Galileo's sunspot letters to Mark Welser


Galileo's letters on sunspots were written to Mark Welser in 1612, and published in 1613 :

Galileo Galilei
Istoria e Dimostrazioni intorno alle Macchie Solari …
(Giacomo Mascardi, Roma, 1613)

A facsimile reproduction of the edition without Scheiner's letters was reprinted in 1967 by Culture et Civilisation, in Brussels.

The High Altitude Observatory has reproduced the title page of this book at its Web site, and the British Library has a more detailed description of their copies of the book.

Jim Mosher has pointed out to me that the book is now available in HTML form at the IMSS website. For instructions on obtaining this version, see here. Jim also points out that the original is available in scanned form from Gallica; this link will bring up the title page, from which you can proceed to the rest of the volume. Gallica even has a second copy scanned at this URL. (If these links don't work you can easily find them by searching Gallica under Auteur = Galileo.) So if you read Italian, you're all set.

A partial English translation appears on pp. 89 – 144 of

Stillman Drake
Discoveries and Opinions of Galileo
(Anchor Books, Garden City, NY, 1957)

Finally, some years after I first wrote this page, Galileo's sunspot letters (as well as many other related documents, including those written by Christoph Scheiner about sunspots) were fully  translated into English and published in

Eileen Reeves and Albert Van Helden
On Sunspots
(University of Chicago Press, Chicago, 2010)

Galileo's observing methods

Initial attempts

On p. 10 of Galileo's book (translated on p. 91 of Drake's), Galileo says “The spots seen at sunset are observed to change place from one evening to the next, descending from the part of the sun then uppermost, and the morning spots ascend from the part then below …”, which suggests that he observed at sunrise and sunset, not in the middle of the day. (On the other hand, it may refer to Scheiner's observations rather than his own; Scheiner published a drawing showing the same spots at sunrise and sunset, in the the original sunspot letters that Welser had asked Galileo to comment on.) This is reported in his first letter to Welser, dated May 4, 1612. (I have quoted here from Drake's translation.)

It's also clear that Galileo observed directly through the telescope, as he says (on p. 13 of Galileo's book, translated on p. 92 of Drake's): “Now consider the fact that when we look at the brilliant solar disk through the telescope, it appears much brighter than the field which surrounds it …”.

In this first letter, Galileo complains of his “inability to make many continued observations,” suggesting that (as anyone readily finds, who tries to observe sunrises and sunsets) the scattered clouds around the horizon that frequently block one's view of the low Sun were preventing him from observing the spots on enough consecutive days to ascertain their motion with confidence. This also suggests that he never tried to observe when the Sun was high; in a Mediterranean climate, such as Galileo had (and such as we have in San Diego), it often happens that the sky is clear in the middle of the day, but overcast in the mornings and evenings. This fact is probably not obvious to the many readers of Galileo's work who do not themselves live in such a climate.

Further evidence of Galileo's initial observing technique is on p. 22 of his book, in a section dealing with specific observations that Drake left untranslated. Here, Galileo refers to drawings of particular sunspot groups: “The spot A which occurred at sunset on the 5th of April … was seen the following day at sunset,” and so on. Likewise, “On the 26th of the same month, at sunset there began to appear … .” (Here, the translation is mine; you will find this passage on p. 99 of On Sunspots.)

In addition, there is a letter from Galileo to Maffeo Barberini, dated 2 June 1612, less than a month after his first letter to Welser; see pp. 981 ff. of

Galileo Galilei
Opere. A cura di Ferdinando Flora
(R. Ricciardi, Milano, 1953)

Here, Galileo says (my translation): “For about eighteen months, I have been watching with the telescope some fairly dark spots in the body of the Sun, when it was near its setting … .” My Italian isn't great, but there's no ambiguity about his phrase “… quando era vicino al suo tramontare …”. [Cf. p. 338 of On Sunspots.] Here he only mentions observing near sunset; and, taken with the sunset observations mentioned in the first letter to Welser, I think this is fair evidence that Galileo only observed the Sun directly near sunset.

There is a good reason for Galileo to have observed only near sunset, and not at sunrise. In Florence, the Sun rises over the Apennines, and would be too high and too bright to look at directly by the time it clears the mountains. So it is obvious that most, if not all, of Galileo's direct observations were made just before sunset, and so were limited to those days on which the western horizon remained clear until then.

Fortunately, Galileo's pupil Benedetto Castelli soon discovered a better way to observe, which made it possible to work in the middle of the day, and hence to obtain continuous daily records of the spots, such as are published later in Galileo's book. This discovery seems to have been made just as Galileo was putting the finishing touches on his first letter to Welser, for it is mentioned only in passing in the closing paragraph, where Galileo asks Welser to pass his greetings to the pseudonymous “Apelles”, “and tell him that in a few days I shall send him some observations and diagrams of sunspots which are absolutely exact both as to their shape and their variation of position from day to day, drawn without a hairsbreadth of error in a very elegant manner discovered by a pupil of mine …”.

And, once his student has developed the projection technique, which allows observations to be made at any time — in particular, in the middle of the day, when the Sun is more likely to be in the clear — Galileo sends with his second letter a set of daily  drawings of the Sun, for every day in June but three! (This is remarkable: climatological data for Florence in June show that, on the average, there are 7 rainy days in that month, and only 66% of the possible hours of sunshine. Many of these observations must have been made between clouds in the middle of the day. For comparison, June is “fog season” in San Diego; the local weatherpeople on TV refer to “June gloom.”) So, switching from direct observation to projection immediately allowed Galileo to follow the motion of the spots across the Sun in detail, day by day.

Projection method

On p. 52 of Galileo's book (translated on p. 115 of Drake's), Galileo explains the new method in his second letter to Welser, dated August 14, 1612 :

… I shall now describe the method of drawing the spots with complete accuracy. This was discovered, as I hinted in my other letter, by a pupil of mine, a monk of Cassino named Benedetto Castelli. …

The method is this: Direct the telescope upon the sun as if you were going to observe that body. Having focused and steadied it, expose a flat white sheet of paper about a foot from the concave lens; upon this will fall a circular image of the sun's disk, with all the spots that are on it arranged and disposed with exactly the same symmetry as in the sun. The more the paper is moved away from the tube, the larger this image will become, and the better the spots will be depicted. Thus they will be seen without damage to the eye, even the smallest of them — which, when observed through the telescope, can scarcely be perceived, and only with fatigue and injury to the eyes.”

He goes on to describe more details of the projection method; but this is enough for present purposes. Note that Galileo was aware of the danger of observing the Sun directly, and took care to avoid it. This agrees with Mulder's remarks on this subject.

From the remarks Galileo himself made about his solar observations, it is apparent that he was cautious about observing the Sun directly through his telescope, that he did so only when the Sun was near the horizon, and that he ceased to look at the Sun directly as soon as the much better projection method was available. And there is no evidence at all that he injured his eyes in this work.

Orthography and typography

The book itself has some features of interest to typographers. Not only are the letters U and V used almost interchangeably (as was common at the time in Latin, as well as Italian); there are several other curiosities. The use of a tilde-like squiggle over a vowel to indicate the omission of the following letter (usually N) was also fairly common.

But not only is a final “ii” often printed as “ij” — a variant I have seen, for example, in Athanasius Kircher's book — it occasionally appears as ÿ, which I have previously seen only in Dutch. Another peculiarity is the use of the character ß for a long-s followed by a short-s. This survives in German, but I have never seen it outside that language before.

Copyright © 2000 – 2003, 2005, 2006, 2012, 2017 Andrew T. Young

Back to the Galileo and solar eye-safety page
Back to the GF home page, or the Overview page