The sun's year, and why a particular street catches it on a particular day

The piece before this one was about the street, where its bearing comes from and why almost nobody who set that bearing was thinking about the sky. This one is about the other half of the henge, the sun, and about the one small piece of geometry that turns some arbitrary street angle into a real date on the calendar. It is the part most henge writing skips over, because it can be made to sound either too obvious to bother with or too hard to follow, and it is neither. It is just a swing, the slow back-and-forth of the sunrise and sunset points along the horizon over a year. Once you can see the swing, every henge date in the world stops being a little bit magic and turns into something you can read straight off.

Azimuth: a street and the sun measured in the same units

Start with the one word that makes the rest easy. Azimuth is a compass bearing, an angle measured clockwise from north, running from zero at due north through ninety at due east, a hundred and eighty at due south, two hundred and seventy at due west, and back around. It is the number a hiker reads off a compass and the number a surveyor stamps on a street. What makes it useful here is that it lets you describe two completely different things, the way a street runs and the way the sun is sitting, in the very same units, so you can ask the plain question of whether they are equal.

Azimuth is just a compass bearing: degrees clockwise from north. A street has one. The sun, at the moment it rises or sets, has one too. A henge is the day those two numbers match.

A street has, near enough, one azimuth along its straight run. West 42nd Street in Manhattan points along about 299 degrees. West Madison Street in Chicago runs along about 269. The Champs-Elysees goes along about 296. Those numbers are fixed. The surveyors set them and they have not moved since. The sun's azimuth is never fixed. It is different at every moment of every day. And the part that matters for us is that the azimuth at the moment of sunrise or sunset is different on every single day of the year. That daily creep of the rise and set points is the engine of the whole thing.

The swing

Here is the swing itself. Over a year the sun does not rise and set at the same spot on the horizon. In the northern hemisphere the sunrise point marches from its farthest south at the winter solstice, up the eastern horizon toward the northeast, reaching its farthest north at the summer solstice, and then back down again. The sunset point does the mirror image of that in the west, swinging from southwest in December to northwest in June and back. At the two equinoxes, halfway along each leg of the trip, the sun rises very close to due east and sets very close to due west, everywhere on earth.

Drag the street's bearing up and down across the sun's swing. A street's bearing is a fixed horizontal line; the days the swinging curve crosses it are the street's henge evenings. Slide the line toward the flat solstice top and the two crossings merge into one long window; leave it in the fast-moving middle and they stay two sharp, separate dates.

Now the two halves lock together. A street's bearing is a fixed horizontal line drawn across that swinging curve. On most days the sun sets at some bearing that does not match the street, and nothing happens. But twice a year, once on the way up and once on the way down, the sun's setting bearing passes straight through the street's bearing, and for those few evenings the sun goes down square along the pavement. That crossing is the henge. Its date is set entirely by where the street's flat line sits on the sun's vertical swing. A street pointed far to the northwest only catches the sun near midsummer, when the swing reaches all the way up to meet it. A street pointed due west catches it at the equinoxes, when the swing runs through the middle. Move the street's bearing and you slide its henge date up or down the calendar. That is the whole mechanism.

Two kinds of street, two kinds of henge

This is where the abstract swing turns into two concrete and genuinely different kinds of henge, and it is worth seeing the difference in the site's own numbers instead of taking it on faith. Streethenge's alignment engine, the same one behind the calculator and the city pages, scans a whole year of real sunrise and sunset bearings for a point and finds the days the sun lines up with a given street. Run it on Chicago and on New York and the contrast jumps out.

Chicago's West Madison Street runs at about 269 degrees, which is to say almost dead west. Its flat line sits right through the middle of the sun's swing, where the curve is steep and moving fast. The engine finds two sharp, short, well-separated henge windows, one in March and one in September, on either side of the two equinoxes.

Chicago, generated from the live engine. West Madison Street runs almost due west, so the sunset curve crosses its bearing steeply, twice, near the two equinoxes. The dates shown are the engine's computed peaks for the reference year.

New York's West 42nd Street runs at about 299 degrees, well around to the northwest. Its flat line sits high up near the very top of the swing, the stretch around the summer solstice where the curve flattens out and the sun's setting bearing barely changes for weeks. The result is quiet and important. Up near that flat top the sun stays within a few degrees of the street's bearing for a long run of the early summer, so the engine reports not two crisp crossings but one long, broad window peaking in the middle of July.

New York, from the same engine. West 42nd Street points well to the northwest, so its bearing meets the sun up near the flat summer peak of the curve, where the sun lingers. The engine marks the single sharpest evening; the famous pair of Manhattanhenge dates sits on either side of this midsummer plateau.

That single-window result is worth sitting with for a second, because it clears up a thing people get confused about. Manhattanhenge is famous for having two dates, in late May and again in mid-July, sitting either side of the June solstice. Both are real. What the engine shows is why they are not two separate sharp events the way Chicago's are. Between them, right across the solstice, the sun's setting bearing sits so close to the street's that the whole stretch counts as one broad alignment. The two celebrated dates are the days the full disc of the sun sits most perfectly on the grid's centerline, on either side of that plateau. The plateau is why Manhattanhenge is a season and Chicagohenge is a pair of appointments.

That is the whole rule behind both cities, and it is the same rule the draggable swing above makes plain. A near-cardinal street meets the fast-moving middle of the sun's swing and gets two sharp equinox windows. A street tilted toward a solstice skims the slow, flat top and gets one long, broad one. Chicago and New York are just those two settings of the same dial.

Why this counts as knowledge, not mysticism

You could stop at the geometry and call the henge solved, a tidy bit of trigonometry. It pays to go carefully with the claims people build on top of it, though, and the archaeoastronomer Clive Ruggles supplies the discipline. In his encyclopedia of the field, Ruggles describes how the study of ancient sky alignments nearly tore itself apart over exactly this temptation. On one side were researchers who "paid great attention to statistical analysis but little to anthropological theory" and decided that ancient monument builders were sophisticated mathematical astronomers. On the other were people who did the reverse and decided "that ancient peoples had no interest in the sky at all." Ruggles' way out is a distinction worth carrying into any henge conversation. Keep separate "our science," the tested methods for choosing and weighing evidence, from "their science," the worldview of the people who actually built the thing.

For a modern henge street, holding to that distinction is bracing. The geometry is "our science," and it is certain. The engine can tell you, to the day, when the sun will line up with a street, because that is pure astronomy and surveying. But the geometry says nothing at all about whether anyone meant it, and it must not be allowed to. A calculated alignment is a fact about where a street and the sun are pointing right now. It is not evidence that a surveyor two centuries ago was thinking about the solstice. Streethenge's own catalog is built on exactly that separation, marking a real local tradition differently from a street that merely, on the numbers, lines up. The pieces on either side of this one lean on that line constantly.

Anthony Aveni, writing for general readers in People and the Sky, gives the other half of why the geometry is worth caring about at all. His theme across a long career in cultural astronomy is that predictable sky events became powerful precisely because they sit in a sweet spot. Rare enough to feel special, regular enough to plan around. He puts the birth of a specifically predictive attitude to the sky in the same Copernican moment Kostof used for the grand manner, the point where people decided the heavens obeyed laws "capable of leading to predictions that could be made manifest and tested." A henge lives in that sweet spot. It comes only a couple of times a year, it can be missed, and yet it can be known in advance to the minute. That combination, and not any mystery, is what pulls people to a street corner with a camera on a particular evening. The geometry is what makes the promise keepable.

The one number that runs the whole thing

Strip it all away and a henge comes down to a single comparison, the fixed azimuth of a street against the swinging azimuth of the setting or rising sun. The street's number came from survey and sale and monument, the way the first piece argued. The sun's number comes from the tilt of the earth's axis and the date. When the two numbers meet you get a henge, and where on the calendar they meet is decided by how far off due west the street happens to point. A near-cardinal street gets its two equinox evenings. A tilted one gets its off-season window, short or long depending on how near it reaches to the solstice. None of that is mystical, and none of it is made smaller by being understood. If anything it is better, a thing you can predict and walk to and watch keep its appointment, which is exactly what the next pieces are for.

Sources

• The azimuth definition, the annual swing of the sunrise and sunset points between the solstices, and the equinox behavior are standard positional astronomy, treated here as general scientific record.
• The Chicago and New York figures are generated directly from Streethenge's own alignment engine, using the catalog bearings for West Madison Street and West 42nd Street; the marked peak dates are the engine's computed windows for the reference year and are reproduced by the site's own calculator. These figures are pinned to the engine's output, so what you read here and what the site computes cannot drift apart.
• Clive Ruggles, Ancient Astronomy: An Encyclopedia of Cosmologies and Myth (ABC-CLIO, 2005), read directly for this piece. The "statistical analysis" versus "no interest in the sky at all" impasse and the "our science" versus "their science" distinction are from the encyclopedia's methodological material.
• Anthony Aveni, People and the Sky: Our Ancestors and the Cosmos (Thames and Hudson, 2008), read directly. The "rare enough to feel special, regular enough to plan around" throughline and the Copernican account of predictive science (predictions "capable of being made manifest and tested") are from the book's opening chapters.