Eight years ago, Sky & Telescope ran a short piece by a stargazer who had cut down a bunch of trees to get a better view of the sky. It provoked a startling number of outraged letters.

Chainsaw astronomy seems to be very controversial, yet the subject arises over and over. Truth be told, in many parts of the world — including my own — trees are the single biggest enemy of stargazing. Sure, light pollution diminishes what you can see, but trees stop it completely. In its natural state, inland New England would have nary a view of the night sky except at the edge of rivers, lakes, and swamps -- and barely even there.

I find myself completely unsympathetic with both extremes of the debate. On the one side, there are people who say they would never cut down a tree just for convenience. On the other, those who say that it's my property, and no tree hugger is going to stop me from doing what I want with it.

I do think that trees demand respect — as does every living thing and even, to some extent, everything inanimate. If you brutalize the world around you, you also brutalize yourself. But I don't view trees as sacred — not as a general rule, anyway. In some ways, that seems just as irresponsible. It's copping out, failing to engage with the concrete reality of individual trees.

Mind you, certainly trees are indeed sacred, though it's we who make them so. At my preferred observing site at my country home, the entire northern sky is blocked by a huge sugar maple that was probably planted when the house was built. But cutting it is completely unthinkable. My grandparents preserved it when they bought the house in 1930, and it's my duty to preserve it for my grandchildren — when and if they are born. It goes beyond my own personal likes and dislikes – even beyond my extended family. It's a link to the boys who lived in that same house and loved that same tree before they went off to fight and die in the Civil War.

On the other hand, I'm perfectly willing to cut down the trees at far end of the field that block my view directly to the south, where it matters most. Those trees are barely older than I am; when my father was a child, those woods were sheep pasture as far as you could see. If I don't cut trees down, then the field will slowly, inexorably, grow in from the edges, and there will be no field at all for my grandchildren.

Mind you, even those trees I'm not about to simply slaughter. I won't cut them faster than I can burn them in our fireplace. At the current rate of progress, I figure it will take me a decade or more to get an extra 15 degrees of unblocked horizon. But that's OK. It's foolish to be in a hurry when you're dealing with organisms whose life spans are measured in centuries.

It's sobering to think that the beavers who recently moved into the swamp below our house have killed more trees in three years than I have in my whole life. Then again, it's a full-time occupation for them, and just a hobby for me.

Sky & Telescope readers often comment how much they enjoy reading Sue French's "Deep-Sky Wonders" column. I certainly agree, but to some extent, that's missing the point. Sue is a good enough writer to make her column entertaining as armchair reading, but its real purpose is to be used outside, at night, by the side of a telescope.

Since I'm Sue's regular editor, I make a point of "doing" her columns whenever I get a chance. Among other things, it lets me make sure that the charts and illustrations that I prepare are adequate for finding the things she talks about. And in any case, it's hard to imagine a better way to spend an hour or two.

I particularly enjoyed the column in the November 2009 issue because of its variety. Sue starts out exploring Pegasus I, a galaxy cluster with a couple of very prominent members (NGC 7619 and 7623) and a whole host of fainter ones. I was pleased that, using my 12.5-inch Dob last Sunday at my semi-dark second home in rural NY, I was able to log at least a strong "maybe" for all the ones listed in the article.

Sue sometimes fails to get the respect that she deserves from hard-core deep-sky observers -- perhaps because she spends so much time with her 4.1-inch refractor, or perhaps because she never goes out of the way to took her own horn. In fact, she generally sees more through her 10-inch scope from her far-from-dark backyard than I can through my 12.5-incher at a considerably darker site. Then again, she probably devotes ten times as much time as I do to observing, so her superior skill is hardly surprising.

After I'd spent more than an hour straining to see the 14th-magnitude galaxies described in the November Deep-Sky Wonders, I got to unwind with a lovely succession of relatvely easy double stars, the magnificent carbon star TX Piscium (which I often view), and a charming asterism that I never would have stumbled on if Sue hadn't mentioned it.

But don't take my word for it. If you own a telescope, why don't you try "doing" Deep-Sky Wonders yourself? Sue almost always lists one or more targets that are easy for novices to enjoy as well at least one target that's bound to be a challenge for even the most experienced observer.

Astronomy is an unusual hobby; it snares lots of different kinds of people for many different reasons. For me, it was a natural outgrowth of my love of the outdoors. The one activity that I love even more than stargazing is hiking in the mountains. And combining hiking and stargazing is best of all!

Most people who hike in the U.S. Northeast would agree that September is the best time to do so. The oppressive heat and bugs of summer are gone, the chance of rain is low and the chance of snow even lower, yet the days are still reasonably long. So for the last weekend in September, I took Friday off to do a three-day hike that I'd long dreamed of and never attempted. For those who know New Hampshire's White Mountains, I went north up the Bonds, west across South Twin and Garfield, and then south down the Franconia Range. Many geologists believe that the striking horseshoe-shaped ridge that I followed is the remains of an ancient volcanic caldera.

The weather proceeded exactly as forecast. Friday started cloudy and drizzly, then turned crisp, cool, and windy. Saturday was predicted to be perfect hiking weather, which it indeed turned out to be — cloudless skies, crystal-clear air, gentle breezes, and temperatures around 40°F. (That's T-shirt weather when you're doing strenuous hiking.)

I spent Friday night at Guyot Shelter, high up in the mountains and as far from civilization as you can get in New Hampshire. I was eager to rise early for two reasons. I wanted to take advantage of Saturday's weather and hike as far as possible. And since this was first-quarter Moon, the sky would be truly dark before dawn — giving me a great chance to see the zodiacal light. And indeed, when I awoke around 4:30 on Saturday morning, there was the zodiacal light shining between two trees — as fine a view of it as I've had for several years.

Packing up as quietly as I could to avoid wakening my shelter-mates, I hiked up to the main trail and started to cook breakfast before the first sign of dawn. I could only see the narrow strip of sky above the trail, but that was enough! The Milky Way from Cassiopeia to Auriga ran directly overhead, intricately veined with dark lanes, with the Double Cluster blazing in its center. Quite a backdrop for breakfast!

By the time I was actually hiking, the sky was getting light. Shortly afterward, I emerged above treeline on Mount Guyot to the most glorious view imaginable. Orion and Canis Major were still prominent in the south, while Mars, Castor, and Pollux made a perfect arc overhead. But Venus dominated the view, blazing above a magnificent sunrise glow that stretched the entire length of the eastern horizon. I put on my down jacket and mittens and snapped endless pictures as the sky grew brighter and the mountains around took on a rosy hue. In the lowlands to the north, isolated hills rose out of a sea of fog as far as I could see.

I started to hike again, ducked into the trees, and by the time I emerged above treeline again on South Twin, the Sun was well above the horizon.

Saturday was a great day of hiking by any standard — I ended up walking 14 hours though some of the finest scenery in the East, at one of the loveliest times of year. I crossed six mountaintops, each with a wonderful view, each very different from all the others. And I certainly got a good workout! But none of the time that I spent walking during broad daylight came close to the magic of those hours between the first sight of the zodiacal light and the disappearance of the last stars.

There's a well-known rule of thumb for estimating light pollution called Walker's Law, formulated by the professional astronomy Merle Walker based on measurements in California. It postulates — among other things — that the level of light pollution is proportional to the population of the town or city that's producing it. As I said in my last blog, this is clearly untrue when comparing cities in different countries. But how well does the rule hold up within Canada and the United States?

If the Light-Pollution Atlas is to be believed, not very well. As the clips at right demonstrate, the Atlas shows a much smaller light-pollution blob for Toronto than for Montreal, though Toronto's population is significantly bigger. And Chicago's light-pollution blob is a smidge bigger than that of Los Angeles, which has nearly twice Chicago's population.

The six biggest metropolitan areas in the U.S., according to the 2000 census, are listed below in order of the size of their light-pollution blobs. This is the number of red and white pixels in the Light Pollution Atlas's world map (as best I can figure it after editing out coastlines) divided by the cosine of the latitude, to compensate for distortion in the map's projection. The number of (corrected) pixels per million varies from 39 for Los Angeles to 105 for the Washington/Baltimore conurbation.

Light Pollution Blobs in the U.S.
Metro Area (2000 CMSA)	Pixels (Corrected)	Population (Millions)	Pixels per Million
New York	937	21.2	44
Chicago	745	9.2	81
Los Angeles	650	16.6	39
Wash/Balt	610	5.8	105
Philly	539	6.2	87
S.F. Bay	347	7.0	50

And here are the four largest metropolitan areas in Canada according to the 2001 census:

Light Pollution Blobs in Canada
Metro Area (2001 CMA)	Pixels (Corrected)	Population (Millions)	Pixels per Million
Montreal	612	3.7	165
Toronto	419	4.7	89
Ottawa	121	1.1	110
Vancouver	94	2.0	47

Numerous caveats apply. As I've said many times, it's not at all clear how well the Light Pollution Atlas corresponds with reality. Maybe the apparent darkness of Los Angeles is due to the fact that it was blanketed in smog when the relevant satellite pictures were taken. Maybe the relative brightness of mostl Canadian cities is due to snow. Maybe there are errors in the methodology. Population statistics are also problematic.

But let's pretend for a moment that the data are correct. What could cause such big discrepancies? One obvious answer is population density. One would expect the New York metro area to have relatively low light pollution per capita because such a large fraction of its population lives in multifamily homes, which are far more efficient in every way than single-family homes. But Los Angeles is the archetypal sprawl city for the entire world, and my figures give it even lower light pollution per capita than New York.

Truth be told, Los Angeles is actually quite densely populated compared to the exurban sprawl that grew around many U.S. cities in the last few decades. L.A. did much of its sprawling in the 50s and 60s, and then was stopped short in most directlons by mountains and federally protected land. Many of the houses date to an era when a 1,500-square-foot home on a quarter-acre lot was considered extravagant. Streetlights are probably the single biggest source of light pollution, and they're proportional to the number of street-miles. The more closely houses are spaced, the fewer streetlights per house.

How about Montreal versus Toronto? A respondent to my earlier blog suggested one possible explanation. Montreal is in Quebec, whose vast hydropower resources make it a net energy exporter. The respondent claims that Hydro-Quebec encourages everyone to use as many lights as possible. Public policy can certainly have a big effect on light pollution, but so can the policies of individual electric utility companies.

Click here to download an Excel spreadsheet with data for many more metropolitan areas in North America and other continents.

A couple of months ago, someone in Cloudy Night's Light-Pollution Forum wondered what's the most light-polluted city on Earth. I guessed that it was somewhere in Asia. A different person said that he was sure that North America headed the list. To make a long story short, I was dead wrong and he was absolutely correct.

It occurred to me that if you measure "most light-polluted" by the area where the Milky Way is difficult or impossible to see, then the Light-Pollution Atlas could provide an objective answer. All I had to do was count the number of red and white squares within each metropolitan area, correct for the fact that places at high latitudes are stretched horizontally, and I would be done.

There are a few caveats. As I said in a recent blog, it's unclear how accurately the Light Pollution Atlas captures actual conditions on the ground. Also, many big cities are on seacoasts, and it was tricky to separate the white line representing the coastline from the white for light pollution. Finally, it's quite possible that the methodology behind the Light Pollution Atlas has some kind of regional bias. But it's hard to believe that any such bias could be big enough to explain the differences that I found.

Nine of the top ten light-polluting metropolitan areas are in North America. Sorting by white squares (rather than red and white combined), Tokyo just slides onto the list in the #10 spot. But Tokyo is by far the world's most populous metropolitan area, almost equal to New York and L.A. combined. No city in Europe even makes the top 20.

It's not surprising that cities in the less developed world don't make the list. Beijing, Shanghai, Delhi, Mumbai, and Kolkatta each have light-pollution blobs comparable to a U.S. city of two million, though each of those metropolises have roughly ten times that number of people. But in these parts of the world, energy is far more expensive relative to income, and people don't splash it around heedlessly. Moreover, most of the 120 million or so people living in the map quadrant at lower left are in villages that have essentially no outdoor lighting at all.

But Europe and Japan have standards of living comparable to the U.S. I estimate that the U.S quadrant is home to about 50 million, the European quadrant to about 80 million, and the Japanese quadrant to about 90 million. Yet according to this map, the amount of light pollution in Europe and Japan is far less than in the U.S. Why is this true? Can those areas be a model for better lighting practice in the U.S.? I'll explore those subjects in a future blog. Meanwhile, I would love to hear comments from people with on-the-ground astronomy experience both in the northeastern U.S. and in Japan and/or northwestern Europe.

In 1983 Edward R. Tufte published an immensely influential book entitled The Visual Display of Quantitative Information. The gist of the book is that graphing data is (or should be) a sophisticated art. If you want to convey useful information to your reader, you have to think about the presentation, not just slap data on a page any which way. Tufte's main peeve is with graphs that present data inefficiently or ineffectively, so that you look at them and don't really know what they're saying. A related subject is that the same information can be presented graphically in different ways to make different points.

At right, I've shown a piece of the North America chart from the Light Pollution Atlas re-mapped to a conical projection. On looking at it, most people's first reaction is "My goodness, almost half of the U.S. is brilliantly lit." Whether intentional or not, that's a direct result of the colors chosen to represent different levels of light pollution. (From darkest to lightest, they're black, gray, blue, green, yellow, orange, red, and white.) The most prominent color in this map is green. That's not surprising, since the human eye is most sensitive to that color, and this is the greenest green that you computer monitor can produce, with all the green pixels firing at maximum, and all the other pixels turned off.

To my mind, this is somewhat misleading. Logically, the colors ought to go from darkest to brightest. But the orange zone appears distinctly darker than the green zone, belying the fact that skies are in fact 9× brighter in the orange than in the green. Shown at right is a very minor tweak, using darker versions of green and yellow. This makes the colors more of a continuum, and it's also more faithful to the underlying reality. In fact, the typical reaction of urbanites or suburbanites on first visiting the green zone is that skies are darker there than they ever imagined could be possible.

The differences may be more apparent in a closeup view. At right is an excerpt from the original map showing the northeastern U.S. and southeastern Canada. Detroit is on the left; Philadelphia and New York are near the bottom right of center; and Ottawa, Montreal, and Québec City (left to right) are above them near the top of the map. Prominently dark are the Great North Woods of Maine and the far greater woods of Canada. Less dark but also notable are the Adirondacks southwest of Montreal and the Alleghenies west-northwest of New York City. It looks as though New Yorkers have to go a long, long way to escape light pollution.

Here's the same region using my revised colors. This more or less matches my perception of the Northeast. It's now apparent that there's an almost unbroken corridor of reasonably dark sky — green at the worst — running down the backbone of the Appalachian Mountains. It's a long drive to a gray area from Boston or New York, but there are pockets of green — areas where the Milky Way is very attractive indeed — not too far away.

Here's a more drastic representation of the same data, which would probably seem appropriate to a typical backyard astronomer with little experience of dark skies. I've cranked green and yellow way down so that they look almost dark and boosted red to be a shade of white. Now the main contrast is between areas where the Milky Way is prominent (yellow and darker) and areas where the Milky Way is faint or invisible (red and white), with orange as a transition zone. This would be my guide if looking for a weekday observing site, where long drives are out of the question. Ideally, I would head for yellow or green, but in practice, I would just try to get out of red or white.

Which of these versions matches your perception of the light pollution around you?

Seventeen months ago, I took a break from my weekly blog, entitled Stargazing. Now I've decided to resuscitate it, though I doubt that I'll have time to write a new entry each week.

The entry that you're reading now was inspired by my ongoing obsession with measuring skyglow — a quantity that's immensely important to all stargazers, but very difficult to quantify. (see my online article if you don't know what skyglow is or why it matters.) If you visit someone's backyard, you can see for yourself what his or her light pollution is like. But how can you describe your skyglow to someone 5,000 miles away that you've met over the internet?

The three methods that seem most promising to me are digital cameras, the Sky Quality Meter (SQM) and SQM-L, and the Light Pollution Atlas, which is perhaps best known through its incorporation into the Clear Sky Chart.

Of these, the only method that can be applied without specialized equipment and skills — indeed, without even setting foot outside — is to find one's color zone in the Light Pollution Atlas. Jonathan Tomshine's Dark Sky Finder website has made this extremely easy for anybody in the U.S. or southern Canada.

But the authors of the Light Pollution Atlas have always been happy to admit that its color zones are tentative. They're based on satellite data collected more than a decade ago, over a long timespan, in varying conditions, and massaged by an experimental mathematical model of how skyglow spreadst.

To check the validity of the color zones around my home near Boston, MA, I went to Groton, a town just outside the red zone, and drove from there to my own home deep in the white zone, measuring skyglow along the way with my SQM-L, using a digital camera as a cross-check.

My measurements, shown at right, cast some doubt on the mathematical model behind color zones. For one thing, my measurements were consistently darker than the ones predicted by the key to the Clear Sky Chart's light-pollution maps. More intriguingly, the skyglow did not increase continuously, as I had expected, but followed an unmistakable step pattern. This suggests that highly local conditions play a larger role in skyglow than I would have guessed. The skyglow that I measured remained essentially constant starting 14 miles from the city center, well out in the red zone, until 7 miles from the city center, deep in the white. Throughout this area, it's dark enough to see the summer Milky Way fairly easily when it's overhead. Then, in the next three miles, the skyglow doubled.

Here's my seat-of-the-pants interpretation. I caution that this needs lots more data before it can be taken as a scientific conclusion. I think that the steps on the light-pollution curve reflect local residential patterns, cultural attitudes, and streetlighting practices.

The first big jump happened when I crossed the line from Carlisle into Bedford. Carlisle is a faux-rural suburb with 2-acre zoning, hardly any streetlights, and no bright private lighting. Unshielded mercury-vapor bulbs are for poor people, and garish "architectural lighting" is for the nouveau riche. Old money needs no glare bombs to feel safe nor any ostentatious display to feel self-confident. Houses in Carlisle have dim outdoor lights or none at all.

Bedford is much more typical of a mid-to-outer suburb, with three times Carlisle's population density, significant commercial development, and regular streetlights along the roads. But houses are still pretty far apart, and the street lighting is designed primarily to supplement automobile headlights. There are pedestrians in Bedford, but distances are too far for most people to do errands on foot.

The reading seven miles from Boston was taken at the western foot of Belmont Hill, an escarpment with lots of conservation land and broad estates. Hardly any of the skyglow there has local origins; it's almost all from the urban area on the other side of the Hill.

Then, at the eastern foot of Belmont Hill, the undeveloped land stops abruptly, and from there all the way into the center of Boston, all the houses are cheek by jowl, and the streets have broad sidewalks and closely spaced streetlights. This is where the skyglows starts to climb really sharply — roughly in proportion to the population density.

The color zones of the Light Pollution Atlas suggest a step pattern, but that's actually an illusion — an artifact inevitable in any color-coded map with coarse contour lines. The underlying mathematical model indicates that the transitions should be gradual. But my measurements suggest that abrupt transitions are the rule rather than the exception, at least in the Boston area. More research is needed!

Click here to see a tab-separated table containing details of my measurements.

I've been writing my blog for just one full year now, and it's time to take a break — for an indefinite amount of time. I'm doing a lot more work on the magazine, and it's increasingly hard to find time to write for our website.

Granted, the blog is just a small part of what I write for the Web; the lion's share of my time goes to observing stories like the recent one on the Geminids. But when push comes to shove, it's the observing stories that are indispensable. I'd much rather have 1000 people outside at night using the star charts that I've posted on our website than the same number of people chit-chatting about my opinions during the day. Opinions are cheap, but facts are facts.

Nonetheless, I've had a lot of fun spouting my opinions over the last year, and I still have a couple dozen topics that I've never gotten around to. Maybe you'll see some of them in the magazine! Meanwhile, here's an index to all the subjects that I've blogged:

Dec 19, 2007	The Scientific Value of Visual Observing
Dec 12, 2007	Holmes: Victim of Its Own Success
Dec 7, 2007	A Night in the Life of an S&T Editor
Nov 30, 2007	The Reliability of Visual Observing
Nov 20, 2007	The Amazing Comet Holmes
Nov 15, 2007	Traveling Without a Scope
Oct 11, 2007	Bye for a While
Sep 28, 2007	Big Binocular Messier Survey
Sep 20, 2007	Do the Planet Limbo
Sep 12, 2007	Calendars
Aug 31, 2007	Ridiculously Small Optics
Aug 29, 2007	Moonset Eclipse
Aug 23, 2007	Astronomical Twilight
Aug 16, 2007	Discussions Restored
Aug 14, 2007	Stellafane
Aug 9, 2007	Some Suburban Messiers
Aug 2, 2007	Twilight
Jul 30, 2007	Microsaccades
Jul 26, 2007	The North America Nebula
Jul 24, 2007	Comet Envy
Jul 20, 2007	Anticipating August
Jul 17, 2007	Pollution and Stargazing
Jul 13, 2007	Galaxies and Clusters and Comet, Oh My!
Jul 5, 2007	Strange Encounters Part II
Jun 26, 2007	Strangers in the Night
Jun 21, 2007	The Day the Sun Stands Still
Jun 14, 2007	Decisions, Decisions
Jun 8, 2007	Desk-Chair Science
Jun 1, 2007	Fear
May 25, 2007	Unexpected Connections
May 15, 2007	Big Sky
May 10, 2007	Coda: Binoculars Versus Starblast
May 1, 2007	Binoculars Part III: One Eye Versus Two
Apr 27, 2007	Three Binoculars: Part II
Apr 23, 2007	A Tale of Three Binoculars: Part I
Apr 13, 2007	Stars and Birds
Apr 4, 2007	How Brightly Shines the Moon?
Mar 31, 2007	Better Late Than Never
Mar 22, 2007	Measuring Skyglow
Mar 28, 2007	School Time
Mar 21, 2007	Dressing Up for an Evening Out
Mar 16, 2007	Equipment
Mar 14, 2007	Waiting for Sagittarius
Mar 7, 2007	The Meaning of Stargazing
Mar 5, 2007	A Spontaneous Star Party
Feb 26, 2007	Celestial Time and Human Time
Feb 22, 2007	Instant Astronomy
Feb 19, 2007	June in February
Feb 16, 2007	Stars and Snowflakes
Feb 13, 2007	Mercury Retrospective
Feb 9, 2007	Keeping Myself Honest
Jan 31, 2007	Hello World

John Henry said to his captain:
A man ain't nothing but a man.
But before I let that steam drill beat me down,
I'll die with my hammer in my hand.

For more than 100 years, it's been obvious that astrophotography has far more scientific value than visual observing. Visual observing reigned supreme in a few niches until recently. For instance, normal cameras can't match the eye at capturing fine details on the planets during brief moments of steady seeing. But with the advent of video cameras and computerized image stacking, even that advantage was lost. It's probably fair to say that there's no astronomy that the human eye can do that can't be done as well or better by electronic imaging. Moreover, electronic devices can make images across the entire electromagnetic spectrum, almost all of which is invisible to the eye.

Most important of all, imaging leaves an objective, auditable trail. Visual observations, by contrast, are notoriously unreliable. The history of astronomy is littered with sightings that proved to be false — dating back to the invention of the telescope and before. Most people have heard of the canals that Percival Lowell fantasized seeing on Mars. But did you know that Galileo saw cities on the Moon? That particular error has been swept under the rug by people eager to present science as an inexorable, inevitable, one-way march to the truth.

Yet amateur astronomers continue to make valuable contributions to astronomy using nothing but their eyes and wits. How is this possible? And how much longer can it go on?

The discoveries that loom largest in the popular imagination are comets. That's no doubt partly due to the fact that the brightest ones are spectacular to look at, and many fainter ones make spectacular photos. But perhaps even more important is that fact that comets are named after their discoverers. We're a society that idolizes the individual — despite the fact that never before in history have individual contributions counted for so little.

But only a small fraction of comets are discovered by amateurs these days, and of those, more are found by imaging than visual observing. I bet that most of the amateur-discovered comets would be found by the pros not long after, and that the time lag wouldn't matter much.

Supernova discoveries don't get nearly as much press, but they probably have more scientific value. Again, these would eventually be found by the pros. But supernovas change a lot faster than comets (usually!), and the early stages of a supernova's outburst are very interesting and important. Even if the amateurs only speed discovery by a few hours, that can have considerable value.

Supernovas are extreme examples of variable stars, and amateurs have played a central role in variable-star observing for a long time. But these days, the lion's share of the good work is done with CCD cameras. A skilled amateur can measure a star's brightness with an error of 0.1% using a CCD camera, compared to 5% or 10% for the best visual estimates.

But Arne Henden, head of the American Association of Variable Star Observers, assures me that visual observers still play a key role in monitoring cataclysmic variables — stars that erupt unpredictably, including supernovas, novas, and other less glamorous categories. That's because people skilled at observing these stars are scattered all around Earth, eager to be mobilized with a moment's notice. That makes it possible to monitor the stars continuously; it's always nighttime somewhere on Earth.

To my mind, the area where visual observing is most important is meteor science. It just so happens that humans can monitor a huge field of view in very dim light and spot anything that moves. This ability, which presumably evolved for avoiding predators, happens to be ideally suited to detecting meteors as well.

And as with variable-star observers, there's a worldwide network of people who like nothing better than lying outside in the freezing cold, at a time when all self-respecting people are asleep, keeping careful track of minuscule blips of moving light. So most of what we know about meteors comes from visual observation.

I have to conclude that visual observers are most valuable when they're acting in concert, not as heroic individuals. And just as John Henry achieved glory by doing a purely mechanical job, visual observers are at their best when they're emulating machines: objective and dispassionate. Really, their main advantage is cost. If you had to pay meteor observers by the hour, it would cost a fortune; machines would do the same job cheaper. But amateurs do it for love, not money.

Well, I've posted the article about 8P/Tuttle, so now I can discuss comets with a clean conscience.

Incidentally — while I'm on the subject — I decided to post the S&T charts in black-on-white format, as PDFs. The way I figure it, color charts are easier to read on the screen, and more attractive in the magazine. But when we post really detailed charts like these, most people are going to print them, so that they can carry them into the field. And on most home printers, charts with dark backgrounds are ugly, smeary, tend to jam the printer, and use lots of expensive ink. So I'm inclined to use traditional bright-on-dark for simple, at-a-glance charts, but black-on-white for detailed charts. What do you think?

But that's not what I set out to write about. Last weekend, I observed Comet Tuttle for the first time under reasonably dark skies. And naturally, I took a look at Comet Holmes too. What's odd is that I was more excited about faint, featureless Tuttle than dazzling Holmes. I'm beginning to take Holmes for granted.

Yes, Holmes is overwhelmingly big and bright, and shows amazing detail too. But it changes so little from one night to the next, either in position or appearance. It's almost as though the sky has acquired a new deep-sky object, a permanent fixture. As far as I can tell, it has dimmed not at all in the last 30 days. At this rate, it's going to remain a naked-eye spectacle for the better part of next year!

There are benefits to not having a backyard to observe in. When I go out with my telescope, I can look for a really good spot instead of having to settle for all the obstructions that plague most backyard stargazers. But it also means that every observing session requires advance planning — I can't just step outside on a whim. And that, in turn, means that I'm totally at the mercy of the weather forecast. An unforecast clear night might as well not happen.

Last night was the reverse: it was forecast to be clear, so I invested the 1.5 hours required to drive round-trip to my astronomy club's observing field. In fact, the clouds started to move in not long after I arrived. But that still left me enough time to accomplish my primary observing goal.

With all the (well-deserved) hoopla about Comet Holmes, it's easy to forget that another major comet is creeping up on us. The January issue of the magazine has a fine story about Comet 8P/Tuttle, but we've barely said a word about it on the Web. And it looks as though I've volunteered myself to post that story.

It's tricky to say when such a Web story should go live. On the one hand, our star charts are better than anything else on the Web, and it's a shame not to make them available. On the other hand, the Web is aimed at a broader audience than the magazine, and we don't want to encourage people to go out and look at a comet that will disappoint them. Only one way to make that judgment — have to look for myself.

I was pleased that I found the comet fairly easily using the chart in the magazine (excerpted at right). And that despite the rather poor suburban conditions: magnitude 19.2 per square arcsecond as measured by my Sky Quality Meter. But a showpiece it's not — not yet, anway. I could see it with direct vision at 40× in my 7-inch scope, but barely. With Comet Holmes as an alternative, only a hard-core comet-hunter is going to seek out 8P/Tuttle. Most hardcore comet-hunters subscribe to S&T, and the ones that don't know how to make their own charts.

So it looks as though I don't have to hustle yet. Comet Tuttle is brightening rapidly, but it'll still be a couple of weeks before it's of much interest to the broader public.

People sometimes ask me whether visual astronomy — looking through a telescope's eyepiece, as opposed to photography or electronic imaging — still has any scientific value. The answer, somewhat surprisingly, is yes. Astronomy still has a few niches where visual observers make significant contributions. I'll give examples in future blog entries.

But visual astronomy has many drawbacks. One problem that's plagued it since Galileo's day is reliability. How can you be sure that another observer's report is honest and accurate? Indeed, how sure can you be of what you see yourself? It's a critical issue for anybody straining to see things on the edge of visibility. If you don't try at all, you're greatly handicapped. Try too hard, and you start to see things that aren't really there.

The most egregious cases are people who lie about what they see. Little better (maybe even worse) are people like Percival Lowell, who popularized the canals of Mars. His sin was lack of objectivity. The canals on Mars were controversial, and Lowell was hell-bent on proving their existence. With an attitude like that, he could hardly help seeing them, despite the fact that they don't really exist.

But even completely honest, objective observers can be fooled by optical illusions. Everyone sees the Moon as being bigger when it's near the horizon than when it's high in the sky. Knowing that this is false makes no difference; the illusion is wired into the human brain.

An interesting controversy erupted recently on the Deep Sky Forum on Cloudy Nights — though the arguments have degenerated somewhat recently. Here's the situation.

Many if not most people see green tints in the brighter parts of M42, the Orion Nebula. There's no controversy about that. There is indeed green light, emitted by excited oxygen and hydrogen atoms, and it's bright enough to stimulate color vision. The observation is accurate.

Some people (including me) have also seen pink or reddish tints when observing this nebula through big telescopes. Enough people have reported this that we can be sure it's not just imagination. And there's also no doubt that a lot of the nebula's light is indeed red. But here's the kicker. Human eyes are notoriously insensitive to red light at low intensity. Laboratory experiments indicate that the red light in M42 isn't bright enough to stimulate color vision. People do indeed sometimes see red at low light levels, but its an illusion that can be created by any color light, not just red.

So it's possible that the red tints of M42 are both accurate and illusory! In other words, the nebula is red, and people see it as red, but there's no causal relationship between those two facts. Of course, it's also possible that the laboratory experiments are wrong, or don't faithfully replicate the real-world situation.

What a mess! It's really hard to think of experiments that can say for sure what's going on. But astronomy is full of such unexplained sightings.

For instance, there are the notorious "transient lunar phenomena," the ashen light on the unlit side of Venus, and reports by binocular observers that the visual disk of the Andromeda Galaxy stretches across 5° of sky. All of these defy scientific explanation, but have been reported by several reliable observers.

I'm a little bemused by all the reports that Comet Holmes is getting harder to see. No doubt that's true, but I can't tell by firsthand experience, because each time I've viewed the comet, the conditions have been radically different from the previous time. (Remember, I was traveling in India when the comet first flared up.) But I've found it easily every time I've looked, and it's always appeared both bigger and brighter than I expected.

Most of my sightings have been through my 10×30 image-stabilized binoculars, which seem to be nearly ideal for this particular job. All along, I've found Comet Holmes a little hard to snag without optical aid because it's in such a crowded star field. And because of the comet's immense size and modest surface brightness, I find it much more prominent through binoculars than through a telescope.

My most recent sighting was on Sunday night, around 3h UT on Nov. 19, from my apartment in Cambridge, MA. The comet was immediately visible through my 10×30 binoculars, a bit bigger than 30', with Alpha Persei (Mirfak) right on its edge. A brighter elliptical blob about 10'×20' extended from the center of the coma to the edge.

This was despite the urban skyglow (brighter than mag 18.0 per square arcsecond) and the first-quarter Moon. I'd rate the overall visibility roughly equal to the Andromeda Galaxy, and much higher than any other galaxy. Vastly brighter than M33, for example!

My urban skies are only about 50% brighter at full Moon than without a Moon, so I'm predicting that I'll be able to see the comet with my binoculars right through the full Moon period. And after that, it should start to get more prominent again — at least when viewed from dark sites. I can't wait to see how big the coma has grown by the end of the year!

Well, here I am, back from India. As I said, I didn't bring a telescope with me. On the whole, I don't regret that decision. My lightest "serious" telescope rig adds up to roughly 10 pounds — a pretty major encumbrance. And the equipment's cost is as much of a problem as its weight and bulk.

Consider, for instance, traveling on a typical Indian bus. Most luggage goes on top, where it's liable to be stolen or crushed — not a very attractive option for a telescope. But carrying it on your lap in a crowded bus isn't especially attractive either.

Instead, I brought a truly minimal set of astronomical equipment: my 10×30 image-stabilized binoculars, a red flashlight, and Orion's Deep Map 600. In fact, Deep Map 600 was the only real "extra." I would have brought binoculars anyway, to look at birds, distant mountains, and so on. And flashlights are essential in India, where the electric power is notoriously unreliable.

The binoculars came very much in handy when Comet Holmes underwent its unexpected and startling outburst. And I was also very glad to have them during our 3-day "camel safari" in Rajasthan. A camel safari is a slightly hokey touristic experience, but it does give some sense what it was like to travel in one of the great camel caravans that used to cross this region for centuries or millenia. Everyone agrees that the highlight is sleeping on the dunes under a genuinely dark sky.

We were at latitude 26° north — about the same as Miami, Florida or Brownsville, Texas. That allowed me to see a big chunk of sky between declination 30° and 50° south that's not accessible from my home in Massachusetts. The binoculars were of limited use in the evening, when the galaxy fields of Sculptor and Fornax were transiting the meridian. But they were great in the early morning for viewing the splendid open clusters of Puppis and Vela. I was particularly struck by the contrasting pair NGC 2477 and NGC 2451, the former a patch of creamy light through binoculars, and the latter coarse and well resolved. Very much like M46 and M47.

But I found myself missing a telescope not for my sake but for my companions. We had a guy from Australia and one from England in our group, and the Englishman had never so much as seen the Milky Way before, much less looked through a telescope. I would have loved to show him a few things.

I missed a telescope even more a couple of weeks earlier in the trip, when we were in the Himalayas. We ended up spending a night unexpectedly in a small village, and we hung around outside in the evening as the glow disappeared from the high peaks and the stars started to come out. Not surprisingly, every child in the village was out there with us. But none of them spoke any English, and our Hindi is extremely rudimentary, so we didn't have much to say to each other. I really, really wished that I had my telescope with me so that I could have spoken to them in the universal language of astronomy!

I normally try to post at least one item per week in my blog, but I missed last week. That's because I've been frantically busy preparing for a four-week vacation.

I'm off to India. Astronomy's not one of my primary goals, but every place on Earth and every human endeavor has some relation to astronomy, the oldest and most universal of all the sciences (except maybe medicine). So whenever and wherever I go, my vocation and avocation travel with me.

Moving 15° farther south potentially opens up new celestial vistas. But the great southern objects in autumn's evening sky are all galaxies, which require a big telescope to do them justice — and I'm not about to schlep a scope along on this trip. Maybe I'll find out how many of the Fornax galaxies are visible through 10×30 binoculars.

My primary observational goals are to show my wife two things she's never seen: Canopus, the sky's second-brightest star, and the zodiacal light. Eta Carinae will also be poking above the horizon just before dawn, but it will be so low that I doubt we'll get a decent view.

Astronomy is also tremendously important in India's cultural heritage. If you think people are crazy about astrology in the West, you should see India! But that's a topic for a whole 'nother article ...

On a more sober note, it was India that gave us the mathematical tools (via the Arabs) that were required to make the jump to modern science. And India is home to what are perhaps the most architecturally striking astronomical observatories in the world: the five Jantar Mantars constructed by Maharajah Jai Singh II (1688-1743).

What intrigues me most about these observatories is how archaic they were when they were built. Though Jai Singh was a Hindu, his observatories are clearly descended from Islamic astronomical tradition. They very much resemble the observatory in Samarkand built by Ulugh Beg, another (far greater) astronomer-king. But whereas Ulugh Beg's observatory was cutting-edge when it was built in the 15th century, telescopes rendered naked-eye observatories obsolete a century before Jai Singh built his. And Jai Singh, a highly cultured man, must surely have known that.

So perhaps the Jantar Mantars were built more for publicity than for practical use. I suppose we'll never know.