Tuesday, June 06, 2006

The Conjunction Project



For those who remember the Moon-Venus-Jupiter conjunction of September 2005, this will be a treat: below I've collected a few links to photos of that event as seen over the course of four days from various places around the world.

111 links to be precise, from 96 photographers in 21 countries.

I've organized the links so that you can check out the photos in roughly the order that they were taken. As the Earth turned in an eastwardly direction, photographers farther to the west got to see the conjunction when dusk fell on their part of their world. So the links are ordered according to the longitudes of the photographers (to the best of my information). This does not produce an exact chronological order, as I discuss in the essay that follows, but it suffices to give a sense of the Moon moving across the sky in its monthlong orbit around the Earth. On each of the four days of the conjunction, the Moon appears in a new place with relation to Jupiter and Venus, as in the two photos above, which I took on September 6th and 7th. And the Moon's progress across the sky within a single night can also be observed, quite easily.

A note on linking. Most links below will take you to photographs that appear in full context, amid galleries and writings about the conjunction. Below I have sometimes linked directly to an individual photo, but there is usually a way to surf backwards or forwards to other pages providing the context. Occasionally no such pages are available, but in those cases where one or more pages from the photographer exist which provide context, and surfing to them is not possible, I've provided an extra link.

Do enjoy.


SEPTEMBER 5, 2005

The conjunction "event" might be said to have begun on this night, in Australia, where Ian Musgrave caught the three bodies in a single photograph from Adelaide, South Australia, on the day after New Moon. Just about 39 hours old, the Moon appears as a slender crescent almost at the horizon. The next three lights up from the bottom are Jupiter, Venus, and the star Spica. See Ian's full blog post for more.

The Moon is hard to capture on the evening after its New phase because it is not visible for very long after nightfall, so closely does it follow the setting Sun into the horizon. You can see the difficulty from another perspective, by looking at Australia as it would appear to an observer standing on the Moon. The image below, courtesy of the U.S. Naval Observatory, shows what the Earth would have looked like on September 5, from the Moon, as Australia rotated into night.



The day before, at New Moon phase, the Moon would have been between the Earth and the Sun, and an observer on the Moon would not have seen the shadow of night on either the far left or far right edges of the Earth. In the image above, however, a very slender shadow has begun to cover eastern Australia. Half of our planet, of course, is always in darkness, but this slender shadow is the narrow part of the Earth's night-time half where the Sun has set but the Moon can still be seen, though it too is about to set: a narrow window of time for Moon-spotting.

As the Moon orbits the Earth, it is moving to the "right" of the image, slowly coming around to the half of the globe that is facing away from the Sun and in nighttime. If you looked from the Moon, each passing minute you would see more of that nighttime half of the Earth, until Full Moon is reached, when the Earth, now placed between the Moon and the Sun, would appear to be dark from the far left edge to the far right.

But we are not going that far: we're only covering four days. Below I've provided images intermittently depicting the Earth's rotation; and underneath each image I've placed links to several photographs taken in those places falling into evening.

A half-day after Australia saw the Moon in the sky along with Venus and Jupiter, the eastern seaboard of North America has fallen into night, and the Moon has moved a little more out of the space between the Sun and the Earth:


The following links from North America show the bodies of the conjunction setting in a diagonal line that's close to horizontal, for a simple reason. Note that the dotted lines representing latitude are wrapping around the right edge of the Earth in varying ways. Imagine one more line running parallel to the top and bottom edges of the rectangle, and cutting the globe exactly in half: it represents the plane of the Earth's orbit around the Sun, or the ecliptic. (The Moon does not orbit right in the plane of the ecliptic, so a view from the Sun would be necessary to draw the line of the ecliptic with greatest precision, but the Moon at this point was very close to the ecliptic and will serve the purpose). The dotted line cutting through Australia is arriving at the right edge of the globe almost parallel to the ecliptic, which means that an Australian observer looking out in the direction of the Sun at other objects in the ecliptic -- such as the Moon, Venus, Jupiter and Spica -- will see them setting in the sky in a nearly straight line down into the horizon. The dotted line cutting through North America is arriving at the right edge of the globe at a shallow angle to the ecliptic, which means that the same bodies will be seen hitting the horizon in a manner that looks almost horizontal.

Eric Schandall facing Liberty State Park from Manhattan, New York, U.S.

Steven D. Adams in Rochester, New York, U.S.

Ronnie Sherrill in Troutman, North Carolina, U.S.

Bob McBroom of Kansas Wind Power in Holton, Kansas, U.S.

Ginger Mayfield of Divide, Colorado, U.S.

Judy A. Mosby in Lovelock, Nevada, U.S.

Edgar of San Leandro, California, U.S.

The next stop is in Australia, only six time zones away -- but since we're crossing the International Dateline, we add a day.


SEPTEMBER 6



Beche-la-mer in Sydney, New South Wales, Australia

Shevill Mathers at the Southern Cross Observatory in Cambridge, Tasmania, Australia

Damian Sparkes of Famous Daisy in Adelaide, South Australia, Australia

Ian Musgrave (from his blog post) in Adelaide, South Australia, Australia



Ami B.B. of Flickr in Israel

Yilmaz Akyol of Kusadasi, Turkey

Domenico Licchelli in Lecce, Italy

Peter Wienerroither (from his website) in Vienna, Austria

Leonard Ellul Mercer in Mdina, Malta



Pete Lawrence in Selsey, United Kingdom

Selsey, which is on the southern coast of England, was the farthest north of all the photos I found, at 51 degrees of latitude. The city barely got a look at the night-time Moon before being rotated behind the Earth, and like all the Northern Hemisphere locations so far listed, it is hard to find in the images. The United Kingdom is in the image directly above, but it is just floating on the edge. The reason, I hope, can be explained simply.

In all these images, you can see that the dotted line representing the equator looks a bit more like a frown than a smile. It looks like the Northern Hemisphere is slightly turned upward, like a snob whose nose is turned up and whose mouth appears like a frown. And indeed, at this time between the March and September equinoxes, the equator is slightly turned upward on the side of the Earth facing the Sun. That is due to the Earth's 23-degree tilt with regard to the ecliptic.

The effect that I'm describing is exaggerated in these images because, as noted already, the Moon does not orbit precisely in the ecliptic. At this point, the Moon is below the ecliptic and heading further down. But the Moon's orbit around the Earth is inclined to the ecliptic at only 5 degrees, and in the image above, it has not yet traveled that far below the ecliptic. The frown is merely exaggerated: an observer on the Sun would see it too.

In any case, the effect of having the Northern Hemisphere turned slightly upward is that north of a certain latitude, astronomers trying to get a look at the Sun or at any bodies near that same line of sight (such as the New Moon, or Venus and Jupiter at this time) are being tilted away from a direct view. Imagine pulling up the equatorial dotted line, into a deeper frown: Greenland and the top of North America would disappear entirely.

Because the Northern Hemisphere was turned a little away from the Sun, we can say in general terms that the Southern Hemisphere got better, longer looks at the conjunction.

Laurent Laveder on the Île-Tudy (near Quimper), Bretagne, France

Pablo César Pérez González in San Agustín, Canary Islands, Spain



Pedro Mohr (see the second photo on the page) in Capela de Santana, southeastern Brazil

Mariano Ribas (from this gallery) at Planetario Galileo Galilei de Buenos Aires, Argentina

Facundo A. Fernández in Rosario, Santa Fe, Argentina



Glane in Nova Scotia, Canada

Steven Pinker (from his gallery) in Cape Cod, Massachusetts, U.S.

Jay Ouellet near Quebec City, Quebec, Canada

Bob Ribokas in Hull, Massachusetts, U.S.

Guillaume Poulin in Bromptonville, Quebec, Canada

Phil Harrington looking out at Belle Terre from Cedar Beach, New York, U.S.

Bill Bradley at Fire Island, Robert Moses State Park, New York, U.S.

Tony Hoffman in Queens, New York, U.S.

Myself in Brooklyn, New York, U.S.

Eric Schandall looking out at Ellis Island from Manhattan, New York, U.S.

A.V. Ketterer (from his page) in Piscataway, New Jersey, U.S.

Jerry Lodriguss in Voorhees, New Jersey, U.S.

Dick (Gussie) Fink-Nottle in Lindenwold, New Jersey, U.S.

Drew Evans (see his gallery) in Holland, Pennsylvania, U.S.

Steven D. Adams in Rochester, New York, U.S.

Gerry Hintermeister in Apex, North Carolina, U.S.

Narayan Kovvali in Durham, North Carolina, U.S.

Tom Carney in Waynesboro, Virginia, U.S.

Steve Browne looking out at Jordan Lake, south of Durham, North Carolina, U.S.

David Illig in Perryopolis, Pennsylvania, U.S.

Robert T. Smith of Stoneville, North Carolina, U.S.

Ronnie Sherrill of Troutman, North Carolina, U.S.

Charles Tilley of Statesville, North Carolina, U.S.

Matt Hawrysko in Canton, Ohio, U.S.

John H. Schmidt, M.D. in Charleston, West Virginia, U.S.

Yurii Pidopryhora in Athens, Ohio, U.S.

Jeff Stevens at Whitmore Lake, Michigan, U.S.

Parag Sahasrabudhe south of Ann Arbor, Michigan, U.S.

Dave Quint in Spring Hill, Tennessee, U.S.

Suzanne in east central Minnesota, U.S.

Dick Locke in The Woodlands, Texas, U.S.

Paco Flores in Monterrey, Nuevo Leon, Mexico

Lloyd Overcash (from his gallery) in Ft. Davis, Texas, U.S.

Thad V'Soske near the Colorado National Monument, U.S.

William Olson of Ammon, Idaho, U.S.

Heron Herodias of Flickr outside Salt Lake City, Utah, U.S.

Joe Orman in the Sierra Estrella mountain range south of Phoenix, Arizona, U.S.

Michael Wilson at the Great Salt Lake, Utah, U.S.



Mike O’Leary of El Cajon, California, U.S.

Kevin Baird in San Diego, California, U.S.

Nathan Hubbard in San Diego, California, U.S.

Kevin north of La Jolla, California, U.S.

Neil Schneiderhan in Orange, California, U.S.

Ed Johnson in Los Angeles, California, U.S.

Ryan in Santa Monica, California, U.S.

Paul Keen in the San Fernando Valley northwest of Los Angeles, California, U.S.

F. Ringwald (from this gallery) at Fresno State, California, U.S.

Andy Skinner at Hensley Lake, California, U.S.

Mattie_Shoes of Flickr outside Sacramento, California, U.S.

Dan Oneal of Santa Cruz, California, U.S.

John Koetsier in Abbotsford, British Columbia, Canada

Cathy near San Bruno, California, U.S.

Dave Ward (download the full size) near Bellingham, Washington, U.S.

Jack Amsden in Medford, Oregon

Malcolm Scrimger (from his gallery) looking out at Vancouver Island from Esquimalt, British Columbia, Canada


SEPTEMBER 7




Brenda Anderson in Masterton, New Zealand

Scott Macleod Liddle in Brisbane, Queensland, Australia

Steve Massey (see the top photo, taken with a camcorder) in Sydney, New South Wales, Australia

Beche-la-mer in Sydney, New South Wales, Australia

snappinhappy of Flickr at the Centrepoint Tower in Sydney, New South Wales, Australia

Shevill Mathers at the Southern Cross Observatory in Cambridge, Tasmania, Australia

Firdaus Webgrrl in Ferntree Gully near Melbourne, Victoria, Australia

Roland Gesthuizen in Melbourne, Victoria, Australia

Marcello Avolio of Gorge Creek Orchards in Dimbulah, Queensland, Australia

This was a unique view of the conjunction because it was so close to the equator. Check out Marcello's single page of 21 full-size photos.

Incidentally, Dimbulah was above the plane of the ecliptic when night fell there. One half of the equator (like one half of the Earth) is always above the ecliptic, so a few places close to the equator are taken above and below the ecliptic every 24 hours. Those are called the tropics, defined as the regions of the Earth within 23 degrees of the equator (corresponding with the 23-degree axial tilt of the Earth).

For a conversation about the conjunction, see this page's audio interview of Graeme White, from JCU’s Centre for Astronomy, by ABC Far North Queensland.

John Kazanas in Brunswick, Victoria, Australia

Shane Ekerbicer of Geelong Stormchasers, Geelong, Victoria, Australia

Damian Sparkes of Famous Daisy in Adelaide, South Australia, Australia

Paul Schilling (from his gallery) in Morphett Vale near Adelaide, South Australia, Australia

Ian Musgrave (with extra photos) in Adelaide, South Australia, Australia



Junichiro Aoyama on a post-typhoon evening in Kyoto prefecture, Japan

Two things about Junichiro's photo.

First, as noted above with regard to the Northern Hemisphere, Junichiro had relatively little time to see the conjunction. In New Zealand, Jupiter did not set until almost 3 hours after sunset. In Japan, Jupiter set 93 minutes after sunset -- a period of which only about half would be dark enough to see the planets.

Secondly, note how close the Moon passes by Spica in Junichiro's original, and how far from Venus. By contrast, the photos from Australia on this night nearly caught the Moon passing over Venus.

The cause is very simple, and it's called parallax. When looking out into space from the half of our globe that sits above the ecliptic, Venus and the Moon are like two tennis balls set out on the desk in front of you, with one close to you and the other closer to the computer screen. You see a space between the two balls, but if you lower your chair, the space starts to diminish. Lower your head all the way to the level of the desk, and you may even see one ball eclipse the other.

In Australia, they nearly did see it. But I have no photos of the Moon eclipsing Venus, because Australia is not far enough below the ecliptic. Shevill Mathers in Tasmania was the farthest south, at 43 degrees of latitude, but he could not have seen the eclipse. As far as I can tell from SkyView Cafe, only the Pacific Ocean below Australia, and the southwestern tip of New Zealand's Stewart Island, would have seen the Moon, just as it was hitting the horizon, eclipsing Venus. Antarctica would have seen the eclipse quite high in the sky, and would have had enough time to see Venus emerge from the eclipse.

At the South Pole, the Moon would have entirely escaped the apparent corridor in the sky between Venus and Spica; and on SkyView I see the possibility of a grazing occultation, with Venus winking in and out behind mountains on the edge of the lunar disk.



Paul Watson in Cape Town, South Africa

Harald Wochner (with other photos) at Lake Constance near the German-Swiss border in Radolfzell, Germany

Patrick Bornet (see the second photo down) in Saint-martin-sur-Nohain, Nièvre, France

Laurent Laveder on the Île-Tudy near Quimper, Bretagne, France

Rafa Gallego in San Roque, Andalusia, Spain



Supersnail of Flickr in Aruba, Kingdom of the Netherlands

Aruba was the closest of all the photos to the equator, at 12.5 degrees North. It is the only photo that clearly shows the three bodies in a classic diagonal line.

A. Pasten, A. Gomez and NOAO/AURA/NSF for the Cerro Tololo Inter-American Observatory near La Serena, Chile

Patrick LaFreniere at Pequawket Pond near Mount Washington Observatory, New Hampshire, U.S.

Myself in Brooklyn, New York, U.S.

Tristan Panek in Wilmington, Delaware, U.S.

Dick Locke in The Woodlands, Texas, U.S.

Judy A. Mosby in Lovelock, Nevada, U.S.

Ed Johnson in Los Angeles, California, U.S.

Jasmine008 of Flickr in Stumptown, Oregon, U.S.


SEPTEMBER 8

MisterPH of Flickr in Brisbane, Queensland, Australia

Shevill Mathers (with an extra photo) at the Southern Cross Observatory in Cambridge, Tasmania, Australia



Shonteen of Flickr in Hsinchu City, Taiwan



F. Ringwald (from this gallery) at Fresno State, California, U.S.


MISSING PHOTOS

There must be many thousands of photos out there. What I have here is a sampling from the internet, mostly via Google, Google Images and Flickr (the latter hosts 42 of the links by itself -- including 10 of the 16 links from California).

There are very large regions, and cities, not represented in this project. As discussed above, there is an astronomical reason when it comes to the Northern Hemipshere. Selsey, U.K. was the farthest north, at 51 degrees latitude. The largest country in the world, Russia, has no photos here, but most of that country lies above Selsey's latitude. Moscow is north of that line, as are Warsaw, Berlin, Brussels and London. The Scandinavian nations lie entirely above it, and so do Holland, the Baltic States, Belarus, Ireland, and Iceland. The same goes for Greenland, Alaska, and most of Canada -- though of course these places are not heavily populated.

But there are large regions missing for no astronomical reason. Connection to the internet, and perhaps the prevalance of digital photography as a hobby and a profession, seem to have been large factors. Most of the 64 links from the contiguous United States came from the western and eastern coasts, where light pollution is thick but internet connection is well-established. All 20 of the links from Australia, the second-best represented nation, came from the eastern half of that country and from many of its largest cities.

Of course, a large majority of the links come from countries where English is the dominant language. Here my own linguistic limitations played a factor. Using the free translations at Babblefish, I did run searches on Google and Flickr for the terms "Moon", "Venus" and "Jupiter" -- and occasionally "Spica" -- in Spanish, French, Dutch, Italian, German, Portuguese, and Russian. (A search in Dutch is what turned up the Aruba photo). I also attempted some tentative searches in the scripts of Japanese, Chinese, Hebrew and Arabic, without any luck. Yet I have to imagine that many more internet photos in the missing regions could be found with a basic command of the corresponding languages.

I don't know why no photos turned up from Hawaii, and I still hope to find photos from there or from any Pacific island not already included.

I found nothing from mainland China and India, each with over 1 billion people. The same goes for large regions of South America and Africa around the equator.

In fact, only two of the photos -- Dimbulah and Aruba -- were taken within the vast 47-degree path that lies between the Tropic of Cancer and the Tropic of Capricorn. The tropics contain the most economically impoverished regions of the world, where at least half -- and I have seen an estimate of three-quarters -- of the global population currently live.


THE LONGITUDE PROBLEM

I have the photos organized according to longitude, which is a bit of conceit. It gives a rough sense of new photographers getting to view the conjunction as the Earth rotated them into darkness but, as became increasingly clear in the images above, lines of longitude did not correspond exactly to the line separating day from night (the terminator) in early September.

The exact dates of the equinoxes and solstices differ from year to year, but in general you can say that a line drawn along the terminator on March 20 and September 22 will correspond to a line of longitude. You can see the correspondence in the images below, which show that neither pole is turned toward the Sun on September 22.




On the September equinox, New York would have seen sunset at the exact moment that it occurred on the same line of longitude (74 degrees West) in a Southern Hemisphere location just as far from the equator (41 degrees).

Now compare with this image, which corresponds to the evening of September 6th in New York:





On that date, New York saw sunset 42 minutes after its southern counterpart.

In short, the conjunction was seen "first" by denizens of the Southern Hemisphere, because they crossed the terminator before their northern counterparts of the same longitude.

Now it may be that 42 minutes makes a large difference, but it is not as large as the three hours of difference that exist between New York and its counterpart on the June and December solstices, when one or the other hemisphere is heavily tilted toward the Sun. This is a typical June 21 view:




Secondly, the order of the photographs was not much impacted, because the longitudes of the Southern Hemisphere photographers did not overlap with those of the North. That was purely by chance, since of course there are longitudes shared by North and South America, by Australia and East Asia, and by Africa and Europe. But on each day of the conjunction, photographers on those southern continents saw the Moon from longitudes east of the corresponding Northern photographers. For example, the South American photographers were all done before the first North American photographer came to the terminator. That means that the Earth's easterwardly rotation, by itself and without a tilt in the Earth's axis, would have carried the Southern hemisphere photographers first into the terminator. The fact that the Earth's axis tilted them in early September "ahead" of their Northern neighbors just made them arrive even earlier at the terminator.

That does present an interesting question. How would the order have looked for a conjunction on the other side of the equinox (e.g., early October), when Northern photographers would be tilted first toward the terminator? I'm going to leave that one alone.

The important thing is that no pair of photographers experienced sunset anything like 42 minutes apart due to different latitudes, except those ordered accurately (e.g., South Africa before New York).

Some differences do exist within single continents. Dimbulah, Australia, has two neighboring cities about 20 degrees to the south that are listed after it on September 7 but actually experienced sunset 10 minutes earlier: Brunswick and Geelong. The day before, Suzanne in Minnesota also had a distant southern neighbor listed after her who experienced sunset before her, by 5 minutes: Dick Locke in The Woodlands, Texas, about 15 degrees to the south. And many other discrepancies exist on an even less significant scale, for instance in the thick cluster of photographs listed from California.

What I really need is a Google Earth overlay that will display the terminator at any given time during those four days of the conjunction. I have all the photographers marked off on Google Earth, so I would need only to spin them into night to get the order -- with the added requirement that Google Earth would enable you to spin the planet on its axis, and to move the clock forward just by spinning the globe, with the terminator being updated continuously. I don't know how educational it would be to use such an application (as opposed to creating one), but it would be supremely cool.


THE PARALLAX PROBLEM, OR: HOW I LEARNED THAT THERE IS NO CONJUNCTION

My original intention for this project was more ambitious than to refer to the photographers according to when they experienced sunset. I had intended to put the photographs themselves in order -- to pick one photo per shoot, whether taken early or late in the evening, and to place it in an exact chronological list of all 111 shoots. The method was to use the Moon's position in the sky with relation to the planets.

Comparing two different days in the photographs, or even two different hours, you can see how much the Moon has traveled in its monthly orbit around the Earth, but the Moon is so close that even a minute's traveling time shifts its position in the sky noticeably, if you measure carefully -- as I tried to do against the laptop screen with a simple ruler. For the days that the Moon was approaching the two planets, I figured that the smaller the distance between it and Venus (or Jupiter), the later the photo; and that after the Moon passed by Venus (or Jupiter), the greater distance marked the later photo.

Of course, the photos came at different scales, so I couldn't just do a straight comparison. It became a matter of measuring two lines in each photo and coming up with a ratio that could be compared against another photo. For photos on September 6, when the three bodies formed a small triangle, I compared a Venus-Moon line with Venus-Jupiter. On the days when the three bodies were closer to forming a line in the sky, it was more straightforward (the pun is really not intended).



Trouble is, trying to distinguish measurements becomes unreliable when using photos of ordinary resolution, since blowing them up to get an exact calculation often leaves the bodies with fuzzy edges. Moreover, the photos come in varying time exposures, with the bodies consequently more "stretched" in some photos than in others, meaning that the center or edge of every body cannot be precisely compared. Then there is the problem of parallax, defined formally as the apparent difference in position or direction of an object as viewed from different points. Let me explain.

The original idea was to differentiate, let's say, between an early-dusk photo taken on September 6th in Cleveland, Ohio and a late-dusk photo taken 650km (400m) farther east in New York City. Two hypothetical photographers from these places, which share the same time zone, take photos in which the conjunction itself looks strikingly similar. Hypothetically, the two photographers have snapped their shutters close to simultaneously. And being in the same time zone, each records his photo as being around 8:30 p.m. local time. Of course, in Cleveland there is more sunlight remaining in the photo, because Cleveland arrived at the terminator 31 minutes after New York did. But which photo was taken first?

Well, reported times are not enough, because watches and clocks are often inaccurate. I planned simply to look at the "conjunctions" in the two photos, and to determine which one looked younger, meaning which one showed the Moon more distant from the planets. And you can do this, if the photos have enough resolution and the calculated difference is outside the margin of error (i.e., if repeated calculations turn up the same result). For the most part I had no trouble, though with some pairs of photos I couldn't be certain. But the chief uncertainty arises from the parallax that occurs because the photographers are viewing from different places.

Let's say I have two people in my field of view: you are two meters in front of me, and I can see someone else about 100 meters somewhere behind you. If I get on a stepladder, you appear lower in my field of vision. The person 100 meters back, however, will hardly appear lower to me. Because I climbed the ladder, you appear to have changed position relative to the more distant person in back: now I can see right above you to that person.

Now, if I see the Moon and two planets, and perhaps a star like Spica, in my field of view as I stand at the "top" of the globe -- the spot farthest above the ecliptic, which is not exactly at the North Pole since the Earth is tilted with relation to the ecliptic -- I can look "over" the Moon to the more distant bodies behind it. In the sky I will see the Moon passing underneath the bodies, and it will pass the planets at a greater distance than it will if I watch from the "bottom" of the globe, near the South Pole.

The same thing occurs at the "side" edges of the Earth. Two meters away from you, if I take several sideward steps to the left, you appear farther to the right side of my field of vision. If a person watches the Moon, Venus and Jupiter from one "side" edge of the Earth, where it is night, and another person watches simultaneously from the opposite edge, where it is day, they will see the Moon in noticeably different positions relative to the planets. Of course, the unaided eye could only watch the conjunction from the "side" edge of the Earth that was in night-time, but even on that slim portion of the Earth -- the shadow in all the "Moon view" images above -- there was a parallax, and a difference in what people saw.

Cleveland and New York, being at different longitudes, are like that person who steps sideways and gets a different field of view: a case of side-to-side parallax, you might call it. The two cities are at the same latitude, so you would think, as I assumed, that they are not like that person who gets a different view by climbing a stepladder: an up-down parallax. But because the Earth is tilted with regard to the ecliptic, we all change our distance from the ecliptic continuously as the Earth rotates us in space. The step ladder analogy does partially apply to Cleveland and New York.

Further complicating the matter is that in the images used here, because the lines of latitude are not being viewed edge-on, those lines are clearly taking a downward curl toward the ecliptic at the rightmost edge of the planet. This is what you can expect, I guess, when you're observing a sphere. So if we regard the two cities at the same point in time, which is higher above the ecliptic? Is New York, when it reaches the terminator, higher or lower than Cleveland as the latter city approaches the terminator from 650 km away? I confess I'm not sure.

To what extent would parallax interfere with the measurements I was making? In a way, the impact would be small. The side-to-side parallax -- which arises chiefly due to different longitudes, we can say in general terms -- has a great potential impact, because the Moon is traveling side-to-side, vaguely speaking, with regard to the ecliptic: its orbit is inclined only 5 degree to the ecliptic. And it's the Moon's orbit that we're measuring when attempting to order the photos, so a side-to-side parallax can be expected to interfere with our attempt to measure the orbit. But this potential impact is minimized because only one edge of the Earth, the one falling into night, was watching this Moon and its two planetary neighbors: so the side-to-side distance between an early-dusk photographer and a late-dusk one is very small.

The up-down parallax -- which arises chiefly due to different latitudes, we can say in general terms -- also has a great potential impact, in that differences in latitude among two simultaneous observers can be immense. But this difference in "height" with regard to the ecliptic cannot greatly effect our view of the Moon's orbit, which is, as we said, nearly side-to-side with regard to the ecliptic.

Perhaps such differences only have an impact when comparing two photos taken within seconds of each other. I don't know if I found any pair of photographs so close together, but some discrepancies did show up in the order. In any case, the photos themselves were not resolved enough to deal with small differences.

Stick with me for one more thought experiment.

Let's say that the photographer in Cleveland actually took his photo one second before the New York photographer took his. Because New York is farther over to the "side" when looking at the three bodies, the effect is that the space between the Moon and the pair of planets will look slightly larger from New York. It's as if I've stepped to the left to get a more direct view of the space between you and the person 100 meters behind you. Well from that apparent closeness in the Cleveland photo between the Moon and the pair of planets, we would normally decide that the Moon has traveled farther in its orbit -- that the Cleveland photo is later. Our perception would be wrong.

So in a number of ways, there really is no conjunction "out there". We have a close analogy when it's said that constellations in the sky would look entirely different from other parts of space, and that constellations don't have an objective existence. But in that case, everyone on Earth is seeing in effect the same thing, and even sensitive instrumentation cannot pick up the parallax due to distances on the Earth's surface. With a conjunction that occurs between our solar system bodies, especially one involving our nearby neighbor the Moon, different "conjunctions" are observed and documented by simultaneous observers at differing latitudes and longitudes: the positions of the bodies with respect to each other are not the same for every observer.

Such variations have long been noted, to be sure, in comparisons of simultaneous photos taken in different places, but what hits home for me is that two simultaneous observers cannot even trust that they share equal perceptions of motion. That's the thing that has come home for me in this project, which has been all about the movement of the Moon. It's one thing to see a unique view of the Moon very close to Venus on September 7th, or even eclipsing that planet, because you're standing at the South Pole, since that is just a question of seeing the bodies of a conjunction in different positions, or arrangements. Our own diverse positions, on the surface of the Earth, naturally have an effect on the positions that we see other bodies take. But it's another to observe the movements of the bodies toward each other -- gravitational movements, or orbits, which are all that make them appear to converge into a conjunction in the first place -- and to measure a movement in a way that is not shared by others standing on our body, largely because we are being continuously rotated to different vantage points in space.

When we watch objects move into a conjunction, we observers are also moving. It's not a straight observation of one objective thing by many static observers. It's a dance between many participants.


I want to thank all the photographers for their photographic work, their stories, and the information they lent to this project. More of the same -- and corrections from anyone -- are most welcome.



EDIT: This post has been edited to update the links; content will remain unedited.

Some links to this post: Ad Astra; Astroblog; Plakboek's Livejournal

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2 Comments:

Anonymous John Koetsier said...

What a cool project!

Thanks for the link, and congrats on doing such a great job of pulling together so many photos from so many places. I wonder what a slideshow of them all might look like ...

June 06, 2006 1:34 PM  
Anonymous Shevill Mathers said...

An incredible effort Kevin, I have never seen this done before and it makes for fascinating reading. I will read it in more detail and over time check out the links. My three successive nights images of the event became part of an article published in the current issue of Tasmania 40 Degrees South Magazine, a glossy coffee table magazine with world-wide distribution. Your global treatment completes the picture.

Kind Regards

Shevill Mathers,
Southern Cross Observatory, Tasmania 42 degrees south.

June 06, 2006 3:45 PM  

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