The truth is that Galileo became blind at the age of 72, from a combination of cataracts and glaucoma [see D. Sobel, “Galileo's Daughter,” (Walker & Co., New York, (1999); p. 354]. This had nothing to do with his telescopic observations of the Sun a quarter of a century earlier, which were initially made only near sunrise and sunset, and later made by projection; in neither case could he have damaged his eyes.
Like much of the supposed “information” on the Web, the story about Galileo's blindness is completely untrue. Who's spreading this nonsense? A lot of folks who should know better:
Here are some of the Web sites that spread the false Galileo story:
A company that wants to sell you filters for observing the Sun and so stands to profit from scaremongering about the matter. They have the same error on another page.
A NASA site about the Sun. [This page vanished from the Web after 2010, but the earlier versions are preserved on the Wayback Machine.]
Another defunct site that fed off NASA but fortunately does not appear to be supported by NASA [again, preserved on the Wayback Machine.] I got this story toned down but they continued to say it is “controversial.” And they continued to claim that “Galileo nearly went blind by looking at the sun” on another page.
The Physics Department of the Univerisity of Tennessee at Knoxville has a Web page on Galileo sating that he “eventually went blind, perhaps from damage suffered by looking at the Sun with his telescope”.
Another commercial site
A site maintained by the Washington University Medical School. They don't even have Galileo's age right at the time he became blind! At least they have added a correction — but without deleting the incorrect information.
Another site aimed at teachers. I have written to them 3 times trying to get this corrected, without success.
A physicist who thinks “his studies on the sun damaged his eyes; by the time of his death, he was blind.”
An amateur astronomer who says “Galileo went blind from studying the sun”…
You'll notice that these people who have the story wrong never cite the refereed literature, or name reliable factual sources. It's unfortunate that many of these Web pages are aimed at schoolteachers and students.
Here's the true story, complete with references to authoritative sources. The best discussion I know of is in a book by Dr. Marten Edsge Mulder, professor emeritus of ophthalmology at the University of Groningen.
On pp. 111 – 112 of his book, Dr. Mulder says that Galileo Galilei
. . . is said to have become blind through solar observations. Apart from the fact, that looking at the comparatively feeble light of the sun, when this is almost entirely below the horizon, cannot be compared with incautious solar observations, when the sun is high, it is moreover almost absolutely excluded, that this could have been the cause of Galilei's blindness. I don't know the source of this statement, but it is certainly incorrect.
Galilei became blind, when he had already attained the age of seventy-three. Before that time, he seems to have seen perfectly well, for there is no mention of bad eyesight in his biographies. …
Now we know that Galilei already in 1612 (at the age of 48) began his studies on sun-spots, which appeared in 1613 . …
Anyone, who is at all familiar with eye diseases, knows that affectations of the sight, caused by incautiously looking at the sun, which often happens at solar eclipses, will at once show themselves by more or less defective vision in the centre of the field of vision, as a so-called central scotoma. If the affection is strong, then reading is sometimes permanently impossible, but the rest of the field of vision remains good. Absolute blindness, so far as I know, has never been caused by it. If Galilei's eyesight had been affected by solar observations, then it would certainly have happened much earlier, when he made his studies on sunspots, at all events before his 70th year, before his lawsuit at Rome.
But at that time, it seems, his sight was perfectly good. He is reported to have read aloud his renunciation.
After his condemnation at Rome, when he was compelled to reside at his villa at Arcetri, near Florence, Galilei does not seem to have made further solar observations, but to have devoted himself mainly to the publication of his studies. If he had done so, if his left eye had become defective by gazing at the sun, then he would certainly have noticed this himself at once, and not afterwards have exposed his right eye to such danger. In any case, he would never have become totally blind.
Further evidence that Galileo's sight was not affected by his solar observations comes from his sunspot letters to Mark Welser. In the first letter, dated May 4, 1612, he mentions observing the Sun directly — but only at sunset. In its final paragraph, he mentions that his pupil Benedetto Castelli has discovered a better way to observe; this projection method is described at length in the second letter, dated Aug. 14. Galileo is so enthusiastic about the superiority of the projection technique that it seems unlikely that he ever observed the Sun directly again. The third letter is followed by numerous drawings of Jupiter and its satellites, observed by Galileo the following year. He could hardly have made these observations if his eyes had been injured in observing the Sun.
I might add that even much later, after becoming blind in one eye, Galileo was still able to observe well enough with the other to discover the libration of the Moon (see Sobel, p. 355).
Furthermore, Galileo says in his first letter to Welser that he has observed the sunspots “for about eighteen months, having shown them to various friends of mine, and at this time [May] last year  I had many prelates and other gentlemen at Rome observe them there.” If his technique of observing the Sun directly through his telescope at sunset were really dangerous, you'd expect that some of these many people would have suffered eye injuries, and complained about it, even if Galileo himself had escaped by chance. And you'd also expect that Galileo would then have taken care to warn Welser to be careful in observing the Sun. But there is no indication of anything like this at all.
Finally, Jim Mosher points out that Galileo eventually became blind in both eyes, which is hardly likely if the cause had been his telescopic observations. The reason — which I have never seen mentioned explicitly, but which makes excellent sense to anyone who has spent years looking through eyepieces — is that telescopic observers habitually observe with just one eye. You'd think he'd have injured his “observing eye” if the solar observations were the cause; but that would have left the other eye OK. Perhaps this notion is implicit in the passage quoted from Mulder, above; but it didn't occur to me until Jim commented on it, and I think it deserves some emphasis.
There are actually some modern investigations into the probable cause of Galileo's blindness. One is summarized in English in
F. C. Blodi
Some famous persons with visual problems as shown on postage stamps
Documenta Ophthalmologica 77, 295–334 (1991)
In his Snyder Prize lecture, in which Galileo's case is only one of 56 examples of famous blind people, Blodi probably relied on
In tema di eziogenesi della cecita de Galileo
Atti del Symposium internazionale di storia, metodologica e filosofia della scienza (Florence, 1967)
for the discussion of Galileo's blindness. Grondona's paper, in turn, depends heavily on the reply of Giovanni Trullio, a Roman surgeon who was asked to advise Galileo about his eye problems. Trullio's letter is available on the IMSS website in the original Latin, and an Italian translation is provided by Grondona. (These three references were turned up by Jim Mosher.) According to Blodi, when all the symptoms are considered,
A more modern interpretation assumes that Galileo suffered from bilateral iridocyclitis as a complication of his rheumatoid disease. This iridocyclitis then produced a complicated cataract, synechiae and an occlusion membrane.
(Iridocyclitis is an inflammation of the iris.) This seems to have replaced the earlier, often repeated, theory that Galileo's blindness was the result of chronic glaucoma.
Various other suggestions about the causes of Galileo's blindness are discussed at the website of the College of Optometrists in London, near the end of their page on Galileo.
The Galileo Project at Rice University. Plenty of accurate information about Galileo by history-of-science expert Albert van Helden, and no nonsense about his blindness being related to his solar work. In particular, there is a fine Web page summarizing the sunspot observations and the controversy surrounding them.
A Galileo website in Florence, Italy (where he did most of his work), giving a short biography.
NASA's Astrophysics Data System reproduces the abstract of a talk given by Thomas A. Hockey at a meeting of the American Astronomical Society in 2011, in which Hockey mentions several blind astronomers, pointing out that “It is a myth that astronomers blind themselves by observing the Sun.”
A brief biography at UCAR, with a good bibliography on Galileo.
Advice about eye care, stating that “Occasionally glancing at the sun usually does not harm your eyes. However, staring for several minutes at the sun or a solar eclipse can damage visual cells in the part of the retina that allows us to see fine detail. This kind of injury, called solar retinopathy, can cause a blind spot in the center of the field of vision. Loss of vision in these cases can be temporary but often is permanent.” [Alas, this disappeared from the Web at the end of 2003; but the Wayback Machine preserved it here.]
Moorfields Eye Hospital in London used to give the straight story on the symptoms of solar retinopathy: “About half those affected recover to normal vision as measured on vision testing charts, although small dark patches may persist. About 10% will have permanent loss of vision to the extent of not being able to read a number plate at 25 yards. Blindness in the sense of loss of all vision does not occur. There is nothing the person affected can do to speed up recovery or to improve vision. There is no medical or surgical treatment for the problem.” But, in redesigning their website, they have removed that page, on the grounds that solar retinopathy is a relatively rare condition. (Fortunately, the page is archived by www.archive.org at this location.)
So, if Galileo didn't go blind from observing the Sun, why are so many people worried about looking at the Sun? Well, as Mulder mentioned above, people do sometimes suffer eye damage at solar eclipses. And it's also true that a few early observers did suffer some minor eye injury from looking at the Sun under unsafe conditions: Isaac Newton seems to have suffered a very small scotoma by looking at the Sun's reflection in a mirror when it was high in the sky; and Thomas Harriot, who discovered sunspots independently, once observed the Sun near noon, and reported that “My sight was after dim for an houre.” John Greaves reported afterimages looking like “a company of crows” for “some days” after making solar observations directly through a telescope.
Still, it's rare to hear of anyone suffering eye damage from just looking at the Sun under normal conditions. Why?
First of all, you need to understand what kind of damage sunlight can inflict on eyes. Most people suppose you will “burn” your eyes by looking at the Sun. This notion is refuted in the technical paper
T. J. White, M. A. Mainster, P. W. Wilson, and J. H. Tips
Chorioretinal temperature increases from solar observation
Bulletin of Mathematical Biophysics 33, 1-17 (1971)
in which the authors state that
Direct thermal damage to living tissues is generally associated with temperature increases of 10-25°C. … These thresholds are substantially higher than the 4°C temperature rise computed for an unassisted solar observation with a 3 mm pupil diameter and [zenith] observation angle. Since 90% of this temperature rise occurs in the first 300 msec of an observation, accidental solar observation on a clear day would be a significant hazard if a 4°C temperature increase were capable of producing a chorioretinal lesion. However, with normal pupil adaptation, the only effect of unaided solar observations, even several seconds in length, is a transient afterimage. Thus, it is clear that the 4°C temperature rise can be safely tolerated. … Solar observations with a dilated pupil may result in chorioretinal temperature increases substantially greater . … Unaided, solar eclipse observations also produce smaller temperature increases … . However, since the iris may be adapted to a larger pupil diameter during eclipse conditions than during unobscured conditions, … the eclipse observation would then be more hazardous than an unobscured observation. …
So, thermal damage (not really a “burn”) is possible under conditions of a partial eclipse, when only a little of the Sun is exposed, and the pupil opens up to adapt to the low overall light level; but it is unlikely in normal daytime conditions.
In agreement with this calculation, there are published instances of people staring at the Sun, even high in the sky, without harm. As an example, I cite the first-hand account of
Bold experiments in physiological optics
Meteorological Magazine 59, 213 (1924)
As there appears to be prevalent a belief that looking at the sun with the naked eye would be injurious … , for some years I have experimented with the sun by looking at it with the naked eye … . On one occasion, on the 21st of June at 12 o'clock noon, I looked steadily at the sun for 15 minutes, changing from one eye to the other at intervals of about 30 seconds, and beyond making my eyes run there was no inconvenient effect. This was done while I was living in Atlanta, Ga., where the sun is fairly strong on the date given. As this took place 12 years ago, and, as at the age of 68 my sight is very good, I am sure that no one need fear trying similar experiments.
I hasten to add that I do NOT recommend trying this yourself! Nevertheless, it is certainly experimental confirmation of the conclusion by White et al. that the heating of the direct solar image “can be safely tolerated” — at least for a few minutes. Note that the Sun was nearly at the zenith when Lowe performed his experiment in Atlanta at noon on the summer solstice.
Further evidence that even prolonged staring at the Sun does not usually produce blindness is given in the work
M. O. M. Tso, F. G. La Piana
The human fovea after sungazing
Trans. Amer. Acad. Ophthalmol. Otolaryngol. 79, pp. OP-788 to OP-795 (1975)
Tso and Piana asked three middle-aged people, each with an eye that was to be surgically removed to prevent the spread of malignant melanoma, to stare directly at the Sun for one hour, a day or two before the operation. To quote from their summary:
Two of the patients sungazed with an undilated pupil, and, 24 hours later, recovered their preexposure visual acuity with no detectable scotoma. One of the patients looked at the sun with a partially dilated pupil, and 24 hours later her visual acuity dropped from 20/20 to 20/25.
But even in that eye, whose pupil was dilated to 4 mm, acuity was back to 20/20 after another day, though the scotoma remained.
After surgery, the eyes were examined under the microscope. Although damage to the retinal pigment epithelium was seen in every case, the photoreceptors appeared perfectly normal. The ages of the patients were 49, 55, and 57 years.
On the other hand, there are also cases of people who stared at the Sun for only a few minutes, when it was much lower in the sky, and suffered long-lasting scotomas:
M. Hope-Ross, S. Travers, D. Mooney
Solar retinopathy following religious rituals
British Journal of Ophthalmology 72, 931-934 (1988)
These authors report only partial recovery of visual acuity in four patients who stared at the Sun in religious rituals. In some cases the exposure was reported to be only a few minutes, with the Sun moderately low in the sky (variously described as “late afternoon” and the like). Although all reported partial recovery of acuity over the course of several weeks, they all still complained of scotomas many months after the injury. These cases indicate that at least some people are quite susceptible to eye damage from staring at the Sun.
Actually, it turns out that the main damage to the eye is photochemical, not thermal. So it is the short wavelengths that are harmful. This is shown in the paper
W. T. Ham, Jr., H. A Mueller, and D. H. Sliney
Retinal sensitivity to damage from short wavelength light
Nature 260, 153-155 (1976)
Ham and his co-workers estimated that “sungazing at bright midnoon for 100 s can produce a threshold lesion.” This may be roughly consistent with Lowe's experience, and is certainly in line with the reports of eye damage in sun-gazing religious pilgrims, who required at least several minutes' exposure without protection to suffer long-lasting eye damage.
A later paper
W. T. Ham, Jr., H. A. Mueller, J. J. Ruffolo,Jr., and D. Guerry III
“Solar retinopathy as a function of wavelength: its significance for protective eyewear”, in “The Effects of Constant Light on Visual Processes” edited by T. P. Williams and B. N. Baker
(Plenum Press, New York, 1980) pp. 319-346
says there is
. . . conclusive evidence that infra-red radiation in the solar spectrum cannot produce a retinal lesion unless one gazes at the sun for 1000 seconds with a 8 mm pupil. If the wavelengths below 700 nm in solar radiation are removed with a filter like the RG-715 Jena glass filter, direct sun gazing can be tolerated for appreciable periods of time.
In summary, … near infra-red solar radiation makes only a negligible contribution to retinal damage.
However, they note that shorter visible wavelengths can be harmful, so that an optical attenuation by a factor of 1000 would be required for safe continuous observation of the Sun. One can hardly disagree with the statement that using a filter attenuating sunlight by a factor of 1000 would be safe.
Evidence that the normal eye is (marginally) able to look briefly at the Sun without harm is shown by the statistical distribution of solar injuries. After all, the near-total eclipses at which eye injury occasionally occurs are visible only a few minutes per century at any given location on Earth; the unobscured Sun is available for viewing every clear day. If we suppose the Sun is up (on the average) for 12 hours a day, that's about 440,000 hours or over 26 million minutes per century that the Sun is up outside of eclipse, compared to a few minutes of dangerous time near totality. So you'd expect eye injuries from unprotected Sun-viewing to be roughly a million times more common than injuries during eclipses.
But in fact, according to the review of such injuries published by Istock in 1985, “the vast majority of solar retinal injuries occur as a result of viewing a solar eclipse without adequate protection.” So it usually requires the special conditions of an eclipse near totality, in which the low level of general illumination allows the pupil to open up instead of contracting (as it normally does when looking at the Sun), to push the visual system over the threshold for damage in a brief exposure.
Even when eclipses are available, such injuries are uncommon. This suggests that some additional factor, such as exposure to eye-dilating drugs, may be involved. (Quite a variety of nasal decongestants and other common drugs, as well as exposure to some pesticides, have been reported to dilate the pupils.)
While there are a handful of cases of solar retinopathy produced by staring at the Sun outside of eclipse, these are nearly all associated with bizarre religious practices, drug use, mental illness, or other abnormal and rare circumstances. Normal people just don't get eye damage from looking at the Sun; the average person looks away when the Sun is “too bright to look at,” and exposure for a few seconds does not seem to be sufficient to damage most eyes — though some people may be unusually susceptible to this kind of injury.
Outside of eclipse, eye injuries from staring at the Sun are rare, because it's so unpleasant to look at the Sun when it is actually too bright to look at safely that any normal person looks away and avoids eye damage.
Eclipse injuries, on the other hand, can occur without the observer being aware of it. But even these are uncommon. For example,
M.Juan-López and M.P.Peña-Corona
Estrategia para prevenir daños a la salud ocasionados por la observación del eclipse solar en México
Salud Pública de México 35, 494-499 (1993)
discuss the incidence of solar retinopathy at the 1991 eclipse in Mexico, and find it to be less than 1 in 100,000.
One of the most famous cases of eye injury from deliberately staring at the Sun is that of Gustav Theodor Fechner, generally regarded as the “Father of Psychophysics.” In 1840, he looked at the Sun through various colored glasses and solutions in a study of after-images. The details of his experiences are published in
G. Th. Fechner
Ueber die subjectiven Nachbilder und Nebenbilder
Poggendorff's Annalen der Physik und Chemie 50, 193-221, 427-470 (1840)
Some of the filters he used were blue and violet in color, which produced a serious eye hazard: the blockage of most of the visible light allows the pupil to expand, but the color of the filter allows most of the photochemically harmful short waves to enter the eye. Worse yet, he viewed the Sun through a hole in the shutter of a darkened room, which — like the dim light of a solar eclipse — would contribute further to the expansion of the eye pupil. Finally, he stared at the Sun “as long as the eyes could bear without excessive irritation.” You could hardly devise conditions more likely to damage the retina photochemically if you tried! Not surprisingly, Fechner seriously injured his eyes in this process. The photophobia resulting from this experience is a classic symptom of solar retinitis.
Yet, after spending three years secluded in a darkened room, he found that his vision had recovered. Such recoveries are actually fairly common, though they are somewhat unusual in cases as severe as Fechner's.
For example, the Mexican study of eclipse scotomas cited above found that all 21 victims “recovered their full visual function after four months.” Another study,
L. S. Atmaca, A. Idil, D. Can
Early and late visual prognosis in solar retinopathy
Graefe's Archiv Clin. Exp. Ophthalmol. 233, 801-804 (1995)
found that about half the victims recovered completely in a few months. Only eyes that initially lost half or more of their visual acuity retained long-lasting damage.
In fact, Sir Isaac Newton seems to have suffered a mild scotoma at age 22 while looking at the Sun. Like Fechner, he suffered photophobia, but shut himself in a darkened room for only a few days, after which his sight returned to normal in a few months. His case was much like those reported in the Mexican study. (The details are given on another page here.)
Because eye injury outside of eclipse is so rare, and because it is caused primarily by the shorter, photochemically active wavelengths, we can expect that no injury at all can be produced when the Sun is low and the harmful wavelengths are largely removed by atmospheric extinction. The papers cited above allow this matter to be investigated quantitatively, bearing in mind the safety factor of 1000 attenuation suggested by Ham et al., and the statement that an unprotected eye can be marginally injured in 100 seconds when the Sun is high.
In fact, in the article “Eye protective techniques for bright light,” published in Ophthalmology 90, 937-944 (1983), David H. Sliney wrote:
When the sun is low in the sky it is yellow or orange indicating that the hazardous blue light has been scattered out of the direct path of sunlight, and the sun may be fixated for many minutes without risk.
It's worth going through the numbers for this situation, because there is a very large and rapid change in the brightness of the Sun near sunset.
For example, the smallest possible atmospheric extinction coefficient at sea level in blue light is about 1/4 stellar magnitude per airmass (the airmass at the zenith is taken to be unity.) When the Sun is 5° above the astronomical horizon , the airmass is about 10, so the blue light is reduced by at least 2.5 magnitudes, or a factor of 10. This would ordinarily not permit a threshold lesion to develop in 100 seconds; if we suppose the damage depends only on the total exposure, then 1000 seconds would be required, assuming the brightness remained constant. But, at low latitudes, the Sun sets 20 minutes or only 1200 seconds after reaching an altitude of 5° — and during this time, its brightness is rapidly decreasing. This suggests that, at low latitudes, staring at the Sun for the full 20 minutes before sunset might be marginally enough to produce a threshold photochemical retinal lesion in an average eye. As there is evidently some variation in sensitivity, not all eyes would necessarily be safe at this point.
A prudent observer might ask for an additional factor of 10 to be safe. This requires waiting until the Sun reaches 20 airmasses, or about 2° altitude, 8 minutes before sunset at the Equator, or 12 minutes before sunset in places like Montreal, Paris, or Rome. At higher latitudes, the Sun is lower and even safer to look at 10 minutes before sunset; so “10 minutes before sunset” seems a safe rule to employ. As the width of the thumb at arm's length is just about 2°, it is a good “rule of thumb” that if you can cover up the image of the Sun with your thumb, extended at arm's length, and still have the lower edge of the thumb touching the sea horizon, you can look at the Sun safely.
A very conservative observer who wanted the full factor of 1000 attenuation of blue sunlight recommended by Ham et al. would wait until the Sun reached 30 airmasses, at an altitude of a little less than a degree (i.e., 2 solar diameters). At this point, “continuous” viewing is safe; but the Sun remains in sight for only 4 more minutes at low latitudes.
A more realistic calculation would allow for the additional attenuation by aerosols, which can be quite strong at the low altitudes mentioned here. In fact, the Sun is so attenuated at short wavelengths that the first people who tried to photograph sunset phenomena were continually frustrated by their inability to record an image of the Sun at the horizon on unsensitized photographic plates: see, for example, the paper by Riccò in the bibliography. The short, photochemically active wavelengths required for photography on unsensitized plates are the same ones responsible for photochemical retinal injury; if the setting Sun cannot be photographed at these wavelengths, it cannot possibly cause retinal injury.
After I did the calculations described above, I found that similar calculations had been made by
D. Sliney and H. Wolbarsht
Safety with Lasers and Other Optical Sources
(Plenum, New York, 1980)
On pp. 205-206, they say:
As sunset approaches, the relative fraction of blue light in this direct solar spectrum dramatically decreases as the sun nears the horizon. … [O]nce the total irradiance falls below 3 mW/cm2 (corresponding to an elevation angle of less than 5° at sunset in relatively clear weather), most people find it reasonably comfortable to look at a sunset which lasts for less than 10 minutes. … [They then go through a detailed calculation that need not be repeated here.] This would also explain why an individual who drives toward the sun at low elevation angles as he goes to and from work does not receive a retinal injury.
So, when the Sun is touching the sea horizon, it is certainly completely safe to look at. This is in accord with the experience of millions of people who have watched many seaside sunsets without harm.
One consequence of the photochemical nature of retinal damage is that younger people are much more likely to suffer damage than older ones, because the lens and other media of the eye gradually become yellower with age, filtering out the most harmful short wavelengths. No doubt this helps explain Lowe's experience of being able to fixate the Sun without harm — an experiment he performed at the age of 56. Tso and La Piana's patients were likewise middle-aged. So it appears that older people are less likely to suffer eye damage from looking at the Sun.
This idea is supported by the age distribution of people who suffer solar retinitis at eclipses. According to the 1985 review article
Timothy H. Istock
Solar retinopathy: A review of the literature and case report
Journal of the American Optometric Association 56, 374-382 (1985)
a survey made after the 1970 eclipse showed that the average age of the 145 cases studied was 20.7 years.
Likewise, the median age of 20 victims of solar retinopathy suffered at the 1976 eclipse reported by
L. Rothkoff, A. Kushelevsky, M. Blumenthal
Solar retinopathy: Visual prognosis in 20 cases
Israel J. Med. Sci. 14, 238-243 (1978)
was only 15.5 years, with all but 3 being 18 or younger. The oldest of the 20 was 40 years old.
In other words, it may be possible for old geezers to look at the Sun for a few minutes, but, kids, don't try this yourself.
The larger solar image on the retina produces more heating than in naked-eye observation, as shown by the calculations of White et al. Still assuming an eye pupil diameter of 3 mm, they find that a 25x telescope would produce a retinal temperature rise of 12°C in one second, and 34°C in 10 seconds. Both of these numbers exceed the threshold for retinal thermal damage. However, they assume the Sun in the zenith; for the Sun only 5° above the astronomical horizon , the heating rates are smaller by a factor of 4, which would push even the 10-second telescopic observation (just) below the threshold for thermal damage. The smaller image produced by low-power binoculars would be safer still.
Thus, while thermal damage to the retina can be produced in a few seconds if a telescope is used when the Sun is high in the sky, it is thermally safe to look at the Sun with binoculars when it is within a few degrees of the astronomical horizon.
The photochemical hazard depends only on the image brightness, which (by a well-known theorem of optics) cannot be increased by an optical system. So, on the whole, using optical aid cannot significantly increase the photochemical hazard to the retina , and (if the instrument's exit pupil is small, and/or the instrument's transmission is significantly less than unity) may even decrease it.
I have argued above that the retina will not be damaged photochemically if the Sun is within a few degrees of the astronomical horizon. This conclusion remains true if optical aid is used, as any optical instrument (e.g., binoculars) can only make the retinal image dimmer, not brighter.
In particular, the glass lenses used in binoculars and telescope eyepieces strongly absorb the shorter wavelengths that are responsible for photochemical damage.
On the other hand, the bright exit pupil of a telescope can produce very rapid heating indeed. That is why solar filters are made to go over the telescope objective, rather than over the eyepiece. For example, a friend of mine once tried to use a dark welder's glass at the eyepiece of his telescope; he had put the glass over the eyepiece, and was just about to look in, when the welder's goggle exploded!
Let's have a look at the quantitative side of this problem. The diameter of the exit pupil of a telescope depends on the magnification of the eyepiece used, and is always the diameter of the entrance pupil (usually, the objective) divided by the magnification. So, with a magnification of 8x (typical for binoculars), the exit pupil is 1/8 the diameter of the objective.
But this means the power density and heating rate go up with the square of the magnification. For 8x binoculars, this factor is 64. But for a typical small telescope with a 100x eyepiece, this factor is 10,000. (No wonder my friend's dark glass filter exploded!)
Clearly, the hazard here is a pupil so bright it will burn the iris of the eye, even if the retinal image is still within safe bounds. And this hazard increases with the square of the magnification of ordinary telescopes and binoculars.
As the zenith solar irradiance at the surface of the Earth is about 0.1 watt/cm2, the brightness of the exit pupil of 8x binoculars is 64 times larger than this, or some 6.4 W/cm2, which is only a little less than the 6.7 W/cm2 retinal irradiance calculated by White et al. for the 25x telescope.
The heating of the retina is almost entirely due to absorption of radiation by the underlying pigment epithelium, which is only 0.01 mm thick. If the iris of the eye had its pigment concentrated in as thin a layer as the retinal pigment epithelium, the heating of the iris would be comparable to the numbers given above. Actually, the pigment in the iris is distributed over a greater depth; so we certainly overestimate the heating in making this comparison. This shows that the heating of the iris is certainly below the thermal damage threshold when binoculars are used to observe the Sun within 5° of the astronomical horizon.
Clearly, both the retina and the iris are below the threshold of injury when the Sun is viewed through binoculars within a few degrees of the astronomical horizon, but not when it is higher in the sky.
However, telescopic observations, using higher magnifications, can easily be hazardous to the iris of the eye, even near the horizon. For a frightening story of a close encounter with this danger, read William Bunker's first-hand account.
But that's only true for modern telescope and binocular designs, whose optical systems are of the Keplerian type. Cheap “opera-glasses” are usually made according to the scheme Galileo used for his telescope, with a negative lens for the eyepiece. In these systems, the exit pupil lies well up inside the tube of the instrument, so the beam is diverging as it reaches the eye, whose iris is not nearly so brightly illuminated as it is with a “good” telescope.
A couple of Galileo's telescopes have survived, and are on display in the History of Science Museum in Florence, Italy. You can see pictures of them and read their optical specifications at the Museum's website. The optical properties of the lenses were carefully measured by
V. Greco, G. Molesini, and F. Quercioli
Optical tests of Galileo's lenses
Nature 358, 101 (1992)
In Room V is a telescope whose objective has a focal length of 1.33 m and a full diameter of 51 mm ( = f/26). However, as Greco et al. point out, the objective is masked down to 26 mm (f/51). The eyepiece gives a magnification of 14x, so the exit pupil is 26/14 = 1.86 mm in diameter, appreciably smaller than the smallest opening of the pupil in the human eye. The location of the exit pupil is inside the telescope tube, about 9 cm in front of the eye lens. (An alternate link for this telescope is here.)
A second telescope has a 0.98 m objective with only a 16mm clear aperture (f/61); the eyepiece (which is not original) magnifies 21 times, so the exit pupil is only 16/21 or only 0.76 mm in diameter, and is about 4.5 cm in front of the eye lens. (An alternate link for this telescope is here.)
These figures show that the amount of light admitted to the eye by Galileo's telescopes was quite small. Even in the center of the field, the image of the Sun seen through the first (14x) telescope would be less than four tenths as bright as the Sun viewed directly with a 3mm pupil. For the second (21x) telescope, the image is barely 6% as bright as would be seen with the naked eye. And these numbers do not allow for additional losses in the lenses by reflection, scattering and absorption, which are appreciable. Furthermore, the unvignetted field is quite small; over much of the useful field of view, the image brightness is still less than these numbers indicate.
Ignoring transmission losses, which must amount to at least 15% for Fresnel reflections at 4 surfaces alone, the retinal image brightness in Galileo's 14x telescope would not be sufficient to heat the retina by 13°C in a 10-sec observation, even if the Sun could be seen in the zenith. With a reasonable allowance for light loss in the telescope, and the additional atmospheric attenuation of the Sun as seen from Florence, where it never approaches the zenith closer than 17°, it appears that this telescope would at worst have been capable only of inflicting a minimal thermal injury to the eye. The second telescope, with its much dimmer image, would certainly have been safe, everywhere in the sky.
In addition, the glass used in these lenses blocks part of the short-wavelength light that is responsible for photochemical damage to the retina, because glass is opaque in the short-wavelength half of the UV-A region. (That's why you can't get a suntan through glass windows.)
So here is an additional reason why Galileo could not have injured his eyes in his solar work. His telescope design produced a small exit pupil and a dim retinal image. And it seems that he used a telescope with a particularly dim image in his solar work (see below).
Details of his observations are given in his letters to Mark Welser. There, he describes first looking at the Sun directly “at sunset”, and later, using the telescope to project the full image of the Sun on a white card. As he states explicitly that the whole of the Sun could be projected on the card, this provides an additional clue about the telescope actually used in the solar work.
Now, the Sun's angular size is a little more than half a degree. That means it could not have been completely seen if the first telescope had been used. However, the Sun would easily fit into the field of view of the second telescope (the one with the dimmer images). As Galileo says, in describing the eyepiece-projection technique to Mark Welser, that the full outline of the Sun is seen on the paper, he could not have used the first telescope in the solar observations, but might have used the second one (or something similar to it) for his solar observations. This suggests, but does not prove, that the solar work was done with a telescope that was actually safe for viewing the Sun directly.
So, did Galileo go blind from looking at the Sun through his telescope? The answer is, “Certainly not.”
Is it possible to injure your eyes by looking at the Sun? The answer is “Yes, but you have to work at it under normal circumstances.”
Can you become totally blind from looking at the Sun with the naked eye? The answer, according to Mulder, and from the cases of solar retinitis in the literature, is “No.”
Is it safe to look at the setting Sun? The answer is clearly “Yes, IF you are careful to wait until the Sun is within its own diameter of the astronomical horizon.” If you have accurate predictions of nominal sunset times, wait until 5 minutes before the expected time of sunset. CAUTION: Sunset times published in newspapers are often incorrect!
Note that the low Sun can even be observed safely with binoculars, if it is touching a sea horizon. However, higher-powered telescopes are likely to be dangerous. And an elevated mountain horizon may put the Sun dangerously above the astronomical one at apparent sunrise or sunset — particularly if you're in a valley near the mountains.
Is solar eye damage permanent? The answer seems to be, “Only if it is severe.” About half the people with small scotomas recover from them completely after some months or years.
I don't intend to minimize the seriousness of solar eye damage; victims often lose the ability to read normal-sized print, for example. But draconian pronouncements that “you should NEVER look at the Sun” or assertions that you can become permanently and completely blind are an over-reaction to the actual hazard. Sunsets can be viewed safely, both with the naked eye and with binoculars, and most people are already aware of this.
The ophthalmological literature is large, and I am an astronomer, not an ophthalmologist. I have tried to give a fair sampling of the primary literature on solar eye injuries here, but may well have missed something important. Likewise, I am not an historian; though I have consulted the literature on Galileo's observations, there could be something I've missed. Anyone with expertise in these areas is invited to add to my list of references on this matter. But, please, stick to the primary, refereed literature; introductory textbooks and other secondary sources often give a garbled account of things. And even some professional articles (such as, for example, Istock's review, and the book by Sliney and Wolbarsht) propagate the false story about Galileo.
In writing this page, I have asked the advice of a number of people in the eye-safety business, notably David Sliney and Martin Mainster. I have also consulted Albert van Helden about the literature on Galileo. Any remaining errors on this page are my own, not theirs.
Copyright © 2000 – 2012, 2015, 2020 Andrew T. Young