Bad Astronomy: Misconceptions and Misuses Revealed, from Astrology to the Moon Landing “Hoax” - Philip Plait (2002)

Part III. Skies at Night Are Big and Bright

Chapter 11. Well, Well: The Difficulty of Daylight Star Sighting

was never a boy scout.

That is probably a good thing. I was a smartass as a kidsome say that's still true-and I'm sure I would have had a hard time of it in the woods with just other boys my age for company. In high school I learned to be a bit of a practical joker, if only to exact revenge for some of the pranks pulled on me.

There is one trick traditionally done by Boy Scouts for which I'm sure I would have fallen. It's usually done late in the afternoon, while the Sun is still well up in the sky. It's best to do it after a long day in the woods, when everyone is exhausted and perhaps not thinking clearly. While sitting around resting, the discussion will turn to the Astronomy Merit Badge. One of the tests for the badge is constellation identification, so after a few minutes of talking, one of the boys (an older one) will get up and say, "Well, I need to practice finding constellations now."

This will, of course, raise some protest, usually by a tenderfoot. "But the Sun is still up," he'll inevitably say. "You can't see stars during the day!"

The older boy then puts on a condescending smile and says, "Of course I can. I just need to use my tube!" He then makes a tube out of rolled-up paper. Peering through it up at the sky, he'll make some comment like, "Ah, there's Orion now." He'll even invite other scouts (older boys, always) to take a look, and they all agree they can see some stars.

The young scout may resist for awhile, but, inevitably, curiosity will prevail. He'll ask to take a look. The older scout hands him the tube, which he obligingly puts up to his eye … and another scout then pours his canteen down in the tube, drenching the young victim.

That victim would certainly have been me. A skeptic and a loudmouth through and through, I would have vehemently protested any attempt to see stars during the day. I would also have been a wet kid.

The thing is, I would have been a right wet kid. Looking at stars through a tube during the day won't work. However, variations of this idea have been around a long time.

I've heard over and over again that it's possible to see stars in the daytime from the bottom of a tall chimney or a deep well. I've never heard a decent explanation as to why this should work, although people make vague claims about the brightness of the sky being greatly diminished in a deep well, making it easier to see stars. The sky is so bright it washes out the stars, they reason. By cutting back on the amount of skylight, stars are easier to see.

This idea certainly sounds reasonable. It also has a long history. The Greek philosopher Aristotle mentions it in passing in one of his essays. No less an author than Charles Dickens also endorsed it in at least one of his works. In his 1837 book, The Pickwick Papers, he opens his twentieth chapter with this tortuous sentence:

In the ground-floor front of a dingy house, at the very farthest end of Freeman's Court, Cornhill, sat the four clerks of Messrs. Dodson & Fogg, two of his Majesty's attorneys of the courts of King's Bench and Common Pleas at Westminster, and solicitors of the High Court of Chancery-the aforesaid clerks catching as favourable glimpses of heaven's light and heaven's sun, in the course of their daily labours, as a man might hope to do, were he placed at the bottom of a reasonably deep well; and without the opportunity of perceiving the stars in the day-time, which the latter secluded situation affords.

Still awake? In other words, the clerks could see stars as easily as someone at the bottom of a well. Evidently, Dickens's publishers paid him by the word.

In a somewhat different version of this legend, Gregory of Tours, the sixth-century saint and historian, wrote in his Libri Miracu- lorum ("Book of Miracles") that the Virgin Mary drew water from a well, which became blessed by her presence. Those who are pious enough can gaze into the water from this well and, if they cloak their heads with cloths to block out the light from the sky, they see the Star of Bethlehem reflected in it. This is a rather neat trick: if you can't see it, you are not devout enough. Back to church with you!

The legend of seeing stars during the daytime is clearly tenacious, having been with us for a long chunk of our written history. I credit its longevity to the vague "scientificness" of the idea: as I pointed out before, it sounds like it might be true. Like eggs standing on end on the equinox, there is enough scientific jargon sprinkled in the legend that it bamboozles people. They don't understand it, so it must be true. The long history also lends support to it, but anecdotes are not conclusive proof! For that we need to turn away from hearsay and look to science.

Let's look closely at the legend: What is it about a chimney that might make it easier to see stars during the day? One obvious aspect is that it's dark at the bottom of a chimney. As your eyes get adapted to the dark, they become more sensitive to light. Perhaps that helps you to see stars.

Unfortunately, it won't work. Imagine you are sitting in the bottom of a tall chimney or smokestack, and it just so happens a star is directly overhead. Let's also imagine you have let your eyes get dark-adapted. But think about it for a moment: if your eyes have adapted to the darkness, and you are more sensitive to light from the star, the darkness also makes you more sensitive to the light from the sky. It won't be any easier to see a star. It's like standing in a loud bar talking to a friend. It's hard to hear him, so you use a hearing aid to increase your hearing sensitivity. But that won't work. You're focusing more sound in your ear from your friend, but you're also increasing the sound you hear from the rest of the bar. Nothing really changes, and it's just as hard to hear your friend.

Unfortunately, this also proves wrong the legend of seeing the Star of Bethlehem reflected from the water in a well. The water might reduce the brightness of the sky, but it reduces the brightness of the star by the exact same amount. You'd do better from the bottom of a chimney. That would change Nativity scenes extensively; a large smokestack next to the animals in the manger would take away a lot of the charm of Christmas.

You can see stars fairly easily at night, but not easily or at all during the day. The reason is just as obvious: at night, the sky is black and dark, but during the day it's very bright. The sky is bright during the day basically because the Sun lights it up. (See chapter 4, "Blue Skies Smiling at Me," for a more thorough explanation.)

The Sun isn't the only source of light illuminating the sky. If you go out at night during a full Moon, only the brightest stars will be visible, struggling to overcome the glaring light from the Moon. City lights also brighten the sky. This is called light pollution, and it's bad near cities, but it's not a good thing even near small towns. That's why astronomers try to build observatories far away from population centers.

During the day the bright sky swamps the rather meager light from the stars. As a matter of fact, on average the clear, daytime sky is roughly six million times brighter than that same patch of sky on a clear, moonless night. No wonder it's so hard to see stars during the daytime! They have to fight a fierce amount of light from the sky itself.

Still, we know it's possible to see the Moon, for example, during the day, so it's possible for some astronomical objects to be bright enough to be seen against the daytime sky. How bright must a star be before we can see it against the sky?

The critical item here is contrast. To see an object against a bright background, the object must be bright enough for your eye to pick it out over the rest of the light coming from all around the object. Tests done early in the twentieth century showed that the eye can pick out a star against the sky background if the object is roughly 50 percent as bright as the background. It may seem weird, at first, that you can see something that's fainter than the light around it. But the light from the star is concentrated in one spot, while the light from the sky is spread out all around it. The contrast with the sky is what makes the star visible.

Back in 1946, scientists performed experiments to see just how bright a star would need to be to poke out over the sky's glow. They mimicked what a human would see during the day as opposed to at night by tuning the amount of background light around an artificial star. They found that the dimmest star that a person could see during the day was about five times brighter than Sirius, the brightest star in the sky (besides the Sun). In other words, even the brightest star in the sky is too faint to be seen during the day (Journal of the Optical Society of America 36, no. 8 [1946]: 480).

Therefore, it's impossible for the unaided human eye to see any stars during the day. You'd think that's the end of the story, but there's still a twist to it. Those tests back in 1946 were done assuming the extra light was coming from the entire sky. If you are at the bottom of a chimney or a well, you aren't seeing the whole sky, just a little piece of it. If you can block out most of the glare from the sky, you can see fainter stars.

Very early in the twentieth century, two astronomers separately tried to figure out the eye's visibility limit, and to determine the faintest a star can be and still be seen against the night sky. They both found that by limiting the amount of sky they saw, they were able to greatly increase their ability to see faint stars. They determined that if you can cut out all but a tiny fraction of the sky, you can actually see stars that are about 10 times brighter than if seen in the whole sky-in which case it's just possible to see Sirius during the day, but that's it. The next brightest star, Canopus, is on the borderline of detectability. Let's be generous and say that both stars can be seen this way. Let's not forget, either, that there are bright planets visible to the naked eye: Mercury, Venus, Mars, and Jupiter can all appear brighter than Canopus or Sirius.

So we've determined that maybe, just maybe, we can just barely see six objects from a chimney, if the narrow opening of the chimney blocks the glare of most of the sky. We've done this by looking at all the advantages of viewing the sky from the bottom of a long, dark shaft. But we must be fair and look at the disadvantages as well.

There is one big one, and it's a deal-killer. Ironically, we looked at it as an advantage before: the narrow opening of the chimney. Before, it was good because it cut out glow from the sky, increasing contrast, making it easier to see stars. However, the small opening means there's less of a chance of a bright star passing into your field of view.

Most people think of the sky as being filled with stars. That's an illusion. You can see roughly 10,000 stars with the unaided eye, and they're spread out over the entire sky. We can estimate the average number of stars you might see through the opening at the top of a chimney. The answer may surprise you: even with a big opening, you will usually see only about 10 to 20 stars on the very darkest and clearest of nights. On a more typical night you might only see one or two stars. So, actually, looking through a chimney makes it a lot harder to see stars even at night. You are cutting out so much of the sky that only a few stars can be seen through the narrow aperture. During the day the odds are far, far worse. There are only six objects that you can see during the day to start with, not 10,000. The odds of one of these being in the chimney opening are remote indeed.

Scientists, of course, don't usually just calculate a number and assume it's correct. They actually go out and test it. An astronomer named J. Allen Hynek did just that and published his results in an issue of Sky and Telescope (no. 10 [1951]: 61). One day he took a few members of his astronomy class to an abandoned smokestack near Ohio University, where he taught. The bright star Vega-the fourth brightest in the sky-passes very close to directly overhead at that latitude, and they timed their experiment so that it would be in their field of view from the bottom of the smokestack. Vega is about half as bright as should be possible to see according to our calculations, but it is still one of the brightest stars in the sky. If it cannot be seen during the day, then certainly the vast majority of stars cannot be seen then, either.

At the appointed time Hynek and his students peered upwards, straining to see a glimmer from the star, but they all failed to observe it. Two students even used binoculars, which should have helped by increasing the contrast even more. They failed to see Vega as well. This is not surprising, really. Vega is too faint. Still, they showed by direct proof that stars are at least extraordinarily difficult to see through a chimney.

Another legend bites the dust, or in this case, the soot. While looking through a narrow opening does increase your ability to see faint objects, it simply doesn't increase it significantly enough to see stars during the day, and that same narrow opening makes it highly unlikely that a bright star will be in a viewable position.

Still, I have no doubts the legend will persist, as they all do. Even a friend of mine, an astronomer of no small status, swears the legend is true. He claims he saw it himself: he once looked up a long chimney during the day and saw a star. David Hughes, in his excellent paper entitled "Seeing Stars (Especially up Chimneys)," notes that a good chimney will have an updraft, even when there is no fire (Quarterly Journal of the Royal Astronomical Society 24 [1983]: 246-257). It's possible that my friend saw bits of debris caught in the draft and briefly lit by the Sun. At a great distance the debris will look tiny, unresolved, and not appear to move very quickly. This could be mistaken for a momentary glimpse of a star. I explained this to my friend, and I explained the idea of a star's brightness versus the sky's surface brightness, and I even talked about the odds of a bright star just happening to be in that extremely tiny line of sight, but he would have none of it. He stands by his story. I guess even the staunchest of scientific minds can have superstitions they don't want to drop. It's an interesting cautionary tale for all of us, I think.

Now, having said all that, I must confess that it is possible to easily see one starlike object during the day: Venus. Venus is roughly 15 times brighter than Sirius, so not only is it possible to see during the day, it's also relatively easy. You need to know just where to look, but it can be done. I've seen it myself on several occasions, in broad daylight. However, extrapolating from seeing Venus during the day to seeing stars from a chimney is a pretty big stretch. In the end, the legend turns out to be just that: a legend.

A final note on this topic: I know for a fact that I would fall for that old boy scout tube trick. Why? Because a variation got me when I was about seven or eight years old, except I was told it was a coordination test. I was supposed to roll up a paper plate, put it in the front of my pants so that it stuck out a few centimeters, balance a rock on my nose, and then tilt my head forward so that the rock fell into the rolled-up paper plate.

As soon as I tilted my head back one of the other kids poured a glass of ice cold water into the paper-plate tube. This incident may have mentally scarred me for life; I still shrink away from picnics featuring paper plates. For all I know, the stunt gave me a core of vehemence against such things, which in turn led to the book you are holding in your hands right now. So, I say to those older kids who played such a mean trick on a naive young kid: thanks!