WHAT GALILEO SAW

The various astronomical objects portrayed on this website differ greatly in size; Mars at its largest, for example, being no larger in apparent diameter than a moderately small sunspot. To aid in understanding the relative resolution achieved with the current telescope on the various objects and how they compare to what Galileo saw, a sampling of our photographs is reproduced here, all at their original scale of about 0.73 arc-sec per pixel, together with drawings of the same objects found in the National Edition of Galileo's works shown, as nearly as possible, at the same scale. Because of their large size, only small portions of the Sun and Moon are shown. It is evident from this comparison that what Galileo saw must have been rather similar to what we see through the modern reproduction of his telescope.

All Galileo's drawings, with the exception of his watercolor of the Moon and the starfield sketches shown at the bottom of the page, have been extracted from the Galileo//Thek@ website hosted by the IMSS in Florence, Italy. It should be noted that, with the exception of innumerable drawings of Jupiter's moons, a great deal of Galileo's legacy of visual observations appears to have been lost. For example he published many plates of sunspots in The Sunspot Letters to Marc Welser, yet few, if any, of the original tracings made at the telescope seem to have survived. Similarly we know from the plate in Il Saggiatore that Galileo made many careful observations of the phases of Venus, yet we do not know of any manuscript pages containing the original drawings from which the plate was made.

Venus

Left: Venus on May 12, 2004. Center: Venus on April 25, 2004. Right: Drawing of the phases of Venus from Galileo's Il Saggiatore (1623). Galileo saw a raggedness along Venus' terminator that is only vaguely suggested in the photographs. His drawings of Venus are reproduced from Volume 6, page 361 of the National Edition. The height of Venus in the center photo is 34 arc-seconds. For additional details regarding the Venus photos see the Venus page.

Mars

Left: Mars on September 13, 2003 when 24 arc-seconds in diameter. Galileo appears not to have detected the surface markings visible in this enhanced photograph. Right: Drawing of Mars from Galileo's Il Saggiatore (1623). This drawing is reproduced from Volume 6, page 361 of the National Edition. For additional details regarding the Mars photo see the Mars page.

Jupiter

Left: Jupiter on February 14, 2003. Right: Drawing of the disk of Jupiter from Galileo's Il Saggiatore (1623). Galileo appears not to have detected the belts visible in this enhanced photograph. His drawing of Jupiter's disk is reproduced from Volume 6, page 361 of the National Edition.

Below top: The moons of Jupiter photographed on November 28, 2002. A normally exposed image of the area around Jupiter's disc has been pasted over the highly overexposed original. The configuration of Jupiter's moons changes continuously. For more details regarding this photo, which shows a tighter than normal grouping, see the Jupiter page.
Below bottom: One of Galileo's numerous drawings of the moons of Jupiter. This particular drawing represents a configuration similar to that shown above, which he saw on March 4, 1613 in the 3rd hour after sunset. It is reproduced from a chart that appears in The Sunspot Letters to Marc Welser (1613). Note that Galileo indicates the (unresolved) moons appeared to him much smaller than the planet, which, in the photograph, is about 40 arc-seconds in diameter. Galileo's telescope offered enough resolution for him to distinguish the tiny moons of Jupiter even as they passed quite close to one another, as they frequently do, a feat which is complicated by the bright glare from the disk of the main planet.

Famed Belgian amateur astronomer Jean Meeus concluded from an examination of the diagrams of Jupiter's moons in Sidereus Nuncius that the glare of unfocused light from Jupiter's disk prevented Galileo, at that time, from seeing the moons unless they were a minimum of 3.5 to 4 Jupiter radii from Jupiter's center (Sky and Telescope, Feb. 1964, pp. 105-106). If this is true, then by 1613 Galileo must have improved his telescope considerably, for in the diagram from which this illustration is taken he shows examples of moons as close as 1.3 Jupiter radii from Jupiter's center (March 1, 1613), and also indicates he was able to resolve the moons at separations of 10 arc-seconds or less from one another (March 6, 1613).

Saturn

Left: Saturn on December 1, 2002. The main ball of Saturn is 20 arc-seconds in diameter. The rings and the dark lobes separating them from the disk of the planet are between 6 and 7 arc-seconds in width. Right: Drawing of Saturn from Galileo's Il Saggiatore (1623). The aspect of Saturn's rings changes drastically as the planet tilts towards and away from our line of sight. In this undated image, Galileo appears to be drawing a configuration very similar to the modern photograph in which the lower part of the ring is in front of the planet. Galileo sees the rings clearly, but perceives them as a flat platter with the round disk of Saturn placed on top of them. Galileo's earliest drawings of Saturn, in which he perceived the rings as balls on either side of the planet's main disk, appear more crude based on our modern understanding of the planet's true appearance. This drawing is reproduced from Volume 6, page 361 of the National Edition.

Mizar

Left: Mizar A and B photographed on April 28, 2004. Right: A simulation of the image of Mizar A and B expected from a diffraction-limited 38 mm aperture singlet refractor. This simulation was produced by full color geometric ray tracing. It differs from the photograph because it assumes a significantly larger aperture. This aperture was chosen not only because it matches the diameter of Galileo's "Discoverer" lens preserved in Florence, but also because the simulated image closely matches Galileo's description (in Latin) of how the two stars appeared to him: he saw them as having apparent diameters of 6 and 4 arc-seconds with a gap of 10 arc-seconds between them (National Edition, Vol. 3, page 877), for a total separation between centers of 15 arc-seconds. The apparent stellar diameters reported by Galileo are similar to what we see in our photograph, indicating his telescope provided resolution on the same order or better. As demonstrated by the simulation, if Galileo's telescope had a 38 mm objective it would indeed be expected to split the stars somewhat more cleanly than ours with its 23 mm objective. It is also interesting to note that the simulation assumes white stars. As in the real telescope, the green color arises because at the selected focal position the red and blue light are highly dispersed by chromatic aberration. Altering the focal position changes the apparent color of the stars as different wavelengths come into and out of focus. This color change is much more apparent to the camera than to the dark-adapted eye, which sees both stars as nearly white. Mizar B revolves around Mizar A with a very long period (thousands of years). The separation between the centers of the stars was probably an arc-second or so less in Galileo's time than in the modern photo. For more details of Galileo's description of these stars see our Mizar Page. For more accurate simulations using a Fourier transform technique, and showing exactly how the separation between the stars would be expected to vary with aperture, see our Focus Page.

Trapezium

Left: The Trapezium region in Orion photographed on December 30, 2004. The Trapezium is the cluster of closely-spaced stars at upper right, including those labeled c, g and i in Galileo's drawing. For comparison with Galileo's drawing, the photo was framed to include the two field stars at lower left that Galileo labeled a and b. Right: Galileo's notebook drawing of the Trapezium region, dated February 4, 1617, enlarged to the same scale. This drawing, together with Galileo's explanation of it (in Latin), appears on Vol. 3, page 880 of the National Edition. Galileo's labels for the the five stars, originally in black, have been changed to red to clarify his diagram. This drawing of the Trapezium star field was made about seven years after Galileo's first crude drawing of the stars comprising Orion's Belt & Sword shown below.

As a graphic demonstration of the resolving power of Galileo's telescope, this very clear and accurate diagram of the Trapezium is perhaps even more impressive than his verbal description of Mizar. However the words accompanying the diagram suggest that the apparent gap he observed was, as suggested by the drawing, much smaller than what he reported between Mizar A & B. The spacing of the Trapezium stars is similar to Mizar, but they are fainter, and therefore more difficult to observe. The current distance from star g to stars c and i is about 13 arc-seconds. The digital camera seems to see a little larger gap between these stars than that portrayed in Galileo's drawing. The camera even faintly reveals an 8-th magnitude eclipsing binary star 9 arc-seconds above and to the left of star i. This star was apparently either too dim for Galileo to see, or not resolved by his telescope.

Galileo portrayed the relative positions of the five stars he shows with stunning accuracy. In a properly scaled drawing the distance from the center of star g to the centers of either star c or i is almost exactly one-tenth the distance from b to g. Galileo is one of the very few astronomers, modern or historic, to have ever drawn this proportion accurately. The size of the Trapezium is almost always, consciously or unconsciously, exaggerated relative to the neighboring stars. Our Trapezium page shows how when Galileo's drawing is laid over the photograph the registration is almost exact. Since the Orion nebula is one of the youngest and closest star-forming regions in the sky, some might argue that the agreement is too good: over 400 years one might expect to see some changes in the relative brightness and/or positions of the stars. Although Orion does indeed include rapidly evolving objects, it happens that the proper motions of the bright stars shown here are all quite small, and the changes in relative position from Galileo's time to the modern epoch are expected to be at most an arc-second or so. On the other hand, star a appears to have become significantly dimmer than it was in Galileo's day.

Perhaps significantly, Galileo makes no mention of having noticed the now well-known gas cloud, M42, surrounding the Trapezium stars. Such a feature is more apparent to the eye, with its wide dynamic range, than to our camera (it cannot be seen in the photo shown above), but as with the belts of Jupiter and the (extremely faint) surface markings on Mars, Galileo does not appear to have excelled at detecting low contrast features.

Moon

Left: A small fragment of the Moon as photographed through the Galilean refractor on November 26, 2002. This is reproduced at the same pixel scale as the other photographs on this page. The three craters in the connected chain are Ptolemaeus (top), Alphonsus and Arzachel (at the bottom with the prominent central peak). Albategnius, the crater thought by some to be featured so prominently in the engravings in Sidereus Nuncius, is the large shadowed crater to the right of Ptolemaeus. The small well-resolved crater above the point where Ptolemaeus and Albategnius appear to touch is Muller. Its official diameter of 22-26 km would make it 12-14 arc-seconds in diameter at the time of this photo. Our camera was not able to detect Ammonius, a 8-9 km (4-5 arc-sec) diameter crater on the floor of Ptolemaeus. Herschel, the very prominent crater at Ptolemaeus' upper left is 40-46 km in diameter. Giorgio Abetti specifically mentions Herschel as an example of a crater in which he and Hale were able to see a fair amount of detail when looking through Galileo's broken lens in 1923.
Right: A fragment of one of Galileo's seven undated watercolor paintings of the Moon in the collection of the National Central Library of Florence (BNCF). This fragment is taken from slightly below center of the image labeled "4" on Galileo's first sheet of six drawings. It is reproduced to the same scale as the photograph based on the diameter of Galileo's lunar disk. The resemblance of the chain of craters drawn by Galileo to that in the photograph may be accidental. If they are the same, then Galileo has drawn them somewhat larger than life, which would be easy to do since he could see only a small part of the Moon at a time. The Moon's phase at the time of the painting would also be slightly different from that at the time of the photograph, with Albategnius being revealed only by the brightly illuminated peak deep in the shadows to the right. Another drawing on the same manuscript page (the one labeled with the sideways "5") appears to show the same chain of craters fully illuminated with the terminator more to the right. The current photograph is evidently somewhere between the two in phase. The blurriness of the present image of Galileo's painting is partly due to its having been captured from the BNCF website and blown up in size to match the desired scale. However, although a better photo might show the texture of the paper a little more clearly, it would be unlikely to reveal much more lunar detail, for the entire Moon's disk, of which this is only a very small portion, is reportedly only a little over 5 centimeters in diameter. Even if he had attempted to draw his pictures on a larger scale, it would have been impossible for Galileo to show more than representative examples of the detail visible through his telescope, for the quantity and range of detail seen over the whole disk of the Moon is quite overwhelming. An accurate representation of what could be seen on the Moon through Galileo's telescopes was not achieved until 1636 with the aid of a professional artist (see below).

Over the years many attempts have been made to identify the craters depicted in Galileo's drawings. Opinions vary widely, even among highly-regarded lunar experts. Some feel Galileo meant only to convey the general character of the lunar surface as seen through his telescope and that the features he shows have no exact correspondence to the actual Moon. Others feel each feature portrayed by Galileo can be identified very exactly. The most thorough study is probably that of Ewen Whitaker published in the Journal for the History of Astronomy 9, 155-169 (1978). Whitaker, a highly-respected research scientist at the University of Arizona's Lunar and Planetary Laboratory, felt he could pinpoint the exact day and hour at which each of Galileo's drawings was made. He believes the watercolor shown above was made at 7 am (Padua time) on the morning of December 18, 1609. Whitaker's dating is based partly on his belief that these paintings, which appear on a single sheet of folded paper bound with a manuscript copy of Sidereus Nuncius, must have been the prototypes for the engravings published in that book. However, there is some doubt about this, and since there is no text accompanying the paintings, even their attribution to Galileo does not seem to known with absolute certainty; although the other panel of the folded sheet bears a horoscope diagram thought to be in Galileo's handwriting. Whitaker's precise dating of Galileo's images relies heavily on his (Whitaker's) purported identification of the features depicted in them. There is considerable reason to suspect that these identifications are much less certain than Whitaker makes them seem, and hence that the dates are also less certain. It seems to be from Whitaker's article that the romantic notion has arisen (now repeated so many times that it is accepted by many as fact) that Galileo, paintbrush in hand, first turned a newly made 20-power telescope on the night sky on the evening of November 30, 1609, instantly recognizing the topography of craters and mountains written in the ragged pattern of light falling on the crescent Moon. While it might be nice to believe this, we suspect Galileo's telescopic explorations of the night sky began in much less orderly fashion, probably with his very first telescopes, certainly sometime been August and December 1609, but with no clearly defined starting point; and that, as with most mortals, his understanding of what he was seeing developed gradually. For more information about the dating of Galileo's Moon observations, please see our separate Moon Page.

A very similar phase of the Moon to the one illustrated above can be seen in this fragment from French artist Claude Mellan's amazing 1636 engraving of the last quarter Moon. This engraving is of interest here because it was made during Galileo's lifetime and there is good evidence to believe it is based on observations made through a telescope constructed from lenses given by Galileo to Mellan's patron, French nobleman Nicolas Peiresc and assembled into a telescope by Peiresc's friend and collaborator Pierre Gassendi. The quality of Mellan's engraving reinforces our belief that the limitations in Galileo's lunar watercolors were mostly a matter of their small format and hurried execution. Mellan's engraving suggests that remarkable detail could indeed be seen through Galileo's telescopes. The features shown in the small portion of the engraving reproduced here (which is shown at one-half the scale of the other illustrations on this page) are virtually identical to what we are able to photograph through our modern replica telescope. Click here or on the image to visit our separate page giving a view of full engraving and a more complete description of how it was made.

Sun

Above Left: A small fragment of the Sun showing a fairly large spot group near disk center as photographed through the Galilean refractor on July 21, 2004. This fragment is reproduced at the same pixel scale as the other photographs on this page. Above Right: A small fragment of an engraving of the Sun for August 21, 1612 from The Sunspot Letters to Marc Welser showing a similar large spot group near the Sun's center reproduced at the same scale (based on the diameter of Galileo's solar disk). Since Galileo traced the spots directly on the projected image their size in the original drawings should have been very accurate.

The resolution (sharpness) achieved in our solar photographs is not quite as good as in the others. Even through a one-inch telescope the resolution of solar images tends to be limited by the blurriness of the Earth's daytime atmosphere. Galileo's observations of the Sun were made from Florence, Italy. He appears to have enjoyed better daytime seeing conditions than we have experienced in Vancouver, Washington. However, the uncanny resemblance of some of his sunspot drawings to modern high resolution photographs with apparent light bridges, umbral dots, and penumbral fibrils may be an artifact due to the texture of the paper on which the engravings were printed.

Below Left: A small fragment of the Sun showing a large spot group near the Sun's limb as photographed through the Galilean refractor on October 25, 2003 (reproduced at the same pixel scale as the other photographs on this page). Below Right: A fragment of the same engraving showing a similar large spot group near the Sun's limb, reproduced at the same scale.

Rice University's Galileo Project has rather nice photographs of 35 of the sunspot plates from The Sunspot Letters to Marc Welser, somewhat different from those in the on-line National Edition reproduced above. Although smaller, they show the penumbra (the delicate shading around the intensely dark centers of the spots) better. The Rice University copies cover the period June 2, 1612 through July 8, 1612 (although labeled as being from 1613 they are actually from 1612). The Sunspot Letters includes three additional plates from August 1612 for a total of 38 engravings covering June 2, 1612 through August 21, 1612. Yet another version of the June 23, 1612 engraving is available on the High Altitude Observatory's Great Moments in Solar Physics page. Very high resolution photographs of three of the plates from the Sunspot Letters are available on the University of Oklahoma's Galileo Sunspots Exhibit.

Small hand drawn sunspot diagrams dating from February 12 through May 3, 1612 can be found among Galileo's notes on pages 253 and 254 of Volume 5 of the National Edition. In these ink drawings Galileo's technique for representing the spots can seen progressing from formless black blobs to shapes with considerable structure, although the never achieve the polish of the engravings in the Sunspot Letters. Corrections and repeated attempts to portray some of the groups suggest these may be original observations rather than a compilation of earlier drawings. Galileo enclosed much finer drawings from May 3 to 11 in a letter dated June 2, 1612 to Maffeo Barberini, the future Pope Urban VIII. These drawings, presumably copies of now lost originals which Galileo would have retained for his own records, are preserved in the Vatican Museum. Curiously, although dated in the same hand, the May 3 drawing sent to Barberini differs significantly from the drawing for the same date on page 254 of the notebook series. It not only shows the same large central group in greater detail but also shows many smaller spots not present in the other drawing while minimizing two groups that figure more prominently in the latter. This suggests that the larger and more detailed drawings, which very closely resemble those in the Sunspot Letters, may have been prepared by a different observer or using different instrumentation, such as projection versus direct observation through the eyepiece.

In addition to his own sunspot observations, Galileo received drawings from others, such as the 26 daily drawings for February 18 - March 23, 1612 enclosed in a letter from his artist friend Cigoli. Sunspots appear to have already been well known in Italy by this time, since Cigoli expresses no surprise at their existence. Indeed, since Cigoli was in Rome and Galileo in Florence, the exchange of drawings was presumably part of an effort to fill in observations for days that may have been cloudy at one location or the other. Cigoli made his February-March drawings, which are not of particularly high quality, by placing a thick green glass over the eyepiece of his telescope. By July Cigoli knew about, and was using, the technique of eyepiece projection with which much more accurate images, such as those in Galileo's Sunspot Letters could be easily recorded, although he does not seem to have learned of it from Galileo. We do not know if any of Cigoli's sunspot drawings made by eyepiece projection have survived.

Galileo's Sunspot Letters were part of a heated public dispute with Jesuit astronomer Christoph Scheiner in Germany over both the nature of sunspots and who had been first to observer them. Unknown to either of them (as told by Albert Van Helden on Rice University's Galileo Project webpages) they had both been preceded by David and Johannes Fabricius, who had already published a little-noticed book about sunspots before the Galileo-Scheiner dispute even began. All of these had been preceded by the English mathematician Thomas Harriot, who, according to Albert Van Helden, made excellent, but unpublished, drawings of the sunspots as early as December 1610.

Each of Cigoli's 26 sunspot drawings is accompanied by a date and time. It may seem surprising to see a sunspot observation on, for example, March 17 at "ore 23" (= hour 23). This only indicates that the conventions of timekeeping are not as obvious as they seem to us today. We are not experts on this, but we understand that there was little standardization in renaissance Europe In some cities the day started at local noon. In others it began/ended at sunrise or, more commonly, sunset. Since Cigoli's hours range from 12 to 23 he is probably taking the local sunset as his 0/24 hour point, and "ore 23" probably means one hour before sunset. Galileo himself seems to have used a variety of systems. He usually notes whether the time recorded for an observation of Jupiter or the Sun is hours after sunset, hours past midnight, or hours before sunrise. Although these systems may seem odd to us, modern astronomers continue (probably mostly unconsciously) to use at least one of them, for their Julian days begin at noon in Greenwich, England even though Universal Time is counted from midnight there.

Cigoli's drawings are also notable for the fact that at the bottom of each drawing he gives his daily sunspot count. Cigoli's counts long precede the modern Wolf sunspot numbers invented some 200 years later. We do not know if Galileo continued the practice of recording a daily sunspot count, but he seems to have been more interested in the spots' motions than in their numbers.

Image Scale

The star fields from Sidereus Nuncius shown below this line are at a completely different scale. In most of those diagrams Galileo represents a field of about 1 degree (60 arc-minutes) diameter in the sky. However, Galileo's diagram of Orion's Belt and Sword covers a diameter of about 5 degrees.

Galileo's Other Drawings

In addition to the "high-resolution" drawings shown above, Galileo is also known for the four sketches of extended star fields which appear in Sidereus Nuncius. Unlike most of the drawings shown above, Galileo had to stitch these together by combining many small fields of view. As with his drawings and engravings of the Earth's Moon, distortions of scale are inevitable. It is fairly easy to correlate Galileo's drawing of the Pleiades with modern photos of this region, but the other three drawings tend to be much more confused. Aside from the Pleiades we have not attempted to reproduce these fields photographically. The crudeness of these very early efforts stands in stark contrast to the exactness of most of Galileo's later observations; although in those cases where the hand drawn prototypes still survive, it is evident that at least part of the crudeness was introduced in the translation of the original observations into printed plates.

Click on the following thumbnails to see the full-size printed versions of the star fields as they appear in the LiberLiber on-line HTML version of Sidereus Nuncius. These images are NOT at the same scale as the other pictures shown on this page. They cover much larger areas of the sky.

This was Galileo's first attempt to demonstrate the ability of his telescope to reveal stars beyond the limit of normal vision. The large open star symbols represent the nine naked eye stars listed in Ptolemy's star catalog. To these Galileo has added 80 new stars. Although he says he has preserved their relative spacings and magnitudes as nearly as possible, identification of these stars with modern photographs is uncertain at best. There are several hand drawn prototypes of this diagram among Galileo's manuscripts, some even indicating the distance, in arc-minutes, between various stars; but it remains a mystery why Galileo chose to depict the particular stars he did. They are rarely the ones one would pick out as the most striking in a modern star chart. Galileo's clearest drawing of Orion's belt and sword is found immediately following his journal entry regarding the configuration of Jupiter's moons on February 7, 1610. In that drawing, the stars of the sword region are particularly easy to identify. He does not seem to have made any special note of the Trapezium stars. The telescope he used to make this drawing may have been incapable of resolving them.

Galileo is here demonstrating how a feature that had been known to the ancients as a fuzzy unresolved patch of "nebulosity" is resolved into individual stars by his telescope. This drawing refers not the modern "Orion Nebula" (M42 -- which is located in Orion's Sword and contains the Trapezium stars) but rather to an open cluster, also known as the "Head of Orion," that appears above Orion's shoulders in the vicinity of the star 39 Lam Ori. It is well outside the field of Galileo's previous drawing. In modern nomenclature this cluster is sometimes referred to as Collinder 69 or Sharpless 2-264. As pointed out by Ewen Whitaker in his article on dating the Moon pictures, a hand drawn original diagram of these 21 stars can be found in Volume 3, page 964 of the National Edition. The original is much clearer and, in addition, shows the relationship of this star field to Orion's shoulder stars, Betelgeuse and Bellatrix. In the process of printing Sidereus Nuncius, the diagram was rotated 90 degrees clockwise and the relative magnitudes of the stars, which were shown with reasonable accuracy in the original, have become distorted to such an extent that their identities are hardly recognizable. By consulting the hand drawn prototype it becomes clear that the bright star in the lower left corner of the present diagram is 37 Phi1 Ori. The star following that, after a row of three small stars, is 39 Lam Ori (Meissa). The bright star near the upper left corner is 40 Phi2 Ori. As with Orion's Belt and Sword, even after understanding the general orientation, not all the fainter stars can be identified with certainty; but judging from their number alone, some must be close to magnitude 9: similar to the faintest stars shown in the Pleiades diagram.

As in the diagram of Orion's Belt and Sword, the large open star symbols represent the bright stars visible to the naked eye. The correlation of most of the fainter stars with those shown in modern photographs is fairly obvious, although, as in the Moon engravings, their positions are somewhat distorted. The limiting magnitude seems to be somewhere between 8 and 9.

This is another diffuse region prominent to the naked eye under dark skies and known since antiquity. Praesepe lies in the constellation Cancer, roughly midway between Regulus and Castor/Pollux. In modern nomenclature it is referred to as M44, or, informally, as the Beehive Cluster. Galileo identifies his two naked-eye stars as the Ascelli (43 Gam Cnc and 47 Del Cnc); but the fainter stars appear to fall on the wrong side of the line joining them. The orientation shown here is that appearing on LiberLiber and in the National Edition. Some editions rotate this plate 180 degrees, but the star pattern is still not recognizable. As with the Head of Orion, the plate may have become distorted in the printing. Galileo's count of 36 stars, in addition to the Ascelli, would seem to correspond to a limiting magnitude of about 8.5, similar to that for the Pleiades.

Images (unless otherwise credited) © Tom Pope and Jim Mosher
Last modified: May 2, 2006