was stated, Galileo would increase the focal ratio by placing over the front lens a cover that had an oval shape hole to it. The greater magnification through a little lens, along with viewing near sunrise or sunset, would also serve to minimize the problem with the Sun’s brilliance.

He states that the fall of 1610 revealed the phases of Venus. This is found in books and implied by Galileo’s own writing, though Galileo words it differently when he wrote about in on Jan. 1, 1611 …

“…around the sun, as do also Mercury and all the other planets – something indeed believed by the Pythagoreans, Copernicus, Kepler, and myself, but not sensibly proved as it now is by Venus and Mercury.”

Actually, Venus only showed a gibbous phase during the last quarter of 1609. He would not likely have noticed a true crescent phase until just after the first few days of January.

Nevertheless, a gibbous phase alone was prove that Ptolemy’s model was incorrect since Ptolemy held that Venus traveled only between the Sun and Earth, thus no gibbous phase could be possible. [Others held Ptolemy’s model but took the position that the path of Venus was only beyond the Sun, so no crescent phase would be possible.]

However, Mercury in July would have been seen with a crescent phase, which suggests he saw both phases for these and now had solid evidence against either view of the Ptolemy system. My guess is that he didn’t see this for Mercury else he would have made it a big point in his writings. [Maybe he did and I’ve missed this point about Mercury’s crescent phase.]

Regarding the Pope’s turn against Galileo, certainly the Dialogue would have ticked the Pope off, and the Pope was unhappy that Galileo had only given one of his three arguments in the closing of the book, but there was another problem. Although many clerics, including cardinals, loved his book, certain others were quickly aligning against him (eg. Jesuit Scheiner). It has been suggested that the Pope was shown the document of 1616 that commanded Galileo not to teach or hold to the Copernican system. This document was unsigned by any party and for good reason. Jesuit Bellarmine had been instructed to get Galileo to agree to abandon his Copernican view. If he did not, then the Dominican commissary would make it formal. This was in document form that required Galileo must not hold, defend, or teach in any way, orally or in writing, the said propositions [Coperniccan views] on pain of imprisonment. And it stated that Galileo agreed. But this phase of the admonishment never took place, which is why it went unsigned. However, the Pope may have mistaken this as evidence that Galileo was violating his agreement. Of course, Galileo had still agreed to abandon the Copernican view, but Galileo argued in his trial that he was not required to not teach it (ie. hypothetically).


I don’t know that Galileo “begged for a lighter sentence”, because I think it was Cardinal Francesco Barberini that commuted the imprisonment at Rome to the Florentine embassy. [Soon, Niccolini managed to get the Pope to agree to move him to the Archbishops home in Siena. Later, he was allowed to live in his house. Interestingly, 3 of the 10 cardinals of the Inquisition would not sign Galileo's sentence, F. Barberini being one of them.]

It was a nice article, but I enjoy Galileo’s history enough to do the nit thing now and then.

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I just ran a Skymap simulation, and it showed no crescent in 1610 at all, except for the last ten days and then barely.

It shows a crescent Venus the summer of 1609 though.

I've often wondered about the historical accuracy of astronomy software.

That matches my software, Starry Night Pro Plus. [Though only about the last 2 or 3 days of Dec. would a crescent be discernable, especially with his scope.]

I feel confident in Starry Night Pro Plus because it accurately portrays the famous May 28, 585 BC (proleptic Julian Calendar) Solar eclipse in Turkey that ended a 15 year war between the Medes and Lydians. More here, which claims NASA confirms an umbral pass (barely) at that location. Give it a try with your software.

Mid July of '09 was inferior conjunction for Venus and it was gibbous by mid October, before he built his telescope. [According to Starry Night]

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Why would it be necessary to have a crescent phase seen? Wouldn't gibbous be enough?

IIRC, A Thousand Pardons taught me this answer. Check with him.

Nevertheless, it's worth a little more elaboration for a thread like this. The gibbous phase for Venus would be enough to thwart the Ptolemy model because it claimed that the orbit of Venus was between Earth and the Sun, thus no gibbous phases were possible. Based on some things I've found, however, he could have placed the orbit of Venus beyond the Sun and still use his model with equal results. Others did prefer Venus to be beyond the Sun. Ptolemy's teleological viewpoint argued that it would be a waste of space to not have Venus between us and the Sun. [I think I'm right on this, but it is only from tid bits here and there that makes me think this is so.]

Thus, discovering both phases killed both versions of the Ptolemy model, and the Church scholars (Jesuits) were quick to change to...... the Tychonic Model. [A model you revealed to me to be surprisingly valid, IIRC. ]

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I guess I'm interested in Galileo's argument/observation/interpretation. Interesting stuff.

I've been reading several books about his life and circumstances. I wish they would make a first rate movie on his life. It is rare to read an article about Galileo and his Church problems and not find some minor or major error.

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