Grapes of Wrath

By Phil Plait | October 5, 2005 9:09 pm

I was reading a recent entry at Astronomy Blog about what happens to a grape when held up to the eyepiece of a solar telescope, and it reminded me of grad school. We had a lab where students tracked sunspots to calculate solar rotation, and they projected an image of the Sun on a piece of paper to trace the sunspot locations. There was a mechanism to hold the paper in front of the eyepiece… but this pair of geniuses moved the apparatus aside so they could look through the telescope at the Sun.

Think about that, revel in it for a moment. They looked at the telescope, saw the piece of metal in front of the eyepiece which was clearly designed to hold a piece of paper (of course the instructions in the manual also clearly outline how to mount your paper), and decided to move it aside. Then they had to bend over at an awkward angle, move the ‘scope around to find the Sun, and actually sight along the telescope at the Sun– basically staring right at the Sun– to find it.

And never, not once, during this entire process did it occur to them that they were trying to look at the Sun though a telescope! Sheesh. The fact that they survived, and didn’t blind themselves in the process, is almost enough to make me doubt evolution.

Even more ironically, getting the Sun in that telescope was a bit tough. As it was, they weren’t good enough at handling it to get the Sun in the eyepiece, so they were unhurt. Had they been better at astronomy, they might have blinded themselves. Reverse evolution, it seems.

Anyway, Astronomy Blog posted a short video of the grape, and the fun is at the very end. Make sure your speakers are turned up, you can actually hear the sizzle.


Comments (34)

  1. P. Edward Murray

    During the Annular Solar Eclipse of 1994 a group from our astronomy club The Bucks-Mont. Astronomical Assoc., Inc. traveled to New York state from our home in Pennsylvania.

    We had a great time taking photos, seeing a bit of the Chromosphere, Crescent Solar and Annular Rings under the trees and Shadow Bands.

    New Yorker students in the area were not permitted to partake of this celestial event. Shades were drawn and they never saw it.

  2. James O’Meara would disagree with you on this one. Least he did in a talk he gave our astronomy club involving myths of visual observing. This was one of them.

    His argument went along these lines. Assume you have a 250mm scope running at 50x (5mm exit pupil) looking at the sun. You also look with your eye. The eye will stop down to about 1mm making a 1mm, 1x scope. Let’s define the energy hitting the fovea as 1 unit per square mm from this set up. The telescope has 250 times the diameter or 62500x the light energy but is running at 50x which spreads this over 2500 times the area so, if the eye was fully open or at least open to 5mm you’d get 62,500/2500 or 25 times the energy hitting a square mm of the fovea. But again the eye would instantly stop down to 1mm as the naked eye did dropping the energy reaching the each square mm of the fovea to 1, same as the naked eye! Since no telescope is 100% light efficient it would actually put less energy per square mm on the fovea than when viewing the sun with the naked eye. Obviously the total energy hitting the retina would be much higher but it is the energy per unit of area that causes the damage, not total energy.

    Now the iris, on the other hand, is taking the rest of this energy and thus could be burned if you held your eye to the eyepiece too long. Unless you are stoned there’s no way you could do this. You’d pull back instantly, before any damage would be done.

    You can substitute any size scope and any power as well as any minimum pupil diameter in the above argument and you get the same results. I just chose one that made the math easy. I can’t remember what O’Meara used in his talk.

    On the other hand, when I was supervisor of a public observatory I wanted to demonstrate the heat of the sun in an unprotected scope to school science teachers. I drew an eye in black ink on magician’s flash paper and put that to the eyepiece. It went up so fast I got burned both by the flash and because I was using a 1mm exit pupil focused on the palm of my hand that was now covered in black paint residue. It blistered nicely but the teachers were impressed! I had tried it before they got there with plain flash paper, no eye drawing, it it was difficult to get it to ignite at a 4mm exit pupil so I went to 1mm and the black eye but didn’t get a chance to try it before they arrived.

  3. Michelle Rochon

    Now a video like that is sure to stop any brilliant brat from doing it…

    Then there’s people like my sister’s ex that boasts up bravely that he looked at the sun with binoculars. How come his eyes are still fine, I have no idea…

  4. Marlayna

    Maybe it was sunset or sunrise?

  5. Evolving Squid

    I find looking at the full moon on my 8″ celestron or 100mm binos hurts my eyes without filters or very dark sunglasses, so I just don’t do it any more (project the moon with the telescope or use a camera, use binos when moon is a good bit less than full). Maybe I’m just a wuss, but I don’t see how anyone can look directly at the sun through an optical instrument without injuring themselves.

    Even if it’s possible in some conditions, it strikes me as a less-than-smart thing to try.

  6. Wow! Do-it-yourself Lasik surgery!

  7. Evolving Squid

    It was all I could do to resist writing “It strikes me as not a very bright thing to try.”

  8. aiabx

    Sometimes science demands stupid behaviour. I’ve performed a number of experiments that should not have been done; seeing how fast I could go on a motorcycle, touching the handle of a short-circuited fridge, testing my capacity for beer and nachos. And I have looked at the sun, with the naked eye and with binoculars. I was watching the transit of Venus from the north shore of Lake Ontario, as the sun was just appearing through heavy mist right above the horizon. I could barely see a thing with the naked eye, so I took a quick glimpse with binos and confirmed the presence of Venus. For the next ten minutes, I (and a couple of dozen other astronomers) were able to clearly see the transit with the naked eye as the sun rose through the mist. As far as I know, we were the first people in history to see a naked eye transit of Venus.

    But I was let down by the grape video. I was hoping for an explosion of boiling grape juice everywhere.
    -Andy B

  9. Maybe they were looking at the sun at night! Heh heh.

  10. Tim G

    Do some solar filters crack too easily?

    When I was in Junior High, my parents bought me a refracting telescope. It came with a solar filter, which I used to look at sunspots.

    Years later, I heard that some “cheap, foreign” solar filters might crack. I now wonder if what I did was dangerous.

  11. Bruce

    Not unlike my favorite lab sign: Do not look into laser with remaining good eye.

  12. Tobin Dax

    Whether or not you’re right, Rick, I’m not about to risk student’s eyes in such an endeavor.

    In our solar observing session st my University, we TA’s would burn a pencil (well, it would smoulder) near the eyepiece of our 12″ refractor. That was definitely memorable for the students and made the point. I’ve also ignited paper towels in that light, but never with tens of students around me. :) We just had the objective lens cleaned over the summer, though, so my favorite part is a thing of the past.

    I’ll have to watch the video when I get home. (Maybe we’ll use it next semester.)

  13. P. Edward Murray

    Rick, you are probably correct in your assumption because all of us look directly at the Sun when we are driving east at sunrise or west at sunset even though the sun is coming through lots of layers of air right at the horizon.

    I think that these short “peeks” probably do degrade your vision over time but not enough to worry about.

    I used one of those eyepiece solar filters too so did a lot of other folks but not often.

    And if you heard it from good ole Steve O’Meara, I would tend to go with that as far as Observational Astronomy.
    He probably has the keenest eyesight in the world or at the very least, one of the top 5.

  14. monolithfoo

    Thats one hot grape! I wonder what it tastes like :)

    I love ‘Galaxy Quest’, I need to get the DVD. BA, have you seen Serenity yet?

  15. Star Girl

    It is true that viewing the sun through a telescope doesn’t cause instant damage after all Galileo often viewed the sun through his telescope. Having said that keep in mind that he did eventually go blind. And from personal experience I would recommend that no eyepiece solar filter ever be used alone. I have a black squiggly line in my left eye caused by a solar filter that cracked from the heat of the sun while using my first telescope (a Sears 60mm refractor.)

    The problem at least in my case was that even after the initial WOW of seeing the filter crack I continued to view the sun through the cracked filter. I was twelve years old and new to astronomy and didn’t know any better. The glare from the crack was uncomfortable but there was no pain so I was unaware of the damage being caused. I was drawing sunspots so was observing for quite some time. It wasn’t until later that day that I became fully aware of the black squiggly line. A line that has never gone away.

    The most aggravating aspect of the damage is that it is smack in the middle of the prime observing area of my left eye. Imagine having an eyelash on all your eyepieces. So everything I observe has that darn line which can be very irritating when you’re trying to make out fine planetary detail, split a close double or find a faint fuzzy.

    Considering that Phil’s site is probably read by many beginning astronomy enthusiasts it is always best to warn people (children and even as Phil’s blog illustrates adults) of the possible dangers of observing the sun. Not to mention that to do otherwise could open Phil up to potential lawsuits.

  16. P. Edward Murray

    Galileo, bless his soul, did eventually go blind though:(
    Sorry to hear about that Stargirl, best wishes to you:)

    Yes and Phil does have to say something just as when anyone who operates a telescope.

  17. DIguana

    This might be a bit off-topic, but you can take a look at this site to see more demonstrations of the sun’s power on everyday objects.

  18. Nigel Depledge

    Star Girl – very sorry to hear of your misfortune. Most of us have done stupid things at one time or another, but get away with it through sheer luck.

    I agree that it is not so clever to look through any ‘scope at the sun without a proper high-quality filter. My understanding of these things is that even the best lenses absorb some of the energy that is passing through them. If that energy has been focussed at all (i.e. if your filter is at the eyepiece, rather than over the objective), it can cause uneven heating of the material through which it passes. In glass, uneven heating always leads to cracking or shattering, unless the glass is Pyrex or Duran.

    Here’s an illustrative example: I once diluted some concentrated sulphuric acid in a glass bottle (which I had thought was Pyrex, but didn’t check), adding the acid to the water (like you oughta). Even so, it got hot enough to crack the bottle, very neatly, all the way around the base of the bottle. I heard it crack, but couldn’t see it, until I picked the bottle up, whereupon the base stayed put and the contents sloshed everywhere. Fortunately, I did it in a fume cupboard, so the spill was mostly contained.

    Back to the point: The best solar filters fit over the end of the ‘scope (as opposed to the eyepiece), where they will be heated evenly by the incident radiation, and hence will be extremely unlikely to fail.

    To answer Rick Johnson’s point, I would contest that the estimate of 1mm for the aperture of the pupil makes a dangerous assumption: that the iris contracts fast enough to be considered instantaneous. Also, I think that not everyone’s eyes can stop the pupil down to 1.0 mm. I think 2.0 mm is a more realistic estimate of the pupil diameter at maximum constriction (which gives four times the cross-sectional area and hence four times the incident radiation reaching the retina). Also, your concern about the iris is unfounded as no light is focussed on it.

    If my understanding is correct (I hope someone will correct me if it is not), the light exiting the eyepiece of the telescope will be in parallel rays when the scope is adjusted for a person to view through the eyepiece – the focussing is done by the eye’s own optical systems (i.e. the cornea and lens). This permits the observer’s eye to focus at “infinity”, which is the best for prolonged observing as it requires the lens to be relaxed (and hence the muscles that deform the lens will also be relaxed).

  19. Richard Board

    Since the topic is solar eclipses, can anyone help me out with this?

    Most of you are probably aware that central Spain and a lot of northern Africa experienced an annular solar eclipse on October 3rd. There is an interesting photo sequence at Astronomy Picture of the Day – – and it makes me wonder.

    Why does the angular size of the sun look so small in this time lapse sequence? The solar disk appears smaller than a full moon. Is it just perspective, or the lens’ focal length? What gives?

    Thanks in advance for any explanations.

  20. Bernard

    This story reminds me of a ‘test’ I have seen many years ago while a student at the University of Toronto. The test in question had about 20 questions to evaluate the abilities of potential astronomers. I can’t recall them all, but clearly remember the ‘telescope skills’ question:

    The candidate can
    a) Find the Crab pulsar visually
    b) Find the Andromeda galaxy
    c) Find Sirius and the Moon
    d) Find the Sun
    e) Has trouble finding the telescope

  21. P. Edward Murray

    Richard, if I’m not mistaken, it has to do with the aperature of the camera lens. It was probably taken with a 50 mm Lens or wider. The wider the field, the smaller angular diameter the Sun/Moon will appear.

  22. Nigel Depledge

    To follow up P.Edward Murray’s comment: I suspect that if you were to fix the camera to a telescope for recording an annular eclipse, you would find that for each exposure, the film would have a little hole burned in it corresponding to the position of the sun at that time.

    Another potential reason why the sun might seem smaller than you expect in the image is the Moon Illusion. The BA has a useful set of links here:

  23. Richard Board

    Thanks for the replys Nigel and P. Edward. I’m still skeptical, though. It would seem logical that taking pictures of the sun through an ordinary camera lens, regardless of aperture or focal length would result in damage to the film, unless strong filters were used. But this image shows no evidence of filtering, since the sky is blue, and other objects/people in the foreground appear normal. Also, I don’t think the Moon Illusion is applicable here, since the images span many degrees across the sky. The Moon Illusion is only noticed when the moon is near the horizon.

    Is it possible this photo montage is faked? I would expect the experts at APOD would be hard to fool, but I’m more easily baffled. Any thoughts, BA?

  24. Nigel Depledge

    I would imagine that the picture was taken with a very small aperture and a fast shutter speed. That aside, it could have had a filter over the lens, provided the filter responded approximately equally to all wavelengths. For instance, it could be a polarising filter, which would not change any of the colours, but would cut out roughly half of the light entering the lens.

    While it is true that the images span a large portion of the sky, you (a) don’t see the sun at the horizon, and (b) you can’t see the moon illusion in photographs, because the film records what is actually there without processing the image. The moon illusion is created by the way your brain processes image data from the eyes, which is why it is an illusion and not a real change in the size of the moon’s (or sun’s, in this case) image.

    I was extending the idea to account for the fact that the image of the sun in the picture is smaller than you expected it to be. Because we most often notice the moon and the sun when they are close to the horizon, and therefore subject to the moon illusion, we carry a certain expectation of the angular size they “should” appear to be. The photograph seems to have confounded your expectation, and I was making what seemed to be a reasonable assumption in an attempt to explain this.

    Does that make it any clearer?

  25. Richard Board

    Nigel – yes, of course I couldn’t observe an “illusion” in a photograph – duh! That never ocurred to me. Thanks for your clarity. This stuff is fun, huh?

  26. P. Edward Murray

    The “Moon Illusion” actually works with Constellations too!

  27. Sun and Moon Photos

    The sun and moon always look smaller in photographs that are not magnified in some way. But so does eveything else! People in pictures are obviously smaller than the people standing in front of you. Why doesn’t that bother you while the tiny sun or moon does? Because you are used to seeing people at different distances which makes them change aparent size, but the sun and moon always look the same!

    (Someone mentioned Serenity. If you haven’t seen it, run, don’t walk, to the theater! Thank me later.)

  28. Nigel Depledge

    Richard, you’re welcome. I often find that the best way to improve my own understanding of something is to try to explain it to someone else.

    Dan – I have a dilemma: watch Firefly on DVD first (still not seen it yet), or see the film first and watch the series later…

  29. monolithfoo

    I recommend watching the DVD first. Then the Movie. The comic book can be skipped though… it does tie up a couple loose threads… I’m a luke warm comic book fan.

  30. monolithfoo

    Just a note. I saw Serenity. I liked it very much. I think I prefer the episodic TV approach. Do not miss Serenity, like Dan says, RUN!

  31. P. Edward Murray

    Something I forgot to mention too.
    In an Annular Eclipse the Moon is actually smaller than the Sun because it is farther away from us than in a Total Solar Eclipse. Annular comes from “Annulus” (sp?) meaning ring around.

    To complicate matters even more, you can even have a “Hybrid” Solar Eclipse where the Eclipse is Annular except along the track where the Earth “Bulges out” actually negating the fact the Moon is too far away. During these Eclipses at that moment in time, usually only seconds, The Moon does in fact cover the Sun completely.

  32. Ed

    This has nothing to do with anything, but have you ever put a grape in a microwave? You have to do it right, cut a grape in half along its equator, don’t cut through completely, but leave a small flap of skin. Now open it up and place the cut ends down on a plate, and microwave for a few seconds. The whole thing catches on fire.

    Here’s some video:

  33. Yeah, the grape in the microwave thing is a lot of fun. It’s more than fire; it looks like a plasma discharge. Don’t leave it in too long! The smell is not so great.

  34. broken twig

    I suppose that if you project the sun onto a white ball all the sunspoats would be in the right place. ie the image will be streached out to look as the sun would.


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