Galileo Galilei Linceo was, among other talents, a solar physicist. While not the first to observe sunspots, he sketched (in 1612) some of the earliest surviving tracings of sunspots. Observing their daily motion, he deduced that a) they were on or near the surface of the sun and b) that the sun was rotating.

Here is an animated GIF of the sunspots highlighting the sun's rotation. This is built from scans of his drawings archived at galileo.rice.edu. Writes Edward Tufte on Ask E.T.: "... That website (by Albert Van Helden and Elizabeth Burr) on Galileo is superb, a model for a teaching and research site. It presents a great amount of material about Galileo and about science in the 17th century. [...] My students in evidence design at Yale over the years have also made sunspot movies, scanning the original engravings from Galileo's Istoria e dimostrazioni intorno alle macchie solari (Rome, 1613). I show one of those student movies in my one-day course.



Galileo's sunspot drawings are also notable for one crucial missing detail-- one that, ironically enough, reinforces his scientific integrity. Over 40 days, he drew what he saw-- except on two days. Those data (June 4, June 30) are missing from his observations.

Why Galileo At All?

For all that Galileo discovered with the Moon, Jupiter, its moons, and physics in general, Tufte suggested (in his talk yesterday) that Galileo's most significant contribution may have been establishing the idea of observationally driven science.

This is what we use in science today. Instead of simply arguing ideas based on earlier concepts, a theory has to match observations. If so, it's sound and you keep it until either new data proves it wrong or a better theory is available. In the latter case, the new theory has to better match existing data-- with the same degree of rigor as the original theory.

Modern science also prefers theories that are disprovable. To paraphrase Karl Popper, if a theory cannot be disproved, it's not science. (Sorry, IPU fans!) That doesn't mean open speculation and wild ideas aren't useful, but they don't pass the bar to being 'science'*.

Any good theory allows predictions. That bit is what gives theories utility-- they don't just explain the past but give us insight into possible future uses. It's this bit that makes astrophysics relevant to society: theories on solar spectral lines led to engineering solutions for everything from lasers to atomics, for example.

Finally, all else being equal, the less complicated theory is favored (Occam's Razor). Put optimistically, the fewer extra assumptions required to make a theory work, the better.

In the context of current trends, it's important to keep all these pieces:

• explains the data
• disprovable
• allows prediction
• simplicity

The Missing Days

As for why that two days are missing from Galileo's data, here are six hypotheses, going from least to most likely.

1. Galileo deliberately discarded them because they didn't match his theory.
2. Galileo made a numbering error, and there really aren't any missing days.
3. Galileo was sick those days and didn't make an observation.
4. Galileo was too busy having an affair to take data those days.
5. Galileo took the day off.
6. The weather was cloudy and he couldn't observe.

1) He cheated?

First, it would be hard for him to cheat, since his process was to make observations, then derive conclusions. He would have to have discarded it after gathering all the data. However, he includes other frames which are not a perfect, seamless match to the sun's rotation. Since all his data is not perfect, it is highly unlikely (science jargon for 'nigh impossible') that any 1 frame would warrant censure.

Further, we can deduce that Galileo had integrity. Leaving a day out is noticeable. He did not fake data for that day, nor renumber the other days. By omitting the data, he (ironically enough) reaffirms that a) the omission is real and b) the rest of his data is trustworthy.

Finally, any unusual data would in itself be worth speculating on, and perhaps have even more publishable value than the rest of the data. Put another way, if I took 10 images of a CME and 1 contained a UFO, my first thought would not be "discard the non-CME". It would be 'woo hoo, book me on Oprah!' And I'd also get 2 papers out of it, not just 1.

2) He erred in numbering?

This is just plain silly given the high precision and great accuracy apparent all of Galileo's other work. They are, after all, 'numbered' with the day and month of the observation. Just as we can discard "he cheated", we can discard the idea that he'd make such a noticeable error.

3) He was sick?

Very unlikely. If he were so ill that he could not make 1 daily sketch, it is likely he would have been ill for more than one day. A gap of 1 frame is too short to support that it was skipped for health reasons.

4) He was getting laid?

I just put this theory in because I like it. Perhaps he was so distracted by an attractive paramour that he never quite got out of bed to make that day's observation. But I must say, as a male scientist, that he would have had plenty of time during a refractory period to make the observation and still return to bed for round two (or eight, given he'd have all of daylight to burn.)

5) It was his day off?

Also unlikely, for the same reasons as the earlier ones. These observations did not take a full day, and he was quite the dedicated scientist. There's no historical evidence of a significant life event (arrest, torture, et cetera) that would majorly disrupt his day, and that he was in the same place a day before and a day after supports the idea he was likely available that day also.

6) Bad weather?

This is really the most likely explanation. If it's very cloudy, you can't see the sun. Ergo, no data. This is also the reason for most modern data gaps with ground observations. One hassle of having an atmosphere is weather-- great for life, bad for observing.

The corollary of establishing Galileo's data integrity, is we can now have mild confidence in our backdated weather prediction. During that span of 40 days, it was extremely cloudy on two of them, clear the rest.

In fact, dare we venture into rampant speculation? I say yes! Science tells us that 397 years ago... it may have rained.

(And that, my friends, is why Galileo was Galileo and I am just... Alex, the daytime astronomer)