Built for the sun, or found in the grid

Twice a year, when the sky stays clear, a small crowd turns up at one particular corner about half an hour before sunset. I have stood in that crowd a few times now, off to the side with everyone else, all of us facing the same way down the street toward the west, waiting for the thing to happen. It happens fast. For a couple of minutes the sun settles into the gap between the buildings, low and much too bright to look at, filling the whole slot of the street. Then it slips behind the far roofs. The light goes ordinary again, and people put their phones away and drift off toward dinner. The photograph everyone carries home is the same photograph. It never tells you the one thing I always end up wondering about on the walk back, which is whether the street was aimed at that sunset a long time ago on purpose, or whether it simply happened to be pointing the right way on the evening the sun came down to meet it.

Nearly always it is the second thing. A very small number of streets in the world were genuinely built for the sun, meaning that somebody chose a particular day in the sky and then turned a road, or a temple, or in one case a whole city, so that it would face that day. Those places are strange and rare, and worth going a long way to stand in. Everything else, which is to say almost every henge street there is, got laid out for money or for defense or for the plain look of a long straight avenue. The sun found it later, sometimes two thousand years later, on an evening nobody involved had ever once thought about. The two come out identical in a picture. Everything that follows rests on keeping them apart, which is harder than it sounds, because in a photograph they really are the same evening.

Built for the sunFound in the grid

The whole distinction in one picture. Flip between the two readings: a built alignment runs from a chosen sky event to a street made to match it; a found alignment runs the other way, the street laid first for survey or sale or display, and the sky event noticed afterward. Same photograph, opposite direction of intent.

Alexandria: a street aimed at a birthday

The clearest of the built ones is a street, which is a piece of luck for a series about streets. Alexander founded Alexandria in 331 BCE and had the city run out along one enormous straight spine, the Canopic Road. A long time afterward, two architectural historians, Luisa Ferro and Giulio Magli, went and measured what was left of that spine. They came back with an angle that is very hard to explain as an accident.

Alexandria's main street, plotted from Ferro and Magli's measurements. The Canopic Road at 65 degrees 15 minutes ignores the easy shoreline choice near 49 degrees and even the summer solstice sunrise near 62 degrees 20 minutes, and lands instead on Alexander's birthday sunrise at 64 degrees 30 minutes and the rising of Regulus, the "king's star," at 65 degrees 20 minutes.

They put the road at 65 degrees 15 minutes. The ordinary guess, that a thing like this points at a solstice, does not survive contact with the number. The summer solstice sunrise in that year and that place stood at about 62 degrees 20 minutes, and almost three degrees is a wide miss for surveyors who knew perfectly well how to hit a solstice when they wanted one. What the angle does sit on is smaller and stranger. Alexander was born on the twentieth of July in 356 BCE. The sunrise on that morning, carried back to that latitude and that century, comes out at 64 degrees 30 minutes, three quarters of a degree from the street. And Regulus, the star that a good part of the old Near East called the king's star, was rising in that same founding year along very nearly the identical line, 65 degrees 20 minutes. A road that faces a dead match to a king's birthday sunrise and the king's own star, in the city that king had just built and named after himself, is not the kind of thing you shrug off.

It is also not a one-off, and that is the part that closes the case. A generation later, one of Alexander's generals founded Seleucia on the Tigris, out on flat open ground where he could have faced the town any way he pleased. He set it at 295 degrees, which is Alexandria's bearing folded over across the meridian, so that instead of the rising of that sun and star the town looked toward their setting. Two hundred years after that, a king called Antiochos who liked to trace his blood back to Alexander built himself a tomb up on the mountain at Nemrud Dag. He turned its eastern terrace to a sunrise and a Regulus rising that fall within a day of the same July date. The same target, kept for three centuries across three separate monuments, is what a tradition looks like from the far end of time. It is why Ferro and Magli are willing to write built for it with no hedging in the phrase at all. Alexandria is the one street in everything that follows where the sun got there first, ahead of the surveyors.

The Domus Aurea: an emperor set at the equinox

The same wish shows up a few centuries later with a roof over it. In the wreck of Nero's enormous palace, the Domus Aurea, three researchers, Robert Hannah, Giulio Magli and Antonella Palmieri, went carefully through the Octagonal Room. It is a domed eight-sided hall with a round hole standing open at the very top of the dome, and they take it to be the rough draft that the Pantheon would later perfect. It is a real feat of building, an octagon a little under fifteen meters across with an eye near six meters wide open to the sky almost ten meters up. Every corner of it is squared to the compass points to within about a degree.

The Octagonal Room of the Domus Aurea. Because the room is set within about a degree of cardinal, the equinox midday sun, standing some forty-seven and a half degrees above the horizon in Nero's Rome, drops a shaft through the oculus onto the same spot on the same days each year. The authors read it as placing the emperor "at the equinoxes as a point of balance in the heavens."

Squaring it that way was not the convenient choice, and the researchers are careful to say so. The room would have sat more naturally facing the lake and the Palatine hill off to the south of west. Facing true north and south and east and west instead was something the builders went out of their way to impose. What they bought with the trouble was a piece of theater that runs on the calendar. At noon on the two equinoxes the sun stands due south and lets a column of light straight down through the eye onto the floor, the same light on the same days every year. The authors read the whole room as a way of standing the emperor at the balance point of the sky, placing him, in their words, "at the equinoxes as a point of balance in the heavens." Sixty years on, Hadrian's Pantheon would say the identical thing at a far grander size, its own eye letting the equinox noon sun down onto the doorway. The order of things is the same as at Alexandria. The day in the sky came first, and the building was bent to fit around it.

Abu Simbel: the case that moved

The oldest of the three is the one that lets the secret out, because it got picked up and set back down again while people were alive to watch. At Abu Simbel, which Magli describes in his book on the sacred landscape of Egypt, the temple that Ramesses the Second had cut back into a cliff was carved so that on two mornings a year the rising sun runs the full length of the passage inside and reaches all the way to the seated figures in the last dark room. That is about as bold as a built alignment gets, a tunnel of solid rock aimed at a sunrise three thousand years before anyone thought to put a number on it. Then in the nineteen-sixties the Aswan High Dam was going to put the whole site under water. A UNESCO project sawed the temple out of the cliff in blocks and rebuilt it on higher ground. However carefully that was done, the old alignment could not be put back exactly, so the morning the light reaches the far room is now generally said to fall about a day off from where it used to. You move the tunnel, and the date moves with it. The day was never really kept in the stone. It lived in the angle between a fixed passage and the sun's slow yearly walk along the horizon, and the sun keeps no record of where the passage happens to be. If that holds for a temple somebody built on purpose, it holds a good deal harder for a street that never gave the sun a thought.

The streets people actually stand on

The streets people gather on to watch a henge are the other kind, and there is nothing poorer about them for it. The word found only tells you which way the thing was made. Turin is the case that stings a little, because its street really is old and really was laid on purpose and still comes out found. Torinohenge happens along Via Giuseppe Garibaldi, which lies on the decumanus maximus, the main east-west line, of the Roman colony of Augusta Taurinorum, cut by surveyors two thousand years ago under a founding ceremony they took seriously. The street was made with enormous deliberation. It was simply made as a survey line for a garrison town, and not for any sunset. The numbers give the whole thing away. The engine puts it near 297 degrees, with its henge evenings falling in May and again in high summer, which are not dates any Roman on the site was ever watching for.

Turin, from the live engine. Via Giuseppe Garibaldi carries the deliberate line of a Roman survey axis two thousand years old, but its henge evenings fall in May and late July, dates no Roman surveyor was aiming at. The street is built; the henge riding on it is found.

Paris says the same from the opposite end of the scale. The Champs-Elysees is one of those grand designed avenues the first piece was about, worked over for three hundred years to carry your eye up the slope to the Arc de Triomphe. It is about as deliberately made as a street on earth can be, and not one decision in all that making had a thing to do with where the sun goes down. The engine puts it near 296 degrees, with its henge evenings in spring and late summer, handed to it by the angle and by nothing else.

Paris, from the live engine. The Champs-Elysees was designed with enormous care, to frame the Arc de Triomphe, not to catch a sunset. Its henge dates fall out of a bearing that was chosen for the view and nothing else.

Set those next to the Manhattan grid from the first piece and the shape of it is plain enough. A Roman survey line, a Baroque view up to a monument, a New York grid drawn to sell building lots. They were made in different centuries for reasons that have nothing in common with one another, and every last one of them catches a sunset it was never once pointed at.

The honest version is the quieter one

There is a temptation, and it sells well, to smudge the two kinds together, to lend Manhattan a little of Alexandria's purpose and let people go on believing that the great avenues were always somehow meant for the light. This project keeps them in separate columns, a street with a real recorded tradition on one side and a street that merely lines up on the numbers on the other. It does that for the same reason the piece began with. The built cases are astonishing exactly because they almost never happen. One man aimed a whole city at a dead king's birthday. Another stood an emperor in a shaft of noon light twice a year on purpose. Masons three thousand years ago drove a sunrise into the back of a cliff. Spread that kind of intention thin across every street in the world that happens to face northwest, and you have turned it into nothing. It is very far from nothing.

What the found streets have instead is quieter, and it is the thing the whole project turns on. The sky keeps its own old schedule, indifferent to every bit of this. It goes on arriving down streets that were laid out for profit and for show, and we have gotten good enough at the arithmetic now to tell you the evening ahead of time. The built ones are people reaching up to touch the sun. The found ones are the sun coming all the way down a street that was busy looking somewhere else, and I have come around to thinking that is the better of the two things to have stood on a corner and watched.

Sources

• Luisa Ferro and Giulio Magli, "The astronomical orientation of the urban plan of Alexandria" (arXiv:1103.0939v2). Every Alexandria measurement here (the 65 degrees 15 minutes street azimuth, the rejected 62 degrees 20 minutes solstice sunrise, the 64 degrees 30 minutes birthday sunrise, the 65 degrees 20 minutes Regulus rising, and the Seleucia and Nemrud Dag comparisons) is from that paper.
• Robert Hannah, Giulio Magli, and Antonella Palmieri, "Nero's 'solar' kingship and the architecture of the Domus Aurea" (arXiv:1312.7583v2). The Octagonal Room dimensions, the roughly one-degree cardinal orientation imposed "by necessity," the equinox noon hierophany, the Pantheon relationship, and the phrase placing the emperor "at the equinoxes as a point of balance in the heavens" are from that paper.
• Giulio Magli, Architecture, Astronomy and Sacred Landscape in Ancient Egypt (Cambridge University Press, 2013), for Abu Simbel's twice-yearly solar penetration. The roughly one-day shift in the illumination dates after the 1960s UNESCO relocation is widely reported general record and is used here as an illustration, not a precise claim.
• Turin, Paris, and New York street bearings and henge dates are generated from Streethenge's own alignment engine. The Roman origin of Turin's Via Giuseppe Garibaldi as the decumanus of Augusta Taurinorum, and the grand-manner design history of the Champs-Elysees, are general historical record, not a citation to one primary source.